SENDMAILTM
INSTALLATION AND OPERATION GUIDE
Eric Allman
Claus Assmann
Gregory Neil Shapiro
Proofpoint, Inc.
For Sendmail Version 8.17
SendmailTM implements a general purpose internetwork mail routing facility under the UNIX® oper-
ating system. It is not tied to any one transport protocol — its function may be likened to a crossbar switch,
relaying messages from one domain into another. In the process, it can do a limited amount of message
header editing to put the message into a format that is appropriate for the receiving domain. All of this is
done under the control of a configuration file.
Due to the requirements of flexibility for sendmail, the configuration file can seem somewhat unap-
proachable. However, there are only a few basic configurations for most sites, for which standard configu-
ration files have been supplied. Most other configurations can be built by adjusting an existing configura-
tion file incrementally.
Sendmail is based on RFC 821 (Simple Mail Transport Protocol), RFC 822 (Internet Mail Headers
Format), RFC 974 (MX routing), RFC 1123 (Internet Host Requirements), RFC 1413 (Identification
server), RFC 1652 (SMTP 8BITMIME Extension), RFC 1869 (SMTP Service Extensions), RFC 1870
(SMTP SIZE Extension), RFC 1891 (SMTP Delivery Status Notifications), RFC 1892 (Multipart/Report),
RFC 1893 (Enhanced Mail System Status Codes), RFC 1894 (Delivery Status Notifications), RFC 1985
(SMTP Service Extension for Remote Message Queue Starting), RFC 2033 (Local Message Transmission
Protocol), RFC 2034 (SMTP Service Extension for Returning Enhanced Error Codes), RFC 2045 (MIME),
RFC 2476 (Message Submission), RFC 2487 (SMTP Service Extension for Secure SMTP over TLS), RFC
2554 (SMTP Service Extension for Authentication), RFC 2821 (Simple Mail Transfer Protocol), RFC 2822
(Internet Message Format), RFC 2852 (Deliver By SMTP Service Extension), RFC 2920 (SMTP Service
Extension for Command Pipelining), and RFC 7505 (A "Null MX" No Service Resource Record for Do-
mains That Accept No Mail). However, since sendmail is designed to work in a wider world, in many cases
it can be configured to exceed these protocols. These cases are described herein.
Although sendmail is intended to run without the need for monitoring, it has a number of features
that may be used to monitor or adjust the operation under unusual circumstances. These features are de-
scribed.
Section one describes how to do a basic sendmail installation. Section two explains the day-to-day
information you should know to maintain your mail system. If you have a relatively normal site, these two
sections should contain sufficient information for you to install sendmail and keep it happy. Section three
has information regarding the command line arguments. Section four describes some parameters that may
DISCLAIMER: This documentation is under modification.
Sendmail is a trademark of Proofpoint, Inc. US Patent Numbers 6865671, 6986037.
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Sendmail Installation and Operation Guide
be safely tweaked. Section five contains the nitty-gritty information about the configuration file. This sec-
tion is for masochists and people who must write their own configuration file. Section six describes config-
uration that can be done at compile time. The appendixes give a brief but detailed explanation of a number
of features not described in the rest of the paper.
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TABLE OF CONTENTS
1. BASIC INSTALLATION ...............................................................................................................
1.1. Compiling Sendmail ..............................................................................................................
1.1.1. Tweaking the Build Invocation .....................................................................................
1.1.2. Creating a Site Configuration File ................................................................................
1.1.3. Tweaking the Makefile .................................................................................................
1.1.4. Compilation and installation ........................................................................................
1.2. Configuration Files ................................................................................................................
1.3. Details of Installation Files ...................................................................................................
1.3.1. /usr/sbin/sendmail .........................................................................................................
1.3.2. /etc/mail/sendmail.cf ....................................................................................................
1.3.3. /etc/mail/submit.cf ........................................................................................................
1.3.4. /usr/bin/newaliases .......................................................................................................
1.3.5. /usr/bin/hoststat ............................................................................................................
1.3.6. /usr/bin/purgestat ..........................................................................................................
1.3.7. /var/spool/mqueue ........................................................................................................
1.3.8. /var/spool/clientmqueue ...............................................................................................
1.3.9. /var/spool/mqueue/.hoststat ..........................................................................................
1.3.10. /etc/mail/aliases* ........................................................................................................
1.3.11. /etc/rc or /etc/init.d/sendmail ......................................................................................
1.3.12. /etc/mail/helpfile .........................................................................................................
1.3.13. /etc/mail/statistics .......................................................................................................
1.3.14. /usr/bin/mailq .............................................................................................................
1.3.15. sendmail.pid ...............................................................................................................
1.3.16. Map Files ....................................................................................................................
2. NORMAL OPERATIONS .............................................................................................................
2.1. The System Log ....................................................................................................................
2.1.1. Format ..........................................................................................................................
2.1.2. Levels ...........................................................................................................................
2.2. Dumping State .......................................................................................................................
2.3. The Mail Queues ...................................................................................................................
2.3.1. Queue Groups and Queue Directories ..........................................................................
2.3.2. Queue Runs ..................................................................................................................
2.3.3. Manual Intervention .....................................................................................................
2.3.4. Printing the queue .........................................................................................................
2.3.5. Forcing the queue .........................................................................................................
2.3.6. Quarantined Queue Items .............................................................................................
2.4. Disk Based Connection Information .....................................................................................
2.5. The Service Switch ................................................................................................................
2.6. The Alias Database ................................................................................................................
2.6.1. Rebuilding the alias database .......................................................................................
2.6.2. Potential problems ........................................................................................................
2.6.3. List owners ...................................................................................................................
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2.7. User Information Database ....................................................................................................
2.8. Per-User Forwarding (.forward Files) ...................................................................................
2.9. Special Header Lines .............................................................................................................
2.9.1. Errors-To: .....................................................................................................................
2.9.2. Apparently-To: .............................................................................................................
2.9.3. Precedence ....................................................................................................................
2.10. IDENT Protocol Support .....................................................................................................
3. ARGUMENTS ...............................................................................................................................
3.1. Queue Interval .......................................................................................................................
3.2. Daemon Mode .......................................................................................................................
3.3. Forcing the Queue .................................................................................................................
3.4. Debugging .............................................................................................................................
3.5. Changing the Values of Options ............................................................................................
3.6. Trying a Different Configuration File ...................................................................................
3.7. Logging Traffic ......................................................................................................................
3.8. Testing Configuration Files ...................................................................................................
3.9. Persistent Host Status Information ........................................................................................
4. TUNING .........................................................................................................................................
4.1. Timeouts ................................................................................................................................
4.1.1. Queue interval ..............................................................................................................
4.1.2. Read timeouts ...............................................................................................................
4.1.3. Message timeouts .........................................................................................................
4.2. Forking During Queue Runs .................................................................................................
4.3. Queue Priorities .....................................................................................................................
4.4. Load Limiting ........................................................................................................................
4.5. Resource Limits .....................................................................................................................
4.6. Measures against Denial of Service Attacks .........................................................................
4.7. Delivery Mode .......................................................................................................................
4.8. Log Level ...............................................................................................................................
4.9. File Modes .............................................................................................................................
4.9.1. To suid or not to suid? .................................................................................................
4.9.2. Turning off security checks ..........................................................................................
4.10. Connection Caching ............................................................................................................
4.11. Name Server Access ............................................................................................................
4.12. Moving the Per-User Forward Files ....................................................................................
4.13. Free Space ...........................................................................................................................
4.14. Maximum Message Size .....................................................................................................
4.15. Privacy Flags .......................................................................................................................
4.16. Send to Me Too ...................................................................................................................
5. THE WHOLE SCOOP ON THE CONFIGURATION FILE ........................................................
5.1. R and S — Rewriting Rules ..................................................................................................
5.1.1. The left hand side .........................................................................................................
5.1.2. The right hand side .......................................................................................................
5.1.3. Semantics of rewriting rule sets ...................................................................................
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5.1.4. Ruleset hooks ...............................................................................................................
5.1.4.1. check_relay ..........................................................................................................
5.1.4.2. check_mail ..........................................................................................................
5.1.4.3. check_rcpt ...........................................................................................................
5.1.4.4. check_data ...........................................................................................................
5.1.4.5. check_other .........................................................................................................
5.1.4.6. check_compat ......................................................................................................
5.1.4.7. check_eoh ............................................................................................................
5.1.4.8. check_eom ...........................................................................................................
5.1.4.9. check_etrn ...........................................................................................................
5.1.4.10. check_expn ........................................................................................................
5.1.4.11. check_vrfy .........................................................................................................
5.1.4.12. clt_features ........................................................................................................
5.1.4.13. trust_auth ...........................................................................................................
5.1.4.14. tls_client ............................................................................................................
5.1.4.15. tls_server ...........................................................................................................
5.1.4.16. tls_rcpt ...............................................................................................................
5.1.4.17. srv_features .......................................................................................................
5.1.4.18. try_tls .................................................................................................................
5.1.4.19. tls_srv_features and tls_clt_features .................................................................
5.1.4.20. authinfo ..............................................................................................................
5.1.4.21. queuegroup ........................................................................................................
5.1.4.22. greet_pause ........................................................................................................
5.1.5. IPC mailers ...................................................................................................................
5.2. D — Define Macro ................................................................................................................
5.3. C and F — Define Classes .....................................................................................................
5.4. M — Define Mailer ...............................................................................................................
5.5. H — Define Header ...............................................................................................................
5.6. O — Set Option .....................................................................................................................
5.7. P — Precedence Definitions ..................................................................................................
5.8. V — Configuration Version Level .........................................................................................
5.9. K — Key File Declaration .....................................................................................................
5.10. Q — Queue Group Declaration ...........................................................................................
5.11. X — Mail Filter (Milter) Definitions ..................................................................................
5.12. The User Database ..............................................................................................................
5.12.1. Structure of the user database .....................................................................................
5.12.2. User database semantics .............................................................................................
5.12.3. Creating the database23 ...............................................................................................
6. OTHER CONFIGURATION .........................................................................................................
6.1. Parameters in devtools/OS/$oscf ...........................................................................................
6.1.1. For Future Releases ......................................................................................................
6.2. Parameters in sendmail/conf.h ..............................................................................................
6.3. Configuration in sendmail/conf.c ..........................................................................................
6.3.1. Built-in Header Semantics ...........................................................................................
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6.3.2. Restricting Use of Email ..............................................................................................
6.3.3. New Database Map Classes .........................................................................................
6.3.4. Queueing Function .......................................................................................................
6.3.5. Refusing Incoming SMTP Connections .......................................................................
6.3.6. Load Average Computation ..........................................................................................
6.4. Configuration in sendmail/daemon.c .....................................................................................
6.5. LDAP .....................................................................................................................................
6.5.1. LDAP Recursion ..........................................................................................................
6.5.1.1. Example ...............................................................................................................
6.6. STARTTLS ............................................................................................................................
6.6.1. Certificates for STARTTLS ..........................................................................................
6.6.2. PRNG for STARTTLS .................................................................................................
6.7. Encoding of STARTTLS and AUTH related Macros ............................................................
6.8. DANE ....................................................................................................................................
6.9. EAI ........................................................................................................................................
6.10. MTA-STS ............................................................................................................................
7. ACKNOWLEDGEMENTS ............................................................................................................
Appendix A. COMMAND LINE FLAGS .........................................................................................
Appendix B. QUEUE FILE FORMATS ............................................................................................
Appendix C. SUMMARY OF SUPPORT FILES ..............................................................................
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1. BASIC INSTALLATION
There are two basic steps to installing sendmail. First, you have to compile and install the binary.
If sendmail has already been ported to your operating system that should be simple. Second, you must
build a run-time configuration file. This is a file that sendmail reads when it starts up that describes the
mailers it knows about, how to parse addresses, how to rewrite the message header, and the settings of
various options. Although the configuration file can be quite complex, a configuration can usually be
built using an M4-based configuration language. Assuming you have the standard sendmail distribu-
tion, see cf/README for further information.
The remainder of this section will describe the installation of sendmail assuming you can use one
of the existing configurations and that the standard installation parameters are acceptable. All path-
names and examples are given from the root of the sendmail subtree, normally /usr/src/usr.sbin/send-
mail on 4.4BSD-based systems.
Continue with the next section if you need/want to compile sendmail yourself. If you have a run-
ning binary already on your system, you should probably skip to section 1.2.
1.1. Compiling Sendmail
All sendmail source is in the sendmail subdirectory. To compile sendmail, “cd” into the send-
mail directory and type
./Build
This will leave the binary in an appropriately named subdirectory, e.g., obj.BSD-OS.2.1.i386. It
works for multiple object versions compiled out of the same directory.
1.1.1. Tweaking the Build Invocation
You can give parameters on the Build command. In most cases these are only used when
the obj.* directory is first created. To restart from scratch, use -c. These commands include:
−L libdirs
A list of directories to search for libraries.
−I incdirs
A list of directories to search for include files.
−E envar=value
Set an environment variable to an indicated value before compiling.
−c
Create a new obj.* tree before running.
−f siteconfig
Read the indicated site configuration file. If this parameter is not specified, Build in-
cludes
all
of
the
files
$BUILDTOOLS/Site/site.$oscf.m4
and
$BUILD-
TOOLS/Site/site.config.m4, where $BUILDTOOLS is normally ../devtools and $oscf is
the same name as used on the obj.* directory. See below for a description of the site
configuration file.
−S
Skip auto-configuration. Build will avoid auto-detecting libraries if this is set. All li-
braries and map definitions must be specified in the site configuration file.
Most other parameters are passed to the
make
program; for details see
$BUILD-
TOOLS/README.
1.1.2. Creating a Site Configuration File
See sendmail/README for various compilation flags that can be set, and dev-
tools/README for details how to set them.
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1.1.3. Tweaking the Makefile
Sendmail supports two different formats for the local (on disk) version of databases, no-
tably the aliases database. At least one of these should be defined if at all possible.
CDB
Constant DataBase (tinycdb).
NDBM
The ‘‘new DBM’’ format, available on nearly all systems around today. This
was the preferred format prior to 4.4BSD. It allows such complex things as
multiple databases and closing a currently open database.
NEWDB
The Berkeley DB package. If you have this, use it. It allows long records,
multiple open databases, real in-memory caching, and so forth. You can de-
fine this in conjunction with NDBM; if you do, old alias databases are read, but
when a new database is created it will be in NEWDB format. As a nasty hack,
if you have NEWDB, NDBM, and NIS defined, and if the alias file name in-
cludes the substring “/yp/”, sendmail will create both new and old versions of
the alias file during a newalias command. This is required because the Sun
NIS/YP system reads the DBM version of the alias file. It’s ugly as sin, but it
works.
If neither of these are defined, sendmail reads the alias file into memory on every invocation.
This can be slow and should be avoided. There are also several methods for remote database ac-
cess:
LDAP
Lightweight Directory Access Protocol.
NIS
Sun’s Network Information Services (formerly YP).
NISPLUS
Sun’s NIS+ services.
NETINFO
NeXT’s NetInfo service.
HESIOD
Hesiod service (from Athena).
Other compilation flags are set in conf.h and should be predefined for you unless you are porting
to a new environment. For more options see sendmail/README.
1.1.4. Compilation and installation
After making the local system configuration described above, You should be able to com-
pile and install the system. The script “Build” is the best approach on most systems:
./Build
This will use uname(1) to create a custom Makefile for your environment.
If you are installing in the standard places, you should be able to install using
./Build install
This should install the binary in /usr/sbin and create links from /usr/bin/newaliases and
/usr/bin/mailq to /usr/sbin/sendmail. On most systems it will also format and install man pages.
Notice: as of version 8.12 sendmail will no longer be installed set-user-ID root by default. If
you really want to use the old method, you can specify it as target:
./Build install-set-user-id
1.2. Configuration Files
Sendmail cannot operate without a configuration file. The configuration defines the mail de-
livery mechanisms understood at this site, how to access them, how to forward email to remote mail
systems, and a number of tuning parameters. This configuration file is detailed in the later portion
of this document.
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The sendmail configuration can be daunting at first. The world is complex, and the mail con-
figuration reflects that. The distribution includes an m4-based configuration package that hides a lot
of the complexity. See cf/README for details.
Our configuration files are processed by m4 to facilitate local customization; the directory cf
of the sendmail distribution directory contains the source files. This directory contains several sub-
directories:
cf
Both site-dependent and site-independent descriptions of hosts. These can be lit-
eral host names (e.g., “ucbvax.mc”) when the hosts are gateways or more general
descriptions (such as “generic-solaris2.mc” as a general description of an SMTP-
connected host running Solaris 2.x. Files ending .mc (‘‘M4 Configuration’’) are
the input descriptions; the output is in the corresponding .cf file. The general
structure of these files is described below.
domain
Site-dependent subdomain descriptions. These are tied to the way your organiza-
tion wants to do addressing. For example, domain/CS.Berkeley.EDU.m4 is our
description for hosts in the CS.Berkeley.EDU subdomain. These are referenced
using the DOMAIN m4 macro in the .mc file.
feature
Definitions of specific features that some particular host in your site might want.
These are referenced using the FEATURE m4 macro. An example feature is
use_cw_file (which tells sendmail to read an /etc/mail/local-host-names file on
startup to find the set of local names).
hack
Local hacks, referenced using the HACK m4 macro. Try to avoid these. The
point of having them here is to make it clear that they smell.
m4
Site-independent m4(1) include files that have information common to all configu-
ration files. This can be thought of as a “#include” directory.
mailer
Definitions of mailers, referenced using the MAILER m4 macro. The mailer types
that are known in this distribution are fax, local, smtp, uucp, and usenet. For ex-
ample, to include support for the UUCP-based mailers, use “MAILER(uucp)”.
ostype
Definitions describing various operating system environments (such as the loca-
tion of support files). These are referenced using the OSTYPE m4 macro.
sh
Shell files used by the m4 build process. You shouldn’t hav e to mess with these.
siteconfig
Local UUCP connectivity information. This directory has been supplanted by the
mailertable feature; any new configurations should use that feature to do UUCP
(and other) routing. The use of this directory is deprecated.
If you are in a new domain (e.g., a company), you will probably want to create a cf/domain
file for your domain. This consists primarily of relay definitions and features you want enabled site-
wide: for example, Berkeley’s domain definition defines relays for BitNET and UUCP. These are
specific to Berkeley, and should be fully-qualified internet-style domain names. Please check to
make certain they are reasonable for your domain.
Subdomains at Berkeley are also represented in the cf/domain directory. For example, the do-
main CS.Berkeley.EDU is the Computer Science subdomain, EECS.Berkeley.EDU is the Electrical
Engineering and Computer Sciences subdomain, and S2K.Berkeley.EDU is the Sequoia 2000 sub-
domain. You will probably have to add an entry to this directory to be appropriate for your domain.
You will have to use or create .mc files in the cf/cf subdirectory for your hosts. This is de-
tailed in the cf/README file.
1.3. Details of Installation Files
This subsection describes the files that comprise the sendmail installation.
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1.3.1. /usr/sbin/sendmail
The binary for sendmail is located in /usr/sbin1. It should be set-group-ID smmsp as de-
scribed in sendmail/SECURITY. For security reasons, /, /usr, and /usr/sbin should be owned by
root, mode 07552.
1.3.2. /etc/mail/sendmail.cf
This is the main configuration file for sendmail3. This is one of the two non-library file
names compiled into sendmail4, the other is /etc/mail/submit.cf.
The configuration file is normally created using the distribution files described above. If
you have a particularly unusual system configuration you may need to create a special version.
The format of this file is detailed in later sections of this document.
1.3.3. /etc/mail/submit.cf
This is the configuration file for sendmail when it is used for initial mail submission, in
which case it is also called ‘‘Mail Submission Program’’ (MSP) in contrast to ‘‘Mail Transfer
Agent’’ (MTA). Starting with version 8.12, sendmail uses one of two different configuration
files based on its operation mode (or the new −A option). For initial mail submission, i.e., if one
of the options −bm (default), −bs, or −t is specified, submit.cf is used (if available), for other
operations sendmail.cf is used. Details can be found in sendmail/SECURITY. submit.cf is
shipped with sendmail (in cf/cf/) and is installed by default. If changes to the configuration need
to be made, start with cf/cf/submit.mc and follow the instruction in cf/README.
1.3.4. /usr/bin/newaliases
The newaliases command should just be a link to sendmail:
rm −f /usr/bin/newaliases
ln −s /usr/sbin/sendmail /usr/bin/newaliases
This can be installed in whatever search path you prefer for your system.
1.3.5. /usr/bin/hoststat
The hoststat command should just be a link to sendmail, in a fashion similar to
newaliases. This command lists the status of the last mail transaction with all remote hosts.
The −v flag will prevent the status display from being truncated. It functions only when the
HostStatusDirectory option is set.
1.3.6. /usr/bin/purgestat
This command is also a link to sendmail. It flushes expired (Timeout.hoststatus) informa-
tion that is stored in the HostStatusDirectory tree.
1This is usually /usr/sbin on 4.4BSD and newer systems; many systems install it in /usr/lib. I understand it is in /usr/ucblib on
System V Release 4.
2Some vendors ship them owned by bin; this creates a security hole that is not actually related to sendmail. Other important di-
rectories that should have restrictive ownerships and permissions are /bin, /usr/bin, /etc, /etc/mail, /usr/etc, /lib, and /usr/lib.
3Actually, the pathname varies depending on the operating system; /etc/mail is the preferred directory. Some older systems in-
stall it in /usr/lib/sendmail.cf, and I’ve also seen it in /usr/ucblib. If you want to move this file, add -D_PATH_SENDMAIL-
CF=\"/file/name\" to the flags passed to the C compiler. Moving this file is not recommended: other programs and scripts know of this
location.
4The system libraries can reference other files; in particular, system library subroutines that sendmail calls probably reference
/etc/passwd and /etc/resolv.conf.
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1.3.7. /var/spool/mqueue
The directory /var/spool/mqueue should be created to hold the mail queue. This directory
should be mode 0700 and owned by root.
The actual path of this directory is defined by the QueueDirectory option of the send-
mail.cf file. To use multiple queues, supply a value ending with an asterisk. For example,
/var/spool/mqueue/qd* will use all of the directories or symbolic links to directories beginning
with ‘qd’ in /var/spool/mqueue as queue directories. Do not change the queue directory struc-
ture while sendmail is running.
If these directories have subdirectories or symbolic links to directories named ‘qf’, ‘df’,
and ‘xf’, then these will be used for the different queue file types. That is, the data files are
stored in the ‘df’ subdirectory, the transcript files are stored in the ‘xf’ subdirectory, and all oth-
ers are stored in the ‘qf’ subdirectory.
If shared memory support is compiled in, sendmail stores the available diskspace in a
shared memory segment to make the values readily available to all children without incurring
system overhead. In this case, only the daemon updates the data; i.e., the sendmail daemon cre-
ates the shared memory segment and deletes it if it is terminated. To use this, sendmail must
have been compiled with support for shared memory (-DSM_CONF_SHM) and the option
SharedMemoryKey must be set. Notice: do not use the same key for sendmail invocations
with different queue directories or different queue group declarations. Access to shared memory
is not controlled by locks, i.e., there is a race condition when data in the shared memory is up-
dated. However, since operation of sendmail does not rely on the data in the shared memory,
this does not negatively influence the behavior.
1.3.8. /var/spool/clientmqueue
The directory /var/spool/clientmqueue should be created to hold the mail queue. This di-
rectory should be mode 0770 and owned by user smmsp, group smmsp.
The actual path of this directory is defined by the QueueDirectory option of the submit.cf
file.
1.3.9. /var/spool/mqueue/.hoststat
This is a typical value for the HostStatusDirectory option, containing one file per host
that this sendmail has chatted with recently. It is normally a subdirectory of mqueue.
1.3.10. /etc/mail/aliases*
The system aliases are held in “/etc/mail/aliases”. A sample is given in “sendmail/aliases”
which includes some aliases which must be defined:
cp sendmail/aliases /etc/mail/aliases
edit /etc/mail/aliases
You should extend this file with any aliases that are apropos to your system.
Normally sendmail looks at a database version of the files, stored either in
“/etc/mail/aliases.dir” and “/etc/mail/aliases.pag” or “/etc/mail/aliases.db” depending on which
database package you are using. The actual path of this file is defined in the AliasFile option of
the sendmail.cf file.
The permissions of the alias file and the database versions should be 0640 to prevent local
denial of service attacks as explained in the top level README in the sendmail distribution. If
the permissions 0640 are used, be sure that only trusted users belong to the group assigned to
those files. Otherwise, files should not even be group readable.
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1.3.11. /etc/rc or /etc/init.d/sendmail
It will be necessary to start up the sendmail daemon when your system reboots. This dae-
mon performs two functions: it listens on the SMTP socket for connections (to receive mail
from a remote system) and it processes the queue periodically to insure that mail gets delivered
when hosts come up.
If necessary, add the following lines to “/etc/rc” (or “/etc/rc.local” as appropriate) in the
area where it is starting up the daemons on a BSD-base system, or on a System-V-based system
in one of the startup files, typically “/etc/init.d/sendmail”:
if [ −f /usr/sbin/sendmail −a −f /etc/mail/sendmail.cf ]; then
(cd /var/spool/mqueue; rm −f xf*)
/usr/sbin/sendmail −bd −q30m &
echo −n ’ sendmail’ >/dev/console
fi
The “cd” and “rm” commands insure that all transcript files have been removed; extraneous
transcript files may be left around if the system goes down in the middle of processing a mes-
sage. The line that actually invokes sendmail has two flags: “−bd” causes it to listen on the
SMTP port, and “−q30m” causes it to run the queue every half hour.
Some people use a more complex startup script, removing zero length qf/hf/Qf files and df
files for which there is no qf/hf/Qf file. Note this is not advisable. For example, see Figure 1 for
an example of a complex script which does this clean up.
1.3.12. /etc/mail/helpfile
This is the help file used by the SMTP HELP command. It should be copied from “send-
mail/helpfile”:
cp sendmail/helpfile /etc/mail/helpfile
The actual path of this file is defined in the HelpFile option of the sendmail.cf file.
1.3.13. /etc/mail/statistics
If you wish to collect statistics about your mail traffic, you should create the file
“/etc/mail/statistics”:
cp /dev/null /etc/mail/statistics
chmod 0600 /etc/mail/statistics
This file does not grow. It is printed with the program “mailstats/mailstats.c.” The actual path
of this file is defined in the S option of the sendmail.cf file.
1.3.14. /usr/bin/mailq
If sendmail is invoked as “mailq,” it will simulate the −bp flag (i.e., sendmail will print
the contents of the mail queue; see below). This should be a link to /usr/sbin/sendmail.
1.3.15. sendmail.pid
sendmail stores its current pid in the file specified by the PidFile option (default is
_PATH_SENDMAILPID). sendmail uses TempFileMode (which defaults to 0600) as the per-
missions of that file to prevent local denial of service attacks as explained in the top level
README in the sendmail distribution. If the file already exists, then it might be necessary to
change the permissions accordingly, e.g.,
chmod 0600 /var/run/sendmail.pid
Note that as of version 8.13, this file is unlinked when sendmail exits. As a result of this
change, a script such as the following, which may have worked prior to 8.13, will no longer
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#!/bin/sh
# remove zero length qf/hf/Qf files
for qffile in qf* hf* Qf*
do
if [ −r $qffile ]
then
if [ ! −s $qffile ]
then
echo −n " <zero: $qffile>" > /dev/console
rm −f $qffile
fi
fi
done
# rename tf files to be qf if the qf does not exist
for tffile in tf*
do
qffile=‘echo $tffile | sed ’s/t/q/’‘
if [ −r $tffile −a ! −f $qffile ]
then
echo −n " <recovering: $tffile>" > /dev/console
mv $tffile $qffile
else
if [ −f $tffile ]
then
echo −n " <extra: $tffile>" > /dev/console
rm −f $tffile
fi
fi
done
# remove df files with no corresponding qf/hf/Qf files
for dffile in df*
do
qffile=‘echo $dffile | sed ’s/d/q/’‘
hffile=‘echo $dffile | sed ’s/d/h/’‘
Qffile=‘echo $dffile | sed ’s/d/Q/’‘
if [ −r $dffile −a ! −f $qffile −a ! −f $hffile −a ! −f $Qffile ]
then
echo −n " <incomplete: $dffile>" > /dev/console
mv $dffile ‘echo $dffile | sed ’s/d/D/’‘
fi
done
# announce files that have been saved during disaster recovery
for xffile in [A-Z]f*
do
if [ −f $xffile ]
then
echo −n " <panic: $xffile>" > /dev/console
fi
done
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Sendmail Installation and Operation Guide
Figure 1 — A complex startup script
work:
# stop & start sendmail
PIDFILE=/var/run/sendmail.pid
kill ‘head -1 $PIDFILE‘
‘tail -1 $PIDFILE‘
because it assumes that the pidfile will still exist even after killing the process to which it refers.
Below is a script which will work correctly on both newer and older versions:
# stop & start sendmail
PIDFILE=/var/run/sendmail.pid
pid=‘head -1 $PIDFILE‘
cmd=‘tail -1 $PIDFILE‘
kill $pid
$cmd
This is just an example script, it does not perform any error checks, e.g., whether the pidfile ex-
ists at all.
1.3.16. Map Files
To prevent local denial of service attacks as explained in the top level README in the
sendmail distribution, the permissions of map files created by makemap should be 0640. The
use of 0640 implies that only trusted users belong to the group assigned to those files. If those
files already exist, then it might be necessary to change the permissions accordingly, e.g.,
cd /etc/mail
chmod 0640 *.db *.pag *.dir
2. NORMAL OPERATIONS
2.1. The System Log
The system log is supported by the syslogd (8) program. All messages from sendmail are
logged under the LOG_MAIL facility5.
2.1.1. Format
Each line in the system log consists of a timestamp, the name of the machine that gener-
ated it (for logging from several machines over the local area network), the word “sendmail:”,
and a message6. Most messages are a sequence of name=value pairs.
The two most common lines are logged when a message is processed. The first logs the
receipt of a message; there will be exactly one of these per message. Some fields may be omit-
ted if they do not contain interesting information. Fields are:
from
The envelope sender address.
size
The size of the message in bytes.
5Except on Ultrix, which does not support facilities in the syslog.
6This format may vary slightly if your vendor has changed the syntax.
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class
The class (i.e., numeric precedence) of the message.
pri
The initial message priority (used for queue sorting).
nrcpts
The number of envelope recipients for this message (after aliasing and for-
warding).
msgid
The message id of the message (from the header).
bodytype
The message body type (7BIT or 8BITMIME), as determined from the enve-
lope.
proto
The protocol used to receive this message (e.g., ESMTP or UUCP)
daemon
The daemon name from the DaemonPortOptions setting.
relay
The machine from which it was received.
There is also one line logged per delivery attempt (so there can be several per message if deliv-
ery is deferred or there are multiple recipients). Fields are:
to
A comma-separated list of the recipients to this mailer.
ctladdr
The ‘‘controlling user’’, that is, the name of the user whose credentials we use
for delivery.
delay
The total delay between the time this message was received and the current
delivery attempt.
xdelay
The amount of time needed in this delivery attempt (normally indicative of the
speed of the connection).
mailer
The name of the mailer used to deliver to this recipient.
relay
The name of the host that actually accepted (or rejected) this recipient.
dsn
The enhanced error code (RFC 2034) if available.
stat
The delivery status.
Not all fields are present in all messages; for example, the relay is usually not listed for local de-
liveries.
2.1.2. Levels
If you have syslogd (8) or an equivalent installed, you will be able to do logging. There is
a large amount of information that can be logged. The log is arranged as a succession of levels.
At the lowest level only extremely strange situations are logged. At the highest level, even the
most mundane and uninteresting events are recorded for posterity. As a convention, log levels
under ten are considered generally “useful;” log levels above 64 are reserved for debugging pur-
poses. Levels from 11−64 are reserved for verbose information that some sites might want.
A complete description of the log levels is given in section ‘‘Log Level’’.
2.2. Dumping State
You can ask sendmail to log a dump of the open files and the connection cache by sending it a
SIGUSR1 signal. The results are logged at LOG_DEBUG priority.
2.3. The Mail Queues
Mail messages may either be delivered immediately or be held for later delivery. Held mes-
sages are placed into a holding directory called a mail queue.
A mail message may be queued for these reasons:
• If a mail message is temporarily undeliverable, it is queued and delivery is attempted later. If the
message is addressed to multiple recipients, it is queued only for those recipients to whom
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Sendmail Installation and Operation Guide
delivery is not immediately possible.
• If the SuperSafe option is set to true, all mail messages are queued while delivery is attempted.
• If the DeliveryMode option is set to queue-only or defer, all mail is queued, and no immediate
delivery is attempted.
• If the load average becomes higher than the value of the QueueLA option and the QueueFactor
(q) option divided by the difference in the current load average and the QueueLA option plus
one is less than the priority of the message, messages are queued rather than immediately deliv-
ered.
• One or more addresses are marked as expensive and delivery is postponed until the next queue
run or one or more address are marked as held via mailer which uses the hold mailer flag.
• The mail message has been marked as quarantined via a mail filter or rulesets.
2.3.1. Queue Groups and Queue Directories
There are one or more mail queues. Each mail queue belongs to a queue group. There is
always a default queue group that is called ‘‘mqueue’’ (which is where messages go by default
unless otherwise specified). The directory or directories which comprise the default queue
group are specified by the QueueDirectory option. There are zero or more additional named
queue groups declared using the Q command in the configuration file.
By default, a queued message is placed in the queue group associated with the first recipi-
ent in the recipient list. A recipient address is mapped to a queue group as follows. First, if
there is a ruleset called ‘‘queuegroup’’, and if this ruleset maps the address to a queue group
name, then that queue group is chosen. That is, the argument for the ruleset is the recipient ad-
dress (i.e., the address part of the resolved triple) and the result should be $# followed by the
name of a queue group. Otherwise, if the mailer associated with the address specifies a queue
group, then that queue group is chosen. Otherwise, the default queue group is chosen.
A message with multiple recipients will be split if different queue groups are chosen by
the mapping of recipients to queue groups.
When a message is placed in a queue group, and the queue group has more than one
queue, a queue is selected randomly.
If a message with multiple recipients is placed into a queue group with the ’r’ option
(maximum number of recipients per message) set to a positive value N, and if there are more
than N recipients in the message, then the message will be split into multiple messages, each of
which have at most N recipients.
Notice: if multiple queue groups are used, do not move queue files around, e.g., into a dif-
ferent queue directory. This may have weird effects and can cause mail not to be delivered.
Queue files and directories should be treated as opaque and should not be manipulated directly.
2.3.2. Queue Runs
sendmail has two different ways to process the queue(s). The first one is to start queue
runners after certain intervals (‘‘normal’’ queue runners), the second one is to keep queue runner
processes around (‘‘persistent’’ queue runners). How to select either of these types is discussed
in the appendix ‘‘COMMAND LINE FLAGS’’. Persistent queue runners have the advantage
that no new processes need to be spawned at certain intervals; they just sleep for a specified time
after they finished a queue run. Another advantage of persistent queue runners is that only one
process belonging to a workgroup (a workgroup is a set of queue groups) collects the data for a
queue run and then multiple queue runner may go ahead using that data. This can significantly
reduce the disk I/O necessary to read the queue files compared to starting multiple queue run-
ners directly. Their disadvantage is that a new queue run is only started after all queue runners
belonging to a group finished their tasks. In case one of the queue runners tries delivery to a
slow recipient site at the end of a queue run, the next queue run may be substantially delayed.
In general this should be smoothed out due to the distribution of those slow jobs, however, for
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sites with small number of queue entries this might introduce noticeable delays. In general, per-
sistent queue runners are only useful for sites with big queues.
2.3.3. Manual Intervention
Under normal conditions the mail queue will be processed transparently. Howev er, you
may find that manual intervention is sometimes necessary. For example, if a major host is down
for a period of time the queue may become clogged. Although sendmail ought to recover grace-
fully when the host comes up, you may find performance unacceptably bad in the meantime. In
that case you want to check the content of the queue and manipulate it as explained in the next
two sections.
2.3.4. Printing the queue
The contents of the queue(s) can be printed using the mailq command (or by specifying
the −bp flag to sendmail):
mailq
This will produce a listing of the queue id’s, the size of the message, the date the message en-
tered the queue, and the sender and recipients. If shared memory support is compiled in, the
flag −bP can be used to print the number of entries in the queue(s), provided a process updates
the data. However, as explained earlier, the output might be slightly wrong, since access to the
shared memory is not locked. For example, ‘‘unknown number of entries’’ might be shown.
The internal counters are updated after each queue run to the correct value again.
2.3.5. Forcing the queue
Sendmail should run the queue automatically at intervals. When using multiple queues, a
separate process will by default be created to run each of the queues unless the queue run is ini-
tiated by a user with the verbose flag. The algorithm is to read and sort the queue, and then to
attempt to process all jobs in order. When it attempts to run the job, sendmail first checks to see
if the job is locked. If so, it ignores the job.
There is no attempt to insure that only one queue processor exists at any time, since there
is no guarantee that a job cannot take forever to process (however, sendmail does include heuris-
tics to try to abort jobs that are taking absurd amounts of time; technically, this violates RFC
821, but is blessed by RFC 1123). Due to the locking algorithm, it is impossible for one job to
freeze the entire queue. However, an uncooperative recipient host or a program recipient that
never returns can accumulate many processes in your system. Unfortunately, there is no com-
pletely general way to solve this.
In some cases, you may find that a major host going down for a couple of days may create
a prohibitively large queue. This will result in sendmail spending an inordinate amount of time
sorting the queue. This situation can be fixed by moving the queue to a temporary place and
creating a new queue. The old queue can be run later when the offending host returns to service.
To do this, it is acceptable to move the entire queue directory:
cd /var/spool
mv mqueue omqueue; mkdir mqueue; chmod 0700 mqueue
You should then kill the existing daemon (since it will still be processing in the old queue direc-
tory) and create a new daemon.
To run the old mail queue, issue the following command:
/usr/sbin/sendmail −C /etc/mail/queue.cf −q
The −C flag specifies an alternate configuration file queue.cf which should refer to the moved
queue directory
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Sendmail Installation and Operation Guide
O QueueDirectory=/var/spool/omqueue
and the −q flag says to just run every job in the queue. You can also specify the moved queue
directory on the command line
/usr/sbin/sendmail −oQ/var/spool/omqueue −q
but this requires that you do not have queue groups in the configuration file, because those are
not subdirectories of the moved directory. See the section about ‘‘Queue Group Declaration’’
for details; you most likely need a different configuration file to correctly deal with this problem.
However, a proper configuration of queue groups should avoid filling up queue directories, so
you shouldn’t run into this problem. If you have a tendency tow ard voyeurism, you can use the
−v flag to watch what is going on.
When the queue is finally emptied, you can remove the directory:
rmdir /var/spool/omqueue
2.3.6. Quarantined Queue Items
It is possible to "quarantine" mail messages, otherwise known as envelopes. Envelopes
(queue files) are stored but not considered for delivery or display unless the "quarantine" state of
the envelope is undone or delivery or display of quarantined items is requested. Quarantined
messages are tagged by using a different name for the queue file, ’hf’ instead of ’qf’, and by
adding the quarantine reason to the queue file.
Delivery or display of quarantined items can be requested using the −qQ flag to sendmail
or mailq. Additionally, messages already in the queue can be quarantined or unquarantined us-
ing the new −Q flag to sendmail. For example,
sendmail -Qreason -q[!][I|R|S][matchstring]
Quarantines the normal queue items matching the criteria specified by the -q[!][I|R|S][match-
string] using the reason given on the −Q flag. Likewise,
sendmail -qQ -Q[reason] -q[!][I|R|S|Q][matchstring]
Change the quarantine reason for the quarantined items matching the criteria specified by the
-q[!][I|R|S|Q][matchstring] using the reason given on the −Q flag. If there is no reason,
unquarantine the matching items and make them normal queue items. Note that the −qQ flag
tells sendmail to operate on quarantined items instead of normal items.
2.4. Disk Based Connection Information
Sendmail stores a large amount of information about each remote system it has connected to
in memory. It is possible to preserve some of this information on disk as well, by using the HostSta-
tusDirectory option, so that it may be shared between several invocations of sendmail. This allows
mail to be queued immediately or skipped during a queue run if there has been a recent failure in
connecting to a remote machine. Note: information about a remote system is stored in a file whose
pathname consists of the components of the hostname in reverse order. For example, the informa-
tion for host.example.com is stored in com./example./host. For top-level domains like com this
can create a large number of subdirectories which on some filesystems can exhaust some limits.
Moreover, the performance of lookups in directory with thousands of entries can be fairly slow de-
pending on the filesystem implementation.
Additionally enabling SingleThreadDelivery has the added effect of single-threading mail
delivery to a destination. This can be quite helpful if the remote machine is running an SMTP
server that is easily overloaded or cannot accept more than a single connection at a time, but can
cause some messages to be punted to a future queue run. It also applies to all hosts, so setting this
because you have one machine on site that runs some software that is easily overrun can cause mail
to other hosts to be slowed down. If this option is set, you probably want to set the MinQueueAge
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option as well and run the queue fairly frequently; this way jobs that are skipped because another
sendmail is talking to the same host will be tried again quickly rather than being delayed for a long
time.
The disk based host information is stored in a subdirectory of the mqueue directory called
.hoststat7. Removing this directory and its subdirectories has an effect similar to the purgestat
command and is completely safe. However, purgestat only removes expired (Timeout.hoststatus)
data. The information in these directories can be perused with the hoststat command, which will in-
dicate the host name, the last access, and the status of that access. An asterisk in the left most col-
umn indicates that a sendmail process currently has the host locked for mail delivery.
The disk based connection information is treated the same way as memory based connection
information for the purpose of timeouts. By default, information about host failures is valid for 30
minutes. This can be adjusted with the Timeout.hoststatus option.
The connection information stored on disk may be expired at any time with the purgestat
command or by invoking sendmail with the −bH switch. The connection information may be
viewed with the hoststat command or by invoking sendmail with the −bh switch.
2.5. The Service Switch
The implementation of certain system services such as host and user name lookup is con-
trolled by the service switch. If the host operating system supports such a switch, and sendmail
knows about it, sendmail will use the native version. Ultrix, Solaris, and DEC OSF/1 are examples
of such systems8.
If the underlying operating system does not support a service switch (e.g., SunOS 4.X, HP-
UX, BSD) then sendmail will provide a stub implementation. The ServiceSwitchFile option points
to the name of a file that has the service definitions. Each line has the name of a service and the
possible implementations of that service. For example, the file:
hosts
dns files nis
aliases files nis
will ask sendmail to look for hosts in the Domain Name System first. If the requested host name is
not found, it tries local files, and if that fails it tries NIS. Similarly, when looking for aliases it will
try the local files first followed by NIS.
Notice: since sendmail must access MX records for correct operation, it will use DNS if it is
configured in the ServiceSwitchFile file. Hence an entry like
hosts
files dns
will not avoid DNS lookups even if a host can be found in /etc/hosts.
Note: in contrast to the sendmail stub implementation some operating systems do not preserve
temporary failures. For example, if DNS returns a TRY_AGAIN status for this setup
hosts
files dns myhostname
but myhostname does not find the requested entry, then a permanent error is returned to sendmail
which obviously can cause problems, e.g., an immediate bounce instead of a deferral.
Service switches are not completely integrated. For example, despite the fact that the host en-
try listed in the above example specifies to look in NIS, on SunOS this won’t happen because the
system implementation of gethostbyname (3) doesn’t understand this.
7This is the usual value of the HostStatusDirectory option; it can, of course, go anywhere you like in your filesystem.
8HP-UX 10 has service switch support, but since the APIs are apparently not available in the libraries sendmail does not use the
native service switch in this release.
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Sendmail Installation and Operation Guide
2.6. The Alias Database
After recipient addresses are read from the SMTP connection or command line they are
parsed by ruleset 0, which must resolve to a {mailer, host, address} triple. If the flags selected by
the mailer include the A (aliasable) flag, the address part of the triple is looked up as the key (i.e.,
the left hand side) in the alias database. If there is a match, the address is deleted from the send
queue and all addresses on the right hand side of the alias are added in place of the alias that was
found. This is a recursive operation, so aliases found in the right hand side of the alias are similarly
expanded.
The alias database exists in two forms. One is a text form, maintained in the file
/etc/mail/aliases. The aliases are of the form
name: name1, name2, ...
Only local names may be aliased; e.g.,
eric@prep.ai.MIT.EDU: eric@CS.Berkeley.EDU
will not have the desired effect (except on prep.ai.MIT.EDU, and they probably don’t want me)9.
Aliases may be continued by starting any continuation lines with a space or a tab or by putting a
backslash directly before the newline. Blank lines and lines beginning with a sharp sign (“#”) are
comments.
The second form is processed by one of the available map types, e.g., ndbm (3)10 the Berkeley
DB library, or cdb. This is the form that sendmail actually uses to resolve aliases. This technique is
used to improve performance.
The control of search order is actually set by the service switch. Essentially, the entry
O AliasFile=switch:aliases
is always added as the first alias entry; also, the first alias file name without a class (e.g., without
“nis:” on the front) will be used as the name of the file for a ‘‘files’’ entry in the aliases switch. For
example, if the configuration file contains
O AliasFile=/etc/mail/aliases
and the service switch contains
aliases nis files nisplus
then aliases will first be searched in the NIS database, then in /etc/mail/aliases, then in the NIS+
database.
You can also use NIS-based alias files. For example, the specification:
O AliasFile=/etc/mail/aliases
O AliasFile=nis:mail.aliases@my.nis.domain
will first search the /etc/mail/aliases file and then the map named “mail.aliases” in “my.nis.domain”.
Warning: if you build your own NIS-based alias files, be sure to provide the −l flag to makedbm(8)
to map upper case letters in the keys to lower case; otherwise, aliases with upper case letters in their
names won’t match incoming addresses.
Additional flags can be added after the colon exactly like a K line — for example:
O AliasFile=nis:−N mail.aliases@my.nis.domain
will search the appropriate NIS map and always include null bytes in the key. Also:
9Actually, any mailer that has the ‘A’ mailer flag set will permit aliasing; this is normally limited to the local mailer.
10The gdbm package does not work.
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O AliasFile=nis:−f mail.aliases@my.nis.domain
will prevent sendmail from downcasing the key before the alias lookup.
2.6.1. Rebuilding the alias database
The hash or dbm version of the database may be rebuilt explicitly by executing the com-
mand
newaliases
This is equivalent to giving sendmail the −bi flag:
/usr/sbin/sendmail −bi
If you have multiple aliases databases specified, the −bi flag rebuilds all the database
types it understands (for example, it can rebuild NDBM databases but not NIS databases).
2.6.2. Potential problems
There are a number of problems that can occur with the alias database. They all result
from a sendmail process accessing the DBM version while it is only partially built. This can
happen under two circumstances: One process accesses the database while another process is re-
building it, or the process rebuilding the database dies (due to being killed or a system crash) be-
fore completing the rebuild.
Sendmail has three techniques to try to relieve these problems. First, it ignores interrupts
while rebuilding the database; this avoids the problem of someone aborting the process leaving a
partially rebuilt database. Second, it locks the database source file during the rebuild — but that
may not work over NFS or if the file is unwritable. Third, at the end of the rebuild it adds an
alias of the form
@: @
(which is not normally legal). Before sendmail will access the database, it checks to insure that
this entry exists11.
2.6.3. List owners
If an error occurs on sending to a certain address, say “x”, sendmail will look for an alias
of the form “owner-x” to receive the errors. This is typically useful for a mailing list where the
submitter of the list has no control over the maintenance of the list itself; in this case the list
maintainer would be the owner of the list. For example:
unix-wizards: eric@ucbarpa, wnj@monet, nosuchuser,
sam@matisse
owner-unix-wizards: unix-wizards-request
unix-wizards-request: eric@ucbarpa
would cause “eric@ucbarpa” to get the error that will occur when someone sends to unix-wiz-
ards due to the inclusion of “nosuchuser” on the list.
List owners also cause the envelope sender address to be modified. The contents of the
owner alias are used if they point to a single user, otherwise the name of the alias itself is used.
For this reason, and to obey Internet conventions, the “owner-” address normally points at the
“-request” address; this causes messages to go out with the typical Internet convention of using
‘‘list-request’’ as the return address.
11The AliasWait option is required in the configuration for this action to occur. This should normally be specified.
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Sendmail Installation and Operation Guide
2.7. User Information Database
This option is deprecated, use virtusertable and genericstable instead as explained in
cf/README. If you have a version of sendmail with the user information database compiled in, and
you have specified one or more databases using the U option, the databases will be searched for a
user:maildrop entry. If found, the mail will be sent to the specified address.
2.8. Per-User Forwarding (.forward Files)
As an alternative to the alias database, any user may put a file with the name “.forward” in his
or her home directory. If this file exists, sendmail redirects mail for that user to the list of addresses
listed in the .forward file. Note that aliases are fully expanded before forward files are referenced.
For example, if the home directory for user “mckusick” has a .forward file with contents:
mckusick@ernie
kirk@calder
then any mail arriving for “mckusick” will be redirected to the specified accounts.
Actually, the configuration file defines a sequence of filenames to check. By default, this is
the user’s .forward file, but can be defined to be more generally using the ForwardPath option. If
you change this, you will have to inform your user base of the change; .forward is pretty well incor-
porated into the collective subconscious.
2.9. Special Header Lines
Several header lines have special interpretations defined by the configuration file. Others
have interpretations built into sendmail that cannot be changed without changing the code. These
built-ins are described here.
2.9.1. Errors-To:
If errors occur anywhere during processing, this header will cause error messages to go to
the listed addresses. This is intended for mailing lists.
The Errors-To: header was created in the bad old days when UUCP didn’t understand the
distinction between an envelope and a header; this was a hack to provide what should now be
passed as the envelope sender address. It should go away. It is only used if the UseErrorsTo
option is set.
The Errors-To: header is officially deprecated and will go away in a future release.
2.9.2. Apparently-To:
RFC 822 requires at least one recipient field (To:, Cc:, or Bcc: line) in every message. If a
message comes in with no recipients listed in the message then sendmail will adjust the header
based on the “NoRecipientAction” option. One of the possible actions is to add an “Apparently-
To:” header line for any recipients it is aware of.
The Apparently-To: header is non-standard and is both deprecated and strongly discour-
aged.
2.9.3. Precedence
The Precedence: header can be used as a crude control of message priority. It tweaks the
sort order in the queue and can be configured to change the message timeout values. The prece-
dence of a message also controls how delivery status notifications (DSNs) are processed for that
message.
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2.10. IDENT Protocol Support
Sendmail supports the IDENT protocol as defined in RFC 1413. Note that the RFC states a
client should wait at least 30 seconds for a response. The default Timeout.ident is 5 seconds as
many sites have adopted the practice of dropping IDENT queries. This has lead to delays process-
ing mail. Although this enhances identification of the author of an email message by doing a ‘‘call
back’’ to the originating system to include the owner of a particular TCP connection in the audit
trail it is in no sense perfect; a determined forger can easily spoof the IDENT protocol. The follow-
ing description is excerpted from RFC 1413:
6. Security Considerations
The information returned by this protocol is at most as trustworthy as the host providing it OR
the organization operating the host. For example, a PC in an open lab has few if any controls
on it to prevent a user from having this protocol return any identifier the user wants. Like-
wise, if the host has been compromised the information returned may be completely erro-
neous and misleading.
The Identification Protocol is not intended as an authorization or access control protocol. At
best, it provides some additional auditing information with respect to TCP connections. At
worst, it can provide misleading, incorrect, or maliciously incorrect information.
The use of the information returned by this protocol for other than auditing is strongly dis-
couraged. Specifically, using Identification Protocol information to make access control deci-
sions - either as the primary method (i.e., no other checks) or as an adjunct to other methods
may result in a weakening of normal host security.
An Identification server may reveal information about users, entities, objects or processes
which might normally be considered private. An Identification server provides service which
is a rough analog of the CallerID services provided by some phone companies and many of
the same privacy considerations and arguments that apply to the CallerID service apply to
Identification. If you wouldn’t run a "finger" server due to privacy considerations you may
not want to run this protocol.
In some cases your system may not work properly with IDENT support due to a bug in the TCP/IP
implementation. The symptoms will be that for some hosts the SMTP connection will be closed al-
most immediately. If this is true or if you do not want to use IDENT, you should set the IDENT
timeout to zero; this will disable the IDENT protocol.
3. ARGUMENTS
The complete list of arguments to sendmail is described in detail in Appendix A. Some important
arguments are described here.
3.1. Queue Interval
The amount of time between forking a process to run through the queue is defined by the −q
flag. If you run with delivery mode set to i or b this can be relatively large, since it will only be rel-
evant when a host that was down comes back up. If you run in q mode it should be relatively short,
since it defines the maximum amount of time that a message may sit in the queue. (See also the
MinQueueAge option.)
RFC 1123 section 5.3.1.1 says that this value should be at least 30 minutes (although that
probably doesn’t make sense if you use ‘‘queue-only’’ mode).
Notice: the meaning of the interval time depends on whether normal queue runners or persis-
tent queue runners are used. For the former, it is the time between subsequent starts of a queue run.
For the latter, it is the time sendmail waits after a persistent queue runner has finished its work to
start the next one. Hence for persistent queue runners this interval should be very low, typically no
more than two minutes.
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3.2. Daemon Mode
If you allow incoming mail over an IPC connection, you should have a daemon running. This
should be set by your /etc/rc file using the −bd flag. The −bd flag and the −q flag may be combined
in one call:
/usr/sbin/sendmail −bd −q30m
An alternative approach is to invoke sendmail from inetd(8) (use the −bs −Am flags to ask
sendmail to speak SMTP on its standard input and output and to run as MTA). This works and al-
lows you to wrap sendmail in a TCP wrapper program, but may be a bit slower since the configura-
tion file has to be re-read on every message that comes in. If you do this, you still need to have a
sendmail running to flush the queue:
/usr/sbin/sendmail −q30m
3.3. Forcing the Queue
In some cases you may find that the queue has gotten clogged for some reason. You can force
a queue run using the −q flag (with no value). It is entertaining to use the −v flag (verbose) when
this is done to watch what happens:
/usr/sbin/sendmail −q −v
You can also limit the jobs to those with a particular queue identifier, recipient, sender, quar-
antine reason, or queue group using one of the queue modifiers. For example, “−qRberkeley” re-
stricts the queue run to jobs that have the string “berkeley” somewhere in one of the recipient ad-
dresses. Similarly, “−qSstring” limits the run to particular senders, “−qIstring” limits it to particular
queue identifiers, and “−qQstring” limits it to particular quarantined reasons and only operated on
quarantined queue items, and “−qGstring” limits it to a particular queue group. The named queue
group will be run even if it is set to have 0 runners. You may also place an ! before the I or R or S
or Q to indicate that jobs are limited to not including a particular queue identifier, recipient or
sender. For example, “−q!Rseattle” limits the queue run to jobs that do not have the string “seattle”
somewhere in one of the recipient addresses. Should you need to terminate the queue jobs currently
active then a SIGTERM to the parent of the process (or processes) will cleanly stop the jobs.
3.4. Debugging
There are a fairly large number of debug flags built into sendmail. Each debug flag has a cat-
egory and a level. Higher levels increase the level of debugging activity; in most cases, this means
to print out more information. The convention is that levels greater than nine are “absurd,” i.e., they
print out so much information that you wouldn’t normally want to see them except for debugging
that particular piece of code.
You should never run a production sendmail server in debug mode. Many of the debug flags
will result in debug output being sent over the SMTP channel unless the option −D is used. This
will confuse many mail programs. However, for testing purposes, it can be useful when sending
mail manually via telnet to the port you are using while debugging.
A debug category is either an integer, like 42, or a name, like ANSI. You can specify a range
of numeric debug categories using the syntax 17-42. You can specify a set of named debug cate-
gories using a glob pattern like “sm_trace_*”. At present, only “*” and “?” are supported in these
glob patterns.
Debug flags are set using the −d option; the syntax is:
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debug-flag:
−d debug-list
debug-list:
debug-option [ , debug-option ]*
debug-option:
debug-categories [ . debug-level ]
debug-categories:
integer | integer − integer | category-pattern
category-pattern:
[a-zA-Z_*?][a-zA-Z0-9_*?]*
debug-level:
integer
where spaces are for reading ease only. For example,
−d12
Set category 12 to level 1
−d12.3
Set category 12 to level 3
−d3−17
Set categories 3 through 17 to level 1
−d3−17.4
Set categories 3 through 17 to level 4
−dANSI
Set category ANSI to level 1
−dsm_trace_*.3
Set all named categories matching sm_trace_* to level 3
For a complete list of the available debug flags you will have to look at the code and the TRACE-
FLAGS file in the sendmail distribution (they are too dynamic to keep this document up to date).
For a list of named debug categories in the sendmail binary, use
ident /usr/sbin/sendmail | grep Debug
3.5. Changing the Values of Options
Options can be overridden using the −o or −O command line flags. For example,
/usr/sbin/sendmail −oT2m
sets the T (timeout) option to two minutes for this run only; the equivalent line using the long option
name is
/usr/sbin/sendmail -OTimeout.queuereturn=2m
Some options have security implications. Sendmail allows you to set these, but relinquishes
its set-user-ID or set-group-ID permissions thereafter12.
3.6. Trying a Different Configuration File
An alternative configuration file can be specified using the −C flag; for example,
/usr/sbin/sendmail −Ctest.cf −oQ/tmp/mqueue
uses the configuration file test.cf instead of the default /etc/mail/sendmail.cf. If the −C flag has no
value it defaults to sendmail.cf in the current directory.
Sendmail gives up set-user-ID root permissions (if it has been installed set-user-ID root) when
you use this flag, so it is common to use a publicly writable directory (such as /tmp) as the queue di-
rectory (QueueDirectory or Q option) while testing.
3.7. Logging Traffic
Many SMTP implementations do not fully implement the protocol. For example, some per-
sonal computer based SMTPs do not understand continuation lines in reply codes. These can be
very hard to trace. If you suspect such a problem, you can set traffic logging using the −X flag. For
example,
12That is, it sets its effective uid to the real uid; thus, if you are executing as root, as from root’s crontab file or during system
startup the root permissions will still be honored.
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/usr/sbin/sendmail −X /tmp/traffic −bd
will log all traffic in the file /tmp/traffic.
This logs a lot of data very quickly and should NEVER be used during normal operations.
After starting up such a daemon, force the errant implementation to send a message to your host.
All message traffic in and out of sendmail, including the incoming SMTP traffic, will be logged in
this file.
3.8. Testing Configuration Files
When you build a configuration table, you can do a certain amount of testing using the “test
mode” of sendmail. For example, you could invoke sendmail as:
sendmail −bt −Ctest.cf
which would read the configuration file “test.cf” and enter test mode. In this mode, you enter lines
of the form:
rwset address
where rwset is the rewriting set you want to use and address is an address to apply the set to. Test
mode shows you the steps it takes as it proceeds, finally showing you the address it ends up with.
You may use a comma separated list of rwsets for sequential application of rules to an input. For
example:
3,1,21,4 monet:bollard
first applies ruleset three to the input “monet:bollard.” Ruleset one is then applied to the output of
ruleset three, followed similarly by rulesets twenty-one and four.
If you need more detail, you can also use the “−d21” flag to turn on more debugging. For ex-
ample,
sendmail −bt −d21.99
turns on an incredible amount of information; a single word address is probably going to print out
several pages worth of information.
You should be warned that internally, sendmail applies ruleset 3 to all addresses. In test mode
you will have to do that manually. For example, older versions allowed you to use
0 bruce@broadcast.sony.com
This version requires that you use:
3,0 bruce@broadcast.sony.com
As of version 8.7, some other syntaxes are available in test mode:
.D x value
defines macro x to have the indicated value. This is useful when debugging rules
that use the $&x syntax.
.C c value
adds the indicated value to class c.
=S ruleset
dumps the contents of the indicated ruleset.
−d debug-spec
is equivalent to the command-line flag.
Version 8.9 introduced more features:
?
shows a help message.
=M
display the known mailers.
$m
print the value of macro m.
$=c
print the contents of class c.
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/mx host
returns the MX records for ‘host’.
/parse address
parse address, returning the value of crackaddr, and the parsed address.
/try mailer addr
rewrite address into the form it will have when presented to the indicated mailer.
/tryflags flags
set flags used by parsing. The flags can be ‘H’ for Header or ‘E’ for Envelope,
and ‘S’ for Sender or ‘R’ for Recipient. These can be combined, ‘HR’ sets flags
for header recipients.
/canon hostname try to canonify hostname.
/map mapname key
look up ‘key’ in the indicated ‘mapname’.
/quit
quit address test mode.
3.9. Persistent Host Status Information
When HostStatusDirectory is enabled, information about the status of hosts is maintained on
disk and can thus be shared between different instantiations of sendmail. The status of the last con-
nection with each remote host may be viewed with the command:
sendmail −bh
This information may be flushed with the command:
sendmail −bH
Flushing the information prevents new sendmail processes from loading it, but does not prevent ex-
isting processes from using the status information that they already have.
4. TUNING
There are a number of configuration parameters you may want to change, depending on the re-
quirements of your site. Most of these are set using an option in the configuration file. For example,
the line “O Timeout.queuereturn=5d” sets option “Timeout.queuereturn” to the value “5d” (five days).
Most of these options have appropriate defaults for most sites. However, sites having very high
mail loads may find they need to tune them as appropriate for their mail load. In particular, sites experi-
encing a large number of small messages, many of which are delivered to many recipients, may find
that they need to adjust the parameters dealing with queue priorities.
All versions of sendmail prior to 8.7 had single character option names. As of 8.7, options have
long (multi-character names). Although old short names are still accepted, most new options do not
have short equivalents.
This section only describes the options you are most likely to want to tweak; read section 5 for
more details.
4.1. Timeouts
All time intervals are set using a scaled syntax. For example, “10m” represents ten minutes,
whereas “2h30m” represents two and a half hours. The full set of scales is:
s
seconds
m
minutes
h
hours
d
days
w
weeks
4.1.1. Queue interval
The argument to the −q flag specifies how often a sub-daemon will run the queue. This is
typically set to between fifteen minutes and one hour. If not set, or set to zero, the queue will
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not be run automatically. RFC 1123 section 5.3.1.1 recommends that this be at least 30 minutes.
Should you need to terminate the queue jobs currently active then a SIGTERM to the parent of
the process (or processes) will cleanly stop the jobs.
4.1.2. Read timeouts
Timeouts all have option names “Timeout.suboption”. Most of these control SMTP oper-
ations. The recognized suboptions, their default values, and the minimum values allowed by
RFC 2821 section 4.5.3.2 (or RFC 1123 section 5.3.2) are:
connect
The time to wait for an SMTP connection to open (the connect(2) system call)
[0, unspecified]. If zero, uses the kernel default. In no case can this option
extend the timeout longer than the kernel provides, but it can shorten it. This
is to get around kernels that provide an absurdly long connection timeout (90
minutes in one case).
iconnect
The same as connect, except it applies only to the initial attempt to connect to
a host for a given message [0, unspecified]. The concept is that this should be
very short (a few seconds); hosts that are well connected and responsive will
thus be serviced immediately. Hosts that are slow will not hold up other deliv-
eries in the initial delivery attempt.
aconnect
[0, unspecified] The overall timeout waiting for all connection for a single de-
livery attempt to succeed. If 0, no overall limit is applied. This can be used to
restrict the total amount of time trying to connect to a long list of host that
could accept an e-mail for the recipient. This timeout does not apply to Fall-
backMXhost, i.e., if the time is exhausted, the FallbackMXhost is tried next.
initial
The wait for the initial 220 greeting message [5m, 5m].
helo
The wait for a reply from a HELO or EHLO command [5m, unspecified].
This may require a host name lookup, so five minutes is probably a reasonable
minimum.
mail†
The wait for a reply from a MAIL command [10m, 5m].
rcpt†
The wait for a reply from a RCPT command [1h, 5m]. This should be long
because it could be pointing at a list that takes a long time to expand (see be-
low).
datainit†
The wait for a reply from a DAT A command [5m, 2m].
datablock†‡
The wait for reading a data block (that is, the body of the message). [1h, 3m].
This should be long because it also applies to programs piping input to send-
mail which have no guarantee of promptness.
datafinal†
The wait for a reply from the dot terminating a message. [1h, 10m]. If this is
shorter than the time actually needed for the receiver to deliver the message,
duplicates will be generated. This is discussed in RFC 1047.
rset
The wait for a reply from a RSET command [5m, unspecified].
quit
The wait for a reply from a QUIT command [2m, unspecified].
misc
The wait for a reply from miscellaneous (but short) commands such as NOOP
(no-operation) and VERB (go into verbose mode). [2m, unspecified].
command†‡
In server SMTP, the time to wait for another command. [1h, 5m].
ident‡
The timeout waiting for a reply to an IDENT query [5s13, unspecified].
13On some systems the default is zero to turn the protocol off entirely.
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lhlo
The wait for a reply to an LMTP LHLO command [2m, unspecified].
auth
The timeout for a reply in an SMTP AUTH dialogue [10m, unspecified].
starttls
The timeout for a reply to an SMTP STARTTLS command and the TLS hand-
shake [1h, unspecified].
fileopen‡
The timeout for opening .forward and :include: files [60s, none].
control‡
The timeout for a complete control socket transaction to complete [2m, none].
hoststatus‡
How long status information about a host (e.g., host down) will be cached be-
fore it is considered stale [30m, unspecified].
resolver.retrans‡ The resolver’s retransmission time interval (in seconds) [varies]. Sets both
Timeout.resolver.retrans.first and Timeout.resolver.retrans.normal.
resolver.retrans.first‡
The resolver’s retransmission time interval (in seconds) for the first attempt to
deliver a message [varies].
resolver.retrans.normal‡
The resolver’s retransmission time interval (in seconds) for all resolver
lookups except the first delivery attempt [varies].
resolver.retry‡
The number of times to retransmit a resolver query. Sets both Timeout.re-
solver.retry.first and Timeout.resolver.retry.normal [varies].
resolver.retry.first‡
The number of times to retransmit a resolver query for the first attempt to de-
liver a message [varies].
resolver.retry.normal‡
The number of times to retransmit a resolver query for all resolver lookups
except the first delivery attempt [varies].
For compatibility with old configuration files, if no suboption is specified, all the timeouts
marked with a dagger (†) are set to the indicated value. All but those marked with a double dag-
ger (‡) apply to client SMTP.
For example, the lines:
O Timeout.command=25m
O Timeout.datablock=3h
sets the server SMTP command timeout to 25 minutes and the input data block timeout to three
hours.
4.1.3. Message timeouts
After sitting in the queue for a few days, an undeliverable message will time out. This is
to insure that at least the sender is aware of the inability to send a message. The timeout is typi-
cally set to five days. It is sometimes considered convenient to also send a warning message if
the message is in the queue longer than a few hours (assuming you normally have good connec-
tivity; if your messages normally took several hours to send you wouldn’t want to do this be-
cause it wouldn’t be an unusual event). These timeouts are set using the Timeout.queuereturn
and Timeout.queuewarn options in the configuration file (previously both were set using the T
option).
If the message is submitted using the NOTIFY SMTP extension, warning messages will
only be sent if NOTIFY=DELAY is specified. The queuereturn and queuewarn timeouts can be
further qualified with a tag based on the Precedence: field in the message; they must be one of
“urgent” (indicating a positive non-zero precedence), “normal” (indicating a zero precedence),
or
“non-urgent”
(indicating
negative
precedences).
For
example,
setting
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“Timeout.queuewarn.urgent=1h” sets the warning timeout for urgent messages only to one hour.
The default if no precedence is indicated is to set the timeout for all precedences. If the message
has a normal (default) precedence and it is a delivery status notification (DSN), Time-
out.queuereturn.dsn and Timeout.queuewarn.dsn can be used to give an alternative warn and
return time for DSNs. The value "now" can be used for -O Timeout.queuereturn to return en-
tries immediately during a queue run, e.g., to bounce messages independent of their time in the
queue.
Since these options are global, and since you cannot know a priori how long another host
outside your domain will be down, a five day timeout is recommended. This allows a recipient
to fix the problem even if it occurs at the beginning of a long weekend. RFC 1123 section
5.3.1.1 says that this parameter should be ‘‘at least 4−5 days’’.
The Timeout.queuewarn value can be piggybacked on the T option by indicating a time
after which a warning message should be sent; the two timeouts are separated by a slash. For
example, the line
OT5d/4h
causes email to fail after five days, but a warning message will be sent after four hours. This
should be large enough that the message will have been tried several times.
4.2. Forking During Queue Runs
By setting the ForkEachJob (Y) option, sendmail will fork before each individual message
while running the queue. This option was used with earlier releases to prevent sendmail from con-
suming large amounts of memory. It should no longer be necessary with sendmail 8.12. If the
ForkEachJob option is not set, sendmail will keep track of hosts that are down during a queue run,
which can improve performance dramatically.
If the ForkEachJob option is set, sendmail cannot use connection caching.
4.3. Queue Priorities
Every message is assigned a priority when it is first instantiated, consisting of the message
size (in bytes) offset by the message class (which is determined from the Precedence: header) times
the “work class factor” and the number of recipients times the “work recipient factor.” The priority
is used to order the queue. Higher numbers for the priority mean that the message will be processed
later when running the queue.
The message size is included so that large messages are penalized relative to small messages.
The message class allows users to send “high priority” messages by including a “Precedence:” field
in their message; the value of this field is looked up in the P lines of the configuration file. Since the
number of recipients affects the amount of load a message presents to the system, this is also in-
cluded into the priority.
The recipient and class factors can be set in the configuration file using the RecipientFactor
(y) and ClassFactor (z) options respectively. They default to 30000 (for the recipient factor) and
1800 (for the class factor). The initial priority is:
pri = msgsize − (class × ClassFactor) + (nrcpt × RecipientFactor)
(Remember, higher values for this parameter actually mean that the job will be treated with lower
priority.)
The priority of a job can also be adjusted each time it is processed (that is, each time an at-
tempt is made to deliver it) using the “work time factor,” set by the RetryFactor (Z) option. This is
added to the priority, so it normally decreases the precedence of the job, on the grounds that jobs
that have failed many times will tend to fail again in the future. The RetryFactor option defaults to
90000.
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4.4. Load Limiting
Sendmail can be asked to queue (but not deliver) mail if the system load average gets too high
using the QueueLA (x) option. When the load average exceeds the value of the QueueLA option,
the delivery mode is set to q (queue only) if the QueueFactor (q) option divided by the difference
in the current load average and the QueueLA option plus one is less than the priority of the message
— that is, the message is queued iff:
pri >
QueueFactor
LA − QueueLA + 1
The QueueFactor option defaults to 600000, so each point of load average is worth 600000 priority
points (as described above).
For drastic cases, the RefuseLA (X) option defines a load average at which sendmail will
refuse to accept network connections. Locally generated mail, i.e., mail which is not submitted via
SMTP (including incoming UUCP mail), is still accepted. Notice that the MSP submits mail to the
MTA via SMTP, and hence mail will be queued in the client queue in such a case. Therefore it is
necessary to run the client mail queue periodically.
4.5. Resource Limits
Sendmail has several parameters to control resource usage. Besides those mentioned in the
previous
section,
there
are
at
least
MaxDaemonChildren,
ConnectionRateThrottle,
MaxQueueChildren, and MaxRunnersPerQueue. The latter two limit the number of sendmail
processes that operate on the queue. These are discussed in the section ‘‘Queue Group Declara-
tion’’. The former two can be used to limit the number of incoming connections. Their appropriate
values depend on the host operating system and the hardware, e.g., amount of memory. In many sit-
uations it might be useful to set limits to prevent to have too many sendmail processes, however,
these limits can be abused to mount a denial of service attack. For example, if MaxDaemonChil-
dren=10 then an attacker needs to open only 10 SMTP sessions to the server, leave them idle for
most of the time, and no more connections will be accepted. If this option is set then the timeouts
used in a SMTP session should be lowered from their default values to their minimum values as
specified in RFC 2821 and listed in section 4.1.2.
4.6. Measures against Denial of Service Attacks
Sendmail has some built-in measures against simple denial of service (DoS) attacks. The
SMTP server by default slows down if too many bad commands are issued or if some commands
are repeated too often within a session. Details can be found in the source file sendmail/srvrsmtp.c
by looking for the macro definitions of MAXBADCOMMANDS, MAXNOOPCOMMANDS,
MAXHELOCOMMANDS, MAXVRFYCOMMANDS, and MAXETRNCOMMANDS. If an
SMTP command is issued more often than the corresponding MAXcmdCOMMANDS value, then
the response is delayed exponentially, starting with a sleep time of one second, up to a maximum of
four minutes (as defined by MAXTIMEOUT). If the option MaxDaemonChildren is set to a
value greater than zero, then this could make a DoS attack even worse since it keeps a connection
open longer than necessary. Therefore a connection is terminated with a 421 SMTP reply code if
the number of commands exceeds the limit by a factor of two and MAXBADCOMMANDS is set
to a value greater than zero (the default is 25).
4.7. Delivery Mode
There are a number of delivery modes that sendmail can operate in, set by the DeliveryMode
(d) configuration option. These modes specify how quickly mail will be delivered. Legal modes
are:
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i
deliver interactively (synchronously)
b
deliver in background (asynchronously)
q
queue only (don’t deliver)
d
defer delivery attempts (don’t deliver)
There are tradeoffs. Mode “i” gives the sender the quickest feedback, but may slow down some
mailers and is hardly ever necessary. Mode “b” delivers promptly but can cause large numbers of
processes if you have a mailer that takes a long time to deliver a message. Mode “q” minimizes the
load on your machine, but means that delivery may be delayed for up to the queue interval. Mode
“d” is identical to mode “q” except that it also prevents lookups in maps including the -D flag from
working during the initial queue phase; it is intended for ‘‘dial on demand’’ sites where DNS
lookups might cost real money. Some simple error messages (e.g., host unknown during the SMTP
protocol) will be delayed using this mode. Mode “b” is the usual default.
If you run in mode “q” (queue only), “d” (defer), or “b” (deliver in background) sendmail will
not expand aliases and follow .forward files upon initial receipt of the mail. This speeds up the re-
sponse to RCPT commands. Mode “i” should not be used by the SMTP server.
4.8. Log Level
The level of logging can be set for sendmail. The default using a standard configuration is
level 9. The levels are approximately as follows (some log types are using different level depending
on various factors):
0
Minimal logging.
1
Serious system failures and potential security problems.
2
Lost communications (network problems) and protocol failures.
3
Other serious failures, malformed addresses, transient forward/include errors, connection
timeouts.
4
Minor failures, out of date alias databases, connection rejections via check_ rulesets.
5
Message collection statistics.
6
Creation of error messages, VRFY and EXPN commands.
7
Delivery failures (host or user unknown, etc.).
8
Successful deliveries and alias database rebuilds.
9
Messages being deferred (due to a host being down, etc.).
10
Database expansion (alias, forward, and userdb lookups) and authentication information.
11
NIS errors and end of job processing.
12
Logs all SMTP connections.
13
Log bad user shells, files with improper permissions, and other questionable situations.
14
Logs refused connections.
15
Log all incoming SMTP commands.
20
Logs attempts to run locked queue files. These are not errors, but can be useful to note if
your queue appears to be clogged.
30
Lost locks (only if using lockf instead of flock).
Additionally, values above 64 are reserved for extremely verbose debugging output. No normal site
would ever set these.
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4.9. File Modes
The modes used for files depend on what functionality you want and the level of security you
require. In many cases sendmail does careful checking of the modes of files and directories to avoid
accidental compromise; if you want to make it possible to have group-writable support files you
may need to use the DontBlameSendmail option to turn off some of these checks.
4.9.1. To suid or not to suid?
Sendmail is no longer installed set-user-ID to root. sendmail/SECURITY explains how to
configure and install sendmail without set-user-ID to root but set-group-ID which is the default
configuration starting with 8.12.
The daemon usually runs as root, unless other measures are taken. At the point where
sendmail is about to exec (2) a mailer, it checks to see if the userid is zero (root); if so, it resets
the userid and groupid to a default (set by the U= equate in the mailer line; if that is not set, the
DefaultUser option is used). This can be overridden by setting the S flag to the mailer for mail-
ers that are trusted and must be called as root. However, this will cause mail processing to be
accounted (using sa (8)) to root rather than to the user sending the mail.
A middle ground is to set the RunAsUser option. This causes sendmail to become the in-
dicated user as soon as it has done the startup that requires root privileges (primarily, opening
the SMTP socket). If you use RunAsUser, the queue directory (normally /var/spool/mqueue)
should be owned by that user, and all files and databases (including user .forward files, alias
files, :include: files, and external databases) must be readable by that user. Also, since sendmail
will not be able to change its uid, delivery to programs or files will be marked as unsafe, e.g.,
undeliverable, in .forward, aliases, and :include: files. Administrators can override this by set-
ting the DontBlameSendmail option to the setting NonRootSafeAddr. RunAsUser is proba-
bly best suited for firewall configurations that don’t hav e regular user logins. If the option is
used on a system which performs local delivery, then the local delivery agent must have the
proper permissions (i.e., usually set-user-ID root) since it will be invoked by the RunAsUser,
not by root.
4.9.2. Turning off security checks
Sendmail is very particular about the modes of files that it reads or writes. For example,
by default it will refuse to read most files that are group writable on the grounds that they might
have been tampered with by someone other than the owner; it will even refuse to read files in
group writable directories. Also, sendmail will refuse to create a new aliases database in an un-
safe directory. You can get around this by manually creating the database file as a trusted user
ahead of time and then rebuilding the aliases database with newaliases.
If you are quite sure that your configuration is safe and you want sendmail to avoid these
security checks, you can turn off certain checks using the DontBlameSendmail option. This
option takes one or more names that disable checks. In the descriptions that follow, “unsafe di-
rectory” means a directory that is writable by anyone other than the owner. The values are:
Safe
No special handling.
AssumeSafeChown
Assume that the chown system call is restricted to root. Since some versions of UNIX
permit regular users to give away their files to other users on some filesystems, send-
mail often cannot assume that a given file was created by the owner, particularly when
it is in a writable directory. You can set this flag if you know that file giveaw ay is re-
stricted on your system.
ClassFileInUnsafeDirPath
When reading class files (using the F line in the configuration file), allow files that are
in unsafe directories.
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DontWarnForwardFileInUnsafeDirPath
Prevent logging of unsafe directory path warnings for non-existent forward files.
ErrorHeaderInUnsafeDirPath
Allow the file named in the ErrorHeader option to be in an unsafe directory.
FileDeliveryToHardLink
Allow delivery to files that are hard links.
FileDeliveryToSymLink
Allow delivery to files that are symbolic links.
ForwardFileInGroupWritableDirPath
Allow .forward files in group writable directories.
ForwardFileInUnsafeDirPath
Allow .forward files in unsafe directories.
ForwardFileInUnsafeDirPathSafe
Allow a .forward file that is in an unsafe directory to include references to program and
files.
GroupReadableKeyFile
Accept a group-readable key file for STARTTLS.
GroupReadableSASLDBFile
Accept a group-readable Cyrus SASL password file.
GroupReadableDefaultAuthInfoFile
Accept a group-readable DefaultAuthInfo file for SASL.
GroupWritableAliasFile
Allow group-writable alias files.
GroupWritableDirPathSafe
Change the definition of “unsafe directory” to consider group-writable directories to be
safe. World-writable directories are always unsafe.
GroupWritableForwardFile
Allow group writable .forward files.
GroupWritableForwardFileSafe
Accept group-writable .forward files as safe for program and file delivery.
GroupWritableIncludeFile
Allow group writable :include: files.
GroupWritableIncludeFileSafe
Accept group-writable :include: files as safe for program and file delivery.
GroupWritableSASLDBFile
Accept a group-writable Cyrus SASL password file.
HelpFileInUnsafeDirPath
Allow the file named in the HelpFile option to be in an unsafe directory.
IncludeFileInGroupWritableDirPath
Allow :include: files in group writable directories.
IncludeFileInUnsafeDirPath
Allow :include: files in unsafe directories.
IncludeFileInUnsafeDirPathSafe
Allow a :include: file that is in an unsafe directory to include references to program
and files.
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InsufficientEntropy
Try to use STARTTLS even if the PRNG for OpenSSL is not properly seeded despite
the security problems.
LinkedAliasFileInWritableDir
Allow an alias file that is a link in a writable directory.
LinkedClassFileInWritableDir
Allow class files that are links in writable directories.
LinkedForwardFileInWritableDir
Allow .forward files that are links in writable directories.
LinkedIncludeFileInWritableDir
Allow :include: files that are links in writable directories.
LinkedMapInWritableDir
Allow map files that are links in writable directories. This includes alias database files.
LinkedServiceSwitchFileInWritableDir
Allow the service switch file to be a link even if the directory is writable.
MapInUnsafeDirPath
Allow maps (e.g., hash, btree, and dbm files) in unsafe directories. This includes alias
database files.
NonRootSafeAddr
Do not mark file and program deliveries as unsafe if sendmail is not running with root
privileges.
RunProgramInUnsafeDirPath
Run programs that are in writable directories without logging a warning.
RunWritableProgram
Run programs that are group- or world-writable without logging a warning.
TrustStickyBit
Allow group or world writable directories if the sticky bit is set on the directory. Do
not set this on systems which do not honor the sticky bit on directories.
WorldWritableAliasFile
Accept world-writable alias files.
WorldWritableForwardfile
Allow world writable .forward files.
WorldWritableIncludefile
Allow world writable :include: files.
WriteMapToHardLink
Allow writes to maps that are hard links.
WriteMapToSymLink
Allow writes to maps that are symbolic links.
WriteStatsToHardLink
Allow the status file to be a hard link.
WriteStatsToSymLink
Allow the status file to be a symbolic link.
4.10. Connection Caching
When processing the queue, sendmail will try to keep the last few open connections open to
avoid startup and shutdown costs. This only applies to IPC and LPC connections.
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When trying to open a connection the cache is first searched. If an open connection is found,
it is probed to see if it is still active by sending a RSET command. It is not an error if this fails; in-
stead, the connection is closed and reopened.
Tw o parameters control the connection cache. The ConnectionCacheSize (k) option defines
the number of simultaneous open connections that will be permitted. If it is set to zero, connections
will be closed as quickly as possible. The default is one. This should be set as appropriate for your
system size; it will limit the amount of system resources that sendmail will use during queue runs.
Never set this higher than 4.
The ConnectionCacheTimeout (K) option specifies the maximum time that any cached con-
nection will be permitted to idle. When the idle time exceeds this value the connection is closed.
This number should be small (under ten minutes) to prevent you from grabbing too many resources
from other hosts. The default is five minutes.
4.11. Name Server Access
Control of host address lookups is set by the hosts service entry in your service switch file. If
you are on a system that has built-in service switch support (e.g., Ultrix, Solaris, or DEC OSF/1)
then your system is probably configured properly already. Otherwise, sendmail will consult the file
/etc/mail/service.switch, which should be created. Sendmail only uses two entries: hosts and
aliases, although system routines may use other services (notably the passwd service for user name
lookups by getpwname).
However, some systems (such as SunOS 4.X) will do DNS lookups regardless of the setting
of the service switch entry. In particular, the system routine gethostbyname(3) is used to look up
host names, and many vendor versions try some combination of DNS, NIS, and file lookup in
/etc/hosts without consulting a service switch. Sendmail makes no attempt to work around this
problem, and the DNS lookup will be done anyway. If you do not have a nameserver configured at
all, such as at a UUCP-only site, sendmail will get a “connection refused” message when it tries to
connect to the name server. If the hosts switch entry has the service “dns” listed somewhere in the
list, sendmail will interpret this to mean a temporary failure and will queue the mail for later pro-
cessing; otherwise, it ignores the name server data.
The same technique is used to decide whether to do MX lookups. If you want MX support,
you must have “dns” listed as a service in the hosts switch entry.
The ResolverOptions (I) option allows you to tweak name server options. The command
line takes a series of flags as documented in resolver(3) (with the leading “RES_” deleted). Each
can be preceded by an optional ‘+’ or ‘−’. For example, the line
O ResolverOptions=+AAONLY −DNSRCH
turns on the AAONLY (accept authoritative answers only) and turns off the DNSRCH (search the
domain path) options. Most resolver libraries default DNSRCH, DEFNAMES, and RECURSE
flags on and all others off. If NETINET6 is enabled, most libraries default to USE_INET6 as well.
You can also include “HasWildcardMX” to specify that there is a wildcard MX record matching
your domain; this turns off MX matching when canonifying names, which can lead to inappropriate
canonifications. Use “WorkAroundBrokenAAAA” when faced with a broken nameserver that re-
turns SERVFAIL (a temporary failure) on T_AAAA (IPv6) lookups during hostname canonifica-
tion. Notice: it might be necessary to apply the same (or similar) options to submit.cf too.
Version level 1 configurations (see the section about ‘‘Configuration Version Level’’) turn
DNSRCH and DEFNAMES off when doing delivery lookups, but leave them on everywhere else.
Version 8 of sendmail ignores them when doing canonification lookups (that is, when using $[ ...
$]), and always does the search. If you don’t want to do automatic name extension, don’t call $[ ...
$].
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The search rules for $[ ... $] are somewhat different than usual. If the name being looked up
has at least one dot, it always tries the unmodified name first. If that fails, it tries the reduced search
path, and lastly tries the unmodified name (but only for names without a dot, since names with a dot
have already been tried). This allows names such as ‘‘utc.CS’’ to match the site in Czechoslovakia
rather than the site in your local Computer Science department. It also prefers A and CNAME
records over MX records — that is, if it finds an MX record it makes note of it, but keeps looking.
This way, if you have a wildcard MX record matching your domain, it will not assume that all
names match.
To completely turn off all name server access on systems without service switch support
(such as SunOS 4.X) you will have to recompile with −DNAMED_BIND=0 and remove −lresolv
from the list of libraries to be searched when linking.
4.12. Moving the Per-User Forward Files
Some sites mount each user’s home directory from a local disk on their workstation, so that
local access is fast. However, the result is that .forward file lookups from a central mail server are
slow. In some cases, mail can even be delivered on machines inappropriately because of a file
server being down. The performance can be especially bad if you run the automounter.
The ForwardPath (J) option allows you to set a path of forward files. For example, the con-
fig file line
O ForwardPath=/var/forward/$u:$z/.forward.$w
would first look for a file with the same name as the user’s login in /var/forward; if that is not found
(or is inaccessible) the file ‘‘.forward.machinename’’ in the user’s home directory is searched. A
truly perverse site could also search by sender by using $r, $s, or $f.
If you create a directory such as /var/forward, it should be mode 1777 (that is, the sticky bit
should be set). Users should create the files mode 0644. Note that you must use the ForwardFileIn-
UnsafeDirPath and ForwardFileInUnsafeDirPathSafe flags with the DontBlameSendmail option to
allow forward files in a world writable directory. This might also be used as a denial of service at-
tack (users could create forward files for other users); a better approach might be to create /var/for-
ward mode 0755 and create empty files for each user, owned by that user, mode 0644. If you do
this, you don’t hav e to set the DontBlameSendmail options indicated above.
4.13. Free Space
On systems that have one of the system calls in the statfs(2) family (including statvfs and us-
tat), you can specify a minimum number of free blocks on the queue filesystem using the MinFree-
Blocks (b) option. If there are fewer than the indicated number of blocks free on the filesystem on
which the queue is mounted the SMTP server will reject mail with the 452 error code. This invites
the SMTP client to try again later.
Beware of setting this option too high; it can cause rejection of email when that mail would
be processed without difficulty.
4.14. Maximum Message Size
To avoid overflowing your system with a large message, the MaxMessageSize option can be
set to set an absolute limit on the size of any one message. This will be advertised in the ESMTP
dialogue and checked during message collection.
4.15. Privacy Flags
The PrivacyOptions (p) option allows you to set certain ‘‘privacy’’ flags. Actually, many of
them don’t giv e you any extra privacy, rather just insisting that client SMTP servers use the HELO
command before using certain commands or adding extra headers to indicate possible spoof
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attempts.
The option takes a series of flag names; the final privacy is the inclusive or of those flags. For
example:
O PrivacyOptions=needmailhelo, noexpn
insists that the HELO or EHLO command be used before a MAIL command is accepted and dis-
ables the EXPN command.
The flags are detailed in section 5.6.
4.16. Send to Me Too
Beginning with version 8.10, sendmail includes by default the (envelope) sender in any list
expansions. For example, if “matt” sends to a list that contains “matt” as one of the members he
will get a copy of the message. If the MeToo option is set to FALSE (in the configuration file or via
the command line), this behavior is changed, i.e., the (envelope) sender is excluded in list expan-
sions.
5. THE WHOLE SCOOP ON THE CONFIGURATION FILE
This section describes the configuration file in detail.
There is one point that should be made clear immediately: the syntax of the configuration file is
designed to be reasonably easy to parse, since this is done every time sendmail starts up, rather than
easy for a human to read or write. The configuration file should be generated via the method described
in cf/README, it should not be edited directly unless someone is familiar with the internals of the
syntax described here and it is not possible to achieve the desired result via the default method.
The configuration file is organized as a series of lines, each of which begins with a single charac-
ter defining the semantics for the rest of the line. Lines beginning with a space or a tab are continuation
lines (although the semantics are not well defined in many places). Blank lines and lines beginning
with a sharp symbol (‘#’) are comments.
5.1. R and S — Rewriting Rules
The core of address parsing are the rewriting rules. These are an ordered production system.
Sendmail scans through the set of rewriting rules looking for a match on the left hand side (LHS) of
the rule. When a rule matches, the address is replaced by the right hand side (RHS) of the rule.
There are several sets of rewriting rules. Some of the rewriting sets are used internally and
must have specific semantics. Other rewriting sets do not have specifically assigned semantics, and
may be referenced by the mailer definitions or by other rewriting sets.
The syntax of these two commands are:
Sn
Sets the current ruleset being collected to n. If you begin a ruleset more than once it appends to the
old definition.
Rlhs rhs comments
The fields must be separated by at least one tab character; there may be embedded spaces in the
fields. The lhs is a pattern that is applied to the input. If it matches, the input is rewritten to the rhs.
The comments are ignored.
Macro expansions of the form $x are performed when the configuration file is read. A literal
$ can be included using $$. Expansions of the form $&x are performed at run time using a some-
what less general algorithm. This is intended only for referencing internally defined macros such as
$h that are changed at runtime.
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5.1.1. The left hand side
The left hand side of rewriting rules contains a pattern. Normal words are simply
matched directly. Metasyntax is introduced using a dollar sign. The metasymbols are:
$*
Match zero or more tokens
$+
Match one or more tokens
$−
Match exactly one token
$=x Match any phrase in class x
$˜x Match any word not in class x
If any of these match, they are assigned to the symbol $n for replacement on the right hand side,
where n is the index in the LHS. For example, if the LHS:
$−:$+
is applied to the input:
UCBARPA:eric
the rule will match, and the values passed to the RHS will be:
$1 UCBARPA
$2 eric
Additionally, the LHS can include $@ to match zero tokens. This is not bound to a $n on
the RHS, and is normally only used when it stands alone in order to match the null input.
5.1.2. The right hand side
When the left hand side of a rewriting rule matches, the input is deleted and replaced by
the right hand side. Tokens are copied directly from the RHS unless they begin with a dollar
sign. Metasymbols are:
$n
Substitute indefinite token n from LHS
$[name$]
Canonicalize name
$(map key $@arguments $:default $)
Generalized keyed mapping function
$>n
“Call” ruleset n
$#mailer
Resolve to mailer
$@host
Specify host
$:user
Specify user
The $n syntax substitutes the corresponding value from a $+, $−, $*, $=, or $˜ match on
the LHS. It may be used anywhere.
A host name enclosed between $[ and $] is looked up in the host database(s) and replaced
by the canonical name14. For example, “$[ftp$]” might become “ftp.CS.Berkeley.EDU” and
“$[[128.32.130.2]$]” would become “vangogh.CS.Berkeley.EDU.” Sendmail recognizes its nu-
meric IP address without calling the name server and replaces it with its canonical name.
The $( ... $) syntax is a more general form of lookup; it uses a named map instead of an
implicit map. If no lookup is found, the indicated default is inserted; if no default is specified
and no lookup matches, the value is left unchanged. The arguments are passed to the map for
possible use.
The $>n syntax causes the remainder of the line to be substituted as usual and then passed
as the argument to ruleset n. The final value of ruleset n then becomes the substitution for this
14This is actually completely equivalent to $(host hostname$). In particular, a $: default can be used.
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rule. The $> syntax expands everything after the ruleset name to the end of the replacement
string and then passes that as the initial input to the ruleset. Recursive calls are allowed. For ex-
ample,
$>0 $>3 $1
expands $1, passes that to ruleset 3, and then passes the result of ruleset 3 to ruleset 0.
The $# syntax should only be used in ruleset zero, a subroutine of ruleset zero, or rulesets
that return decisions (e.g., check_rcpt). It causes evaluation of the ruleset to terminate immedi-
ately, and signals to sendmail that the address has completely resolved. The complete syntax for
ruleset 0 is:
$#mailer $@host $:user
This specifies the {mailer, host, user} 3-tuple (triple) necessary to direct the mailer. Note: the
third element ( user ) is often also called address part. If the mailer is local the host part may be
omitted15. The mailer must be a single word, but the host and user may be multi-part. If the
mailer is the built-in IPC mailer, the host may be a colon (or comma) separated list of hosts.
Each is separately MX expanded and the results are concatenated to make (essentially) one long
MX list. Hosts separated by a comma have the same MX preference, and for each colon sepa-
rated host the MX preference is increased. The user is later rewritten by the mailer-specific en-
velope rewriting set and assigned to the $u macro. As a special case, if the mailer specified has
the F=@ flag specified and the first character of the $: value is “@”, the “@” is stripped off, and
a flag is set in the address descriptor that causes sendmail to not do ruleset 5 processing.
Normally, a rule that matches is retried, that is, the rule loops until it fails. A RHS may
also be preceded by a $@ or a $: to change this behavior. A $@ prefix causes the ruleset to re-
turn with the remainder of the RHS as the value. A $: prefix causes the rule to terminate imme-
diately, but the ruleset to continue; this can be used to avoid continued application of a rule. The
prefix is stripped before continuing.
The $@ and $: prefixes may precede a $> spec; for example:
R$+
$: $>7 $1
matches anything, passes that to ruleset seven, and continues; the $: is necessary to avoid an in-
finite loop.
Substitution occurs in the order described, that is, parameters from the LHS are substi-
tuted, hostnames are canonicalized, “subroutines” are called, and finally $#, $@, and $: are pro-
cessed.
5.1.3. Semantics of rewriting rule sets
There are six rewriting sets that have specific semantics. Five of these are related as de-
picted by figure 1.
Ruleset three should turn the address into “canonical form.” This form should have the
basic syntax:
local-part@host-domain-spec
Ruleset three is applied by sendmail before doing anything with any address.
If no “@” sign is specified, then the host-domain-spec may be appended (box “D” in Fig-
ure 1) from the sender address (if the C flag is set in the mailer definition corresponding to the
sending mailer).
15You may want to use it for special “per user” extensions. For example, in the address “jgm+foo@CMU.EDU”; the “+foo”
part is not part of the user name, and is passed to the local mailer for local use.
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Ruleset zero is applied after ruleset three to addresses that are going to actually specify re-
cipients. It must resolve to a {mailer, host, address} triple. The mailer must be defined in the
mailer definitions from the configuration file. The host is defined into the $h macro for use in
the argv expansion of the specified mailer. Notice: since the envelope sender address will be
used if a delivery status notification must be send, i.e., it may specify a recipient, it is also run
through ruleset zero. If ruleset zero returns a temporary error 4xy then delivery is deferred.
This can be used to temporarily disable delivery, e.g., based on the time of the day or other vary-
ing parameters. It should not be used to quarantine e-mails.
Rulesets one and two are applied to all sender and recipient addresses respectively. They
are applied before any specification in the mailer definition. They must never resolve.
Ruleset four is applied to all addresses in the message. It is typically used to translate in-
ternal to external form.
In addition, ruleset 5 is applied to all local addresses (specifically, those that resolve to a
mailer with the ‘F=5’ flag set) that do not have aliases. This allows a last minute hook for local
names.
5.1.4. Ruleset hooks
A few extra rulesets are defined as “hooks” that can be defined to get special features.
They are all named rulesets. The “check_*” forms all give accept/reject status; falling off the
end or returning normally is an accept, and resolving to $#error is a reject or quarantine. Quar-
antining is chosen by specifying quarantine in the second part of the mailer triplet:
$#error $@ quarantine $: Reason for quarantine
Many of these can also resolve to the special mailer name $#discard; this accepts the message
as though it were successful but then discards it without delivery. Note, this mailer cannot be
chosen as a mailer in ruleset 0. Note also that all “check_*” rulesets have to deal with tempo-
rary failures, especially for map lookups, themselves, i.e., they should return a temporary error
code or at least they should make a proper decision in those cases.
5.1.4.1. check_relay
The check_relay ruleset is called after a connection is accepted by the daemon. It is
not called when sendmail is started using the −bs option. It is passed
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client.host.name $| client.host.address
where $| is a metacharacter separating the two parts. This ruleset can reject connections
from various locations. Note that it only checks the connecting SMTP client IP address and
hostname. It does not check for third party message relaying. The check_rcpt ruleset dis-
cussed below usually does third party message relay checking.
5.1.4.2. check_mail
The check_mail ruleset is passed the user name parameter of the SMTP MAIL com-
mand. It can accept or reject the address.
5.1.4.3. check_rcpt
The check_rcpt ruleset is passed the user name parameter of the SMTP RCPT com-
mand. It can accept or reject the address.
5.1.4.4. check_data
The check_data ruleset is called after the SMTP DAT A command, its parameter is the
number of recipients. It can accept or reject the command.
5.1.4.5. check_other
The check_other ruleset is invoked for all unknown SMTP commands and for com-
mands which do not have specific rulesets, e.g., NOOP and VERB. Internal checks, e.g.,
those explained in "Measures against Denial of Service Attacks", are performed first. The
ruleset is passed
entire-SMTP-command $| SMTP-reply-first-digit
where $| is a metacharacter separating the two parts. For example,
VERB $| 2
reflects receiving the "VERB" SMTP command and the intent to return a "2XX" SMTP suc-
cess reply. Alternatively,
JUNK TYPE=I $| 5
reflects receiving the unknown "JUNK TYPE=I" SMTP command and the intent to return a
"5XX" SMTP failure reply. If the ruleset returns the SMTP reply code 421:
$#error $@ 4.7.0 $: 421 bad command
the session is terminated. Note: it is a bad idea to return the original command in the error
text to the client as that might be abused for certain attacks. The ruleset cannot override a
rejection triggered by the built-in rules.
5.1.4.6. check_compat
The check_compat ruleset is passed
sender-address $| recipient-address
where $| is a metacharacter separating the addresses. It can accept or reject mail transfer be-
tween these two addresses much like the checkcompat() function. Note: while other check_*
rulesets are invoked during the SMTP mail receiption stage (i.e., in the SMTP server),
check_compat is invoked during the mail delivery stage.
5.1.4.7. check_eoh
The check_eoh ruleset is passed
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number-of-headers $| size-of-headers
where $| is a metacharacter separating the numbers. These numbers can be used for size
comparisons with the arith map. The ruleset is triggered after all of the headers have been
read. It can be used to correlate information gathered from those headers using the macro
storage map. One possible use is to check for a missing header. For example:
Kstorage macro
HMessage-Id: $>CheckMessageId
SCheckMessageId
# Record the presence of the header
R$*
$: $(storage {MessageIdCheck} $@ OK $) $1
R< $+ @ $+ >
$@ OK
R$*
$#error $: 553 Header Error
Scheck_eoh
# Check the macro
R$*
$: < $&{MessageIdCheck} >
# Clear the macro for the next message
R$*
$: $(storage {MessageIdCheck} $) $1
# Has a Message-Id: header
R< $+ >
$@ OK
# Allow missing Message-Id: from local mail
R$*
$: < $&{client_name} >
R< >
$@ OK
R< $=w >
$@ OK
# Otherwise, reject the mail
R$*
$#error $: 553 Header Error
Keep in mind the Message-Id: header is not a required header and is not a guaranteed spam
indicator. This ruleset is an example and should probably not be used in production.
5.1.4.8. check_eom
The check_eom ruleset is called after the end of a message, its parameter is the mes-
sage size. It can accept or reject the message.
5.1.4.9. check_etrn
The check_etrn ruleset is passed the parameter of the SMTP ETRN command. It can
accept or reject the command.
5.1.4.10. check_expn
The check_expn ruleset is passed the user name parameter of the SMTP EXPN com-
mand. It can accept or reject the address.
5.1.4.11. check_vrfy
The check_vrfy ruleset is passed the user name parameter of the SMTP VRFY com-
mand. It can accept or reject the command.
5.1.4.12. clt_features
The clt_features ruleset is called with the server’s host name before sendmail connects
to it (only if sendmail is compiled with STARTTLS or SASL). This ruleset should return $#
followed by a list of options (single characters delimited by white space). If the return value
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Sendmail Installation and Operation Guide
starts with anything else it is silently ignored. Generally upper case characters turn off a fea-
ture while lower case characters turn it on. Options ‘D’/‘M’ cause the client to not use
DANE/MTA-STS, respectively, which is useful to interact with MTAs that have broken
DANE/MTA-STS setups by simply not using it. Note: The d option in tls_clt_features to
turn off DANE does not work when the server does not even offer STARTTLS.
5.1.4.13. trust_auth
The trust_auth ruleset is passed the AUTH= parameter of the SMTP MAIL command.
It is used to determine whether this value should be trusted. In order to make this decision,
the ruleset may make use of the various ${auth_*} macros. If the ruleset does resolve to the
“error” mailer the AUTH= parameter is not trusted and hence not passed on to the next relay.
5.1.4.14. tls_client
The tls_client ruleset is called when sendmail acts as server: after a STARTTLS com-
mand has been issued and the TLS handshake was performed, and from check_mail. The
parameter is the value of ${verify} and STARTTLS or MAIL, respectively. If the ruleset
does resolve to the “error” mailer, the appropriate error code is returned to the client, for
STARTTLS this happens for (most) subsequent commands.
5.1.4.15. tls_server
The tls_server ruleset is called when sendmail acts as client after a STARTTLS com-
mand (should) have been issued. The parameter is the value of ${verify}. If the ruleset does
resolve to the “error” mailer, the connection is aborted (treated as non-deliverable with a per-
manent or temporary error).
5.1.4.16. tls_rcpt
The tls_rcpt ruleset is called each time before a RCPT command is sent. The parame-
ter is the current recipient. If the ruleset does resolve to the “error” mailer, the RCPT com-
mand is suppressed (treated as non-deliverable with a permanent or temporary error). This
ruleset allows to require encryption or verification of the recipient’s MTA even if the mail is
somehow redirected to another host. For example, sending mail to luke@endmail.org may
get redirected to a host named death.star and hence the tls_server ruleset won’t apply. By
introducing per recipient restrictions such attacks (e.g., via DNS spoofing) can be made im-
possible. See cf/README how this ruleset can be used.
5.1.4.17. srv_features
The srv_features ruleset is called with the connecting client’s host name when a client
connects to sendmail. This ruleset should return $# followed by a list of options (single
characters delimited by white space). If the return value starts with anything else it is
silently ignored. Generally upper case characters turn off a feature while lower case charac-
ters turn it on. Option ‘S’ causes the server not to offer STARTTLS, which is useful to inter-
act with MTAs/MUAs that have broken STARTTLS implementations by simply not offering
it. ‘V’ turns off the request for a client certificate during the TLS handshake. Options ‘A’
and ‘P’ suppress SMTP AUTH and PIPELINING, respectively. ‘c’ is the equivalent to Au-
thOptions=p, i.e., it doesn’t permit mechanisms susceptible to simple passive attack (e.g.,
PLAIN, LOGIN), unless a security layer is active. Option ‘l’ requires SMTP AUTH for a
connection. Options ’B’, ’D’, ’E’, and ’X’ suppress SMTP VERB, DSN, ETRN, and
EXPN, respectively. If a client sends one of the (HTTP) commands GET, POST, CON-
NECT, or USER the connection is immediately terminated in the following cases: if sent as
first command, if sent as first command after STARTTLS, or if the ’h’ option is set.
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SMM:08-45
A
Do not offer AUTH
a
Offer AUTH (default)
B
Do not offer VERB
b
Offer VERB (default)
C
Do not require security layer for
plaintext AUTH (default)
c
Require security layer for plaintext AUTH
D
Do not offer DSN
d
Offer DSN (default)
E
Do not offer ETRN
e
Offer ETRN (default)
h
Terminate session after HTTP commands
L
Do not require AUTH (default)
l
Require AUTH
P
Do not offer PIPELINING
p
Offer PIPELINING (default)
S
Do not offer STARTTLS
s
Offer STARTTLS (default)
V
Do not request a client certificate
v
Request a client certificate (default)
X
Do not offer EXPN
x
Offer EXPN (default)
Note: the entries marked as ‘‘(default)’’ may require that some configuration has been made,
e.g., SMTP AUTH is only available if properly configured. Moreover, many options can be
changed on a global basis via other settings as explained in this document, e.g., via Daemon-
PortOptions.
The ruleset may return ‘$#temp’ to indicate that there is a temporary problem deter-
mining the correct features, e.g., if a map is unavailable. In that case, the SMTP server is-
sues a temporary failure and does not accept email.
5.1.4.18. try_tls
The try_tls ruleset is called when sendmail connects to another MTA. The argument
for the ruleset is the name of the server. If the ruleset does resolve to the “error” mailer,
sendmail does not try STARTTLS even if it is offered. This is useful to deal with START-
TLS interoperability issues by simply not using it.
5.1.4.19. tls_srv_features and tls_clt_features
The tls_clt_features ruleset is called right before sendmail issues the STARTTLS com-
mand to another MTA and the tls_srv_features ruleset is called when a client sends the
STARTTLS command to sendmail. The arguments for the rulesets are the host name and IP
address of the other side separated by $| (which is a metacharacter). They should return a
list of key=value pairs separated by semicolons; the list can be empty if no options should be
applied to the connection. Av ailable keys are and their allowed values are:
Options
A comma separated list of SSL related options. See ServerSSLOptions and ClientSS-
LOptions for details, as well as SSL_set_options(3) and note this warning: Options al-
ready set before are not cleared!
CipherList
Specify cipher list for STARTTLS (does not apply to TLSv1.3), see ciphers(1) for possi-
ble values. This overrides the global CipherList for the session.
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Sendmail Installation and Operation Guide
CertFile
File containing a certificate.
Ke yFile
File containing the private key for the certificate.
Flags
Currently the only valid flags are
R to require a CRL for each encountered certificate during verification (by default a
missing CRL is ignored),
c and C which basically clears/sets the option TLSFallbacktoClear for just this session,
respectively,
d to turn off DANE which is obviously only valid for tls_clt_features and requires DANE
to be compiled in. This might be needed in case of a misconfiguration, e.g., specifying
invalid TLSA RRs.
Example:
Stls_srv_features
R$* $| 10.$+
$: cipherlist=HIGH
Notes:
Errors in these features (e.g., unknown keys or inv alid values) are logged and the cur-
rent session is aborted to avoid using STARTTLS with features that should have been
changed.
The keys are case-insensitive.
Both CertFile and Ke yFile must be specified together; specifying only one is an error.
5.1.4.20. authinfo
The authinfo ruleset is called when sendmail tries to authenticate to another MTA.
The arguments for the ruleset are the host name and IP address of the server separated by $|
(which is a metacharacter). It should return $# followed by a list of tokens that are used for
SMTP AUTH. If the return value starts with anything else it is silently ignored. Each token
is a tagged string of the form: "TDstring" (including the quotes), where
T
Tag which describes the item
D
Delimiter: ’:’ simple text follows
’=’ string is base64 encoded
string
Value of the item
Valid values for the tag are:
U
user (authorization) id
I
authentication id
P
password
R
realm
M
list of mechanisms delimited by spaces
If this ruleset is defined, the option DefaultAuthInfo is ignored (even if the ruleset does not
return a ‘‘useful’’ result).
5.1.4.21. queuegroup
The queuegroup ruleset is used to map a recipient address to a queue group name.
The input for the ruleset is the recipient address (i.e., the address part of the resolved triple)
The ruleset should return $# followed by the name of a queue group. If the return value
starts with anything else it is silently ignored. See the section about ‘‘Queue Groups and
Queue Directories’’ for further information.
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5.1.4.22. greet_pause
The greet_pause ruleset is used to specify the amount of time to pause before sending
the initial SMTP 220 greeting. The arguments for the ruleset are the host name and IP ad-
dress of the client separated by $| (which is a metacharacter). If any traffic is received dur-
ing that pause, an SMTP 554 rejection response is given instead of the 220 greeting and all
SMTP commands are rejected during that connection. This helps protect sites from open
proxies and SMTP slammers. The ruleset should return $# followed by the number of mil-
liseconds (thousandths of a second) to pause. If the return value starts with anything else or
is not a number, it is silently ignored. Note: this ruleset is not invoked (and hence the feature
is disabled) when smtps (SMTP over SSL) is used, i.e., the s modifier is set for the daemon
via DaemonPortOptions, because in this case the SSL handshake is performed before the
greeting is sent.
5.1.5. IPC mailers
Some special processing occurs if the ruleset zero resolves to an IPC mailer (that is, a
mailer that has “[IPC]” listed as the Path in the M configuration line. The host name passed af-
ter “$@” has MX expansion performed if not delivering via a named socket; this looks the name
up in DNS to find alternate delivery sites.
The host name can also be provided as a dotted quad or an IPv6 address in square brack-
ets; for example:
[128.32.149.78]
or
[IPv6:2002:c0a8:51d2::23f4]
This causes direct conversion of the numeric value to an IP host address.
The host name passed in after the “$@” may also be a colon or comma separated list of
hosts. Each is separately MX expanded and the results are concatenated to make (essentially)
one long MX list. Hosts separated by a comma have the same MX preference, and for each
colon separated host the MX preference is increased. The intent here is to create “fake” MX
records that are not published in DNS for private internal networks.
As a final special case, the host name can be passed in as a text string in square brackets:
[ucbvax.berkeley.edu]
This form avoids the MX mapping. N.B.: This is intended only for situations where you have a
network firewall or other host that will do special processing for all your mail, so that your MX
record points to a gateway machine; this machine could then do direct delivery to machines
within your local domain. Use of this feature directly violates RFC 1123 section 5.3.5: it should
not be used lightly.
5.2. D — Define Macro
Macros are named with a single character or with a word in {braces}. The names ‘‘x’’ and
‘‘{x}’’ denote the same macro for every single character ‘‘x’’. Single character names may be se-
lected from the entire ASCII set, but user-defined macros should be selected from the set of upper
case letters only. Lower case letters and special symbols are used internally. Long names beginning
with a lower case letter or a punctuation character are reserved for use by sendmail, so user-defined
long macro names should begin with an upper case letter.
The syntax for macro definitions is:
Dx val
where x is the name of the macro (which may be a single character or a word in braces) and val is
the value it should have. There should be no spaces given that do not actually belong in the macro
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Sendmail Installation and Operation Guide
value.
Macros are interpolated using the construct $x, where x is the name of the macro to be inter-
polated. This interpolation is done when the configuration file is read, except in M lines. The spe-
cial construct $&x can be used in R lines to get deferred interpolation.
Conditionals can be specified using the syntax:
$?x text1 $| text2 $.
This interpolates text1 if the macro $x is set and non-null, and text2 otherwise. The “else” ($|)
clause may be omitted.
The following macros are defined and/or used internally by sendmail for interpolation into
argv’s for mailers or for other contexts. The ones marked † are information passed into sendmail16,
the ones marked ‡ are information passed both in and out of sendmail, and the unmarked macros are
passed out of sendmail but are not otherwise used internally. These macros are:
$a
The origination date in RFC 822 format. This is extracted from the Date: line.
$b
The current date in RFC 822 format.
$c
The hop count. This is a count of the number of Received: lines plus the value of the −h com-
mand line flag.
$d
The current date in UNIX (ctime) format.
$e†
(Obsolete; use SmtpGreetingMessage option instead.) The SMTP entry message. This is
printed out when SMTP starts up. The first word must be the $j macro as specified by RFC
821. Defaults to “$j Sendmail $v ready at $b”. Commonly redefined to include the configu-
ration version number, e.g., “$j Sendmail $v/$Z ready at $b”
$f
The envelope sender (from) address.
$g
The sender address relative to the recipient. For example, if $f is “foo”, $g will be “host!foo”,
“foo@host.domain”, or whatever is appropriate for the receiving mailer.
$h
The recipient host. This is set in ruleset 0 from the $@ field of a parsed address.
$i
The queue id, e.g., “f344MXxp018717”.
$j‡
The “official” domain name for this site. This is fully qualified if the full qualification can be
found. It must be redefined to be the fully qualified domain name if your system is not con-
figured so that information can find it automatically.
$k
The UUCP node name (from the uname system call).
$l†
(Obsolete; use UnixFromLine option instead.) The format of the UNIX from line. Unless
you have changed the UNIX mailbox format, you should not change the default, which is
“From $g $d”.
$m
The domain part of the gethostname return value. Under normal circumstances, $j is equiva-
lent to $w.$m.
$n† The name of the daemon (for error messages). Defaults to “MAILER-DAEMON”.
$o† (Obsolete: use OperatorChars option instead.) The set of “operators” in addresses. A list of
characters which will be considered tokens and which will separate tokens when doing pars-
ing. For example, if “@” were in the $o macro, then the input “a@b” would be scanned as
three tokens: “a,” “@,” and “b.” Defaults to “.:@[]”, which is the minimum set necessary to
do RFC 822 parsing; a richer set of operators is “.:%@!/[]”, which adds support for UUCP,
the %-hack, and X.400 addresses.
16As of version 8.6, all of these macros have reasonable defaults. Previous versions required that they be defined.
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SMM:08-49
$p
Sendmail’s process id.
$r
Protocol used to receive the message. Set from the −p command line flag or by the SMTP
server code.
$s
Sender’s host name. Set from the −p command line flag or by the SMTP server code (in
which case it is set to the EHLO/HELO parameter).
$t
A numeric representation of the current time in the format YYYYMMDDHHmm (4 digit
year 1900-9999, 2 digit month 01-12, 2 digit day 01-31, 2 digit hours 00-23, 2 digit minutes
00-59).
$u
The recipient user.
$v
The version number of the sendmail binary.
$w‡ The hostname of this site. This is the root name of this host (but see below for caveats).
$x
The full name of the sender.
$z
The home directory of the recipient.
$_
The validated sender address. See also ${client_resolve}.
${addr_type}
The type of the address which is currently being rewritten. This macro contains up to three
characters, the first is either ‘e’ or ‘h’ for envelope/header address, the second is a space, and
the third is either ‘s’ or ‘r’ for sender/recipient address.
${alg_bits}
The maximum keylength (in bits) of the symmetric encryption algorithm used for a TLS con-
nection. This may be less than the effective keylength, which is stored in ${cipher_bits}, for
‘‘export controlled’’ algorithms.
${auth_authen}
The client’s authentication credentials as determined by authentication (only set if success-
ful). The format depends on the mechanism used, it might be just ‘user’, or ‘user@realm’, or
something similar (SMTP AUTH only).
${auth_author}
The authorization identity, i.e. the AUTH= parameter of the SMTP MAIL command if sup-
plied.
${auth_type}
The mechanism used for SMTP authentication (only set if successful).
${auth_ssf}
The keylength (in bits) of the symmetric encryption algorithm used for the security layer of a
SASL mechanism.
${bodytype}
The message body type (7BIT or 8BITMIME), as determined from the envelope.
${cert_fp}
The fingerprint of the presented certificate (STARTTLS only). Note: this macro is only de-
fined if the option CertFingerprintAlgorithm is set, in which case the specified fingerprint
algorithm is used. The valid algorithms depend on the OpenSSL version, but usually md5,
sha1, and sha256 are available. See
openssl dgst -h
for a list.
${cert_issuer}
The DN (distinguished name) of the CA (certificate authority) that signed the presented cer-
tificate (the cert issuer) (STARTTLS only).
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Sendmail Installation and Operation Guide
${cert_md5}
The MD5 hash of the presented certificate (STARTTLS only). Note: this macro is only de-
fined if the option CertFingerprintAlgorithm is not set.
${cert_subject}
The DN of the presented certificate (called the cert subject) (STARTTLS only).
${cipher}
The cipher suite used for the connection, e.g., EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-
CBC-SHA, DES-CBC-MD5, DES-CBC3-SHA (STARTTLS only).
${cipher_bits}
The effective keylength (in bits) of the symmetric encryption algorithm used for a TLS con-
nection.
${client_addr}
The IP address of the SMTP client. IPv6 addresses are tagged with "IPv6:" before the ad-
dress. Defined in the SMTP server only.
${client_connections}
The number of open connections in the SMTP server for the client IP address.
${client_flags}
The flags specified by the Modifier= part of ClientPortOptions where flags are separated
from each other by spaces and upper case flags are doubled. That is, Modifier=hA will be
represented as "h AA" in ${client_flags}, which is required for testing the flags in rulesets.
${client_name}
The host name of the SMTP client. This may be the client’s bracketed IP address in the form
[ nnn.nnn.nnn.nnn ] for IPv4 and [ IPv6:nnnn:...:nnnn ] for IPv6 if the client’s IP address is
not resolvable, or if it is resolvable but the IP address of the resolved hostname doesn’t match
the original IP address. Defined in the SMTP server only. See also ${client_resolve}.
${client_port}
The port number of the SMTP client. Defined in the SMTP server only.
${client_ptr}
The result of the PTR lookup for the client IP address.
Note: this is the same as
${client_name} if and only if ${client_resolve} is OK. Defined in the SMTP server only.
${client_rate}
The number of incoming connections for the client IP address over the time interval specified
by ConnectionRateWindowSize.
${client_resolve}
Holds the result of the resolve call for ${client_name}. Possible values are:
OK
resolved successfully
FAIL
permanent lookup failure
FORGED forward lookup doesn’t match reverse lookup
TEMP
temporary lookup failure
Defined in the SMTP server only. sendmail performs a hostname lookup on the IP address of
the connecting client. Next the IP addresses of that hostname are looked up. If the client IP
address does not appear in that list, then the hostname is maybe forged. This is reflected as
the value FORGED for ${client_resolve} and it also shows up in $_ as "(may be forged)".
${cn_issuer}
The CN (common name) of the CA that signed the presented certificate (STARTTLS only).
Note: if the CN cannot be extracted properly it will be replaced by one of these strings based
on the encountered error:
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SMM:08-51
BadCertificateContainsNUL
CN contains a NUL character
BadCertificateTooLong
CN is too long
BadCertificateUnknown
CN could not be extracted
In the last case, some other (unspecific) error occurred.
${cn_subject}
The CN (common name) of the presented certificate (STARTTLS only). See ${cn_issuer}
for possible replacements.
${currHeader}
Header value as quoted string (possibly truncated to MAXNAME). This macro is only avail-
able in header check rulesets.
${daemon_addr}
The IP address the daemon is listening on for connections.
${daemon_family}
The network family if the daemon is accepting network connections. Possible values include
“inet”, “inet6”, “iso”, “ns”, “x.25”
${daemon_flags}
The flags for the daemon as specified by the Modifier= part of DaemonPortOptions whereby
the flags are separated from each other by spaces, and upper case flags are doubled. That is,
Modifier=Ea will be represented as "EE a" in ${daemon_flags}, which is required for testing
the flags in rulesets.
${daemon_info}
Some information about a daemon as a text string.
For example, “SMTP+queue-
ing@00:30:00”.
${daemon_name}
The name of the daemon from DaemonPortOptions Name= suboption. If this suboption is
not set, "Daemon#", where # is the daemon number, is used.
${daemon_port}
The port the daemon is accepting connection on. Unless DaemonPortOptions is set, this
will most likely be “25”.
${deliveryMode}
The current delivery mode sendmail is using. It is initially set to the value of the Delivery-
Mode option.
${dsn_envid}
The envelope id parameter (ENVID=) passed to sendmail as part of the envelope.
${dsn_notify}
Value of DSN NOTIFY= parameter (never, success, failure, delay, or empty string).
${dsn_ret}
Value of DSN RET= parameter (hdrs, full, or empty string).
${envid}
The envelope id parameter (ENVID=) passed to sendmail as part of the envelope.
${hdrlen}
The length of the header value which is stored in ${currHeader} (before possible truncation).
If this value is greater than or equal to MAXNAME the header has been truncated.
${hdr_name}
The name of the header field for which the current header check ruleset has been called. This
is useful for a default header check ruleset to get the name of the header; the macro is only
available in header check rulesets.
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${if_addr}
The IP address of the interface of an incoming connection unless it is in the loopback net.
IPv6 addresses are tagged with "IPv6:" before the address.
${if_addr_out}
The IP address of the interface of an outgoing connection unless it is in the loopback net.
IPv6 addresses are tagged with "IPv6:" before the address.
${if_family}
The IP family of the interface of an incoming connection unless it is in the loopback net.
${if_family_out}
The IP family of the interface of an outgoing connection unless it is in the loopback net.
${if_name}
The hostname associated with the interface of an incoming connection. This macro can be
used for SmtpGreetingMessage and HReceived for virtual hosting. For example:
O SmtpGreetingMessage=$?{if_name}${if_name}$|$j$. MTA
${if_name_out}
The name of the interface of an outgoing connection.
${load_avg}
The current load average.
${mail_addr}
The address part of the resolved triple of the address given for the SMTP MAIL command.
Defined in the SMTP server only.
${mail_host}
The host from the resolved triple of the address given for the SMTP MAIL command. Defined
in the SMTP server only.
${mail_mailer}
The mailer from the resolved triple of the address given for the SMTP MAIL command. De-
fined in the SMTP server only.
${msg_id}
The value of the Message-Id: header.
${msg_size}
The value of the SIZE= parameter, i.e., usually the size of the message (in an ESMTP dia-
logue), before the message has been collected, thereafter the message size as computed by
sendmail (and can be used in check_compat).
${nbadrcpts}
The number of bad recipients for a single message.
${nrcpts}
The number of validated recipients for a single message. Note: since recipient validation hap-
pens after check_rcpt has been called, the value in this ruleset is one less than what might be
expected.
${ntries}
The number of delivery attempts.
${opMode}
The current operation mode (from the −b flag).
${quarantine}
The quarantine reason for the envelope, if it is quarantined.
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${queue_interval}
The queue run interval given by the −q flag. For example, −q30m would set ${queue_inter-
val} to “00:30:00”.
${rcpt_addr}
The address part of the resolved triple of the address given for the SMTP RCPT command.
Defined in the SMTP server only after a RCPT command.
${rcpt_host}
The host from the resolved triple of the address given for the SMTP RCPT command. Defined
in the SMTP server only after a RCPT command.
${rcpt_mailer}
The mailer from the resolved triple of the address given for the SMTP RCPT command. De-
fined in the SMTP server only after a RCPT command.
${server_addr}
The address of the server of the current outgoing SMTP connection. For LMTP delivery the
macro is set to the name of the mailer. (only if sendmail is compiled with STARTTLS or
SASL.)
${server_name}
The name of the server of the current outgoing SMTP or LMTP connection. (only if send-
mail is compiled with STARTTLS or SASL.)
${time}
The output of the time(3) function, i.e., the number of seconds since 0 hours, 0 minutes, 0 sec-
onds, January 1, 1970, Coordinated Universal Time (UTC).
${tls_version}
The TLS/SSL version used for the connection, e.g., TLSv1.2, TLSv1; defined after START-
TLS has been used.
${total_rate}
The total number of incoming connections over the time interval specified by Connection-
RateWindowSize.
${verify}
The result of the verification of the presented cert; only defined after STARTTLS has been
used (or attempted). Possible values are:
TRUSTED
verification via DANE succeeded.
DANE_FAIL
verification via DANE failed.
OK
verification succeeded.
NO
no cert presented.
NOT
no cert requested.
FAIL
cert presented but could not be verified,
e.g., the signing CA is missing.
NONE
STARTTLS has not been performed.
CLEAR
STARTTLS has been disabled internally for a clear text delivery attempt.
TEMP
temporary error occurred.
PROT OCOL
some protocol error occurred
at the ESMTP level (not TLS).
CONFIG
tls_*_features failed due to a syntax error.
SOFTWARE
STARTTLS handshake failed,
which is a fatal error for this session,
the e-mail will be queued.
There are three types of dates that can be used. The $a and $b macros are in RFC 822 for-
mat; $a is the time as extracted from the “Date:” line of the message (if there was one), and $b is
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Sendmail Installation and Operation Guide
the current date and time (used for postmarks). If no “Date:” line is found in the incoming message,
$a is set to the current time also. The $d macro is equivalent to the $b macro in UNIX (ctime) for-
mat.
The macros $w, $j, and $m are set to the identity of this host. Sendmail tries to find the fully
qualified name of the host if at all possible; it does this by calling gethostname(2) to get the current
hostname and then passing that to gethostbyname(3) which is supposed to return the canonical ver-
sion of that host name.17 Assuming this is successful, $j is set to the fully qualified name and $m is
set to the domain part of the name (everything after the first dot). The $w macro is set to the first
word (everything before the first dot) if you have a lev el 5 or higher configuration file; otherwise, it
is set to the same value as $j. If the canonification is not successful, it is imperative that the config
file set $j to the fully qualified domain name18.
The $f macro is the id of the sender as originally determined; when mailing to a specific host
the $g macro is set to the address of the sender relative to the recipient. For example, if I send to
“bollard@matisse.CS.Berkeley.EDU” from the machine “vangogh.CS.Berkeley.EDU” the $f macro
will be “eric” and the $g macro will be “eric@vangogh.CS.Berkeley.EDU.”
The $x macro is set to the full name of the sender. This can be determined in several ways. It
can be passed as flag to sendmail. It can be defined in the NAME environment variable. The third
choice is the value of the “Full-Name:” line in the header if it exists, and the fourth choice is the
comment field of a “From:” line. If all of these fail, and if the message is being originated locally,
the full name is looked up in the /etc/passwd file.
When sending, the $h, $u, and $z macros get set to the host, user, and home directory (if lo-
cal) of the recipient. The first two are set from the $@ and $: part of the rewriting rules, respec-
tively.
The $p and $t macros are used to create unique strings (e.g., for the “Message-Id:” field).
The $i macro is set to the queue id on this host; if put into the timestamp line it can be extremely
useful for tracking messages. The $v macro is set to be the version number of sendmail; this is nor-
mally put in timestamps and has been proven extremely useful for debugging.
The $c field is set to the “hop count,” i.e., the number of times this message has been pro-
cessed. This can be determined by the −h flag on the command line or by counting the timestamps
in the message.
The $r and $s fields are set to the protocol used to communicate with sendmail and the send-
ing hostname. They can be set together using the −p command line flag or separately using the −M
or −oM flags.
The $_ is set to a validated sender host name. If the sender is running an RFC 1413 compli-
ant IDENT server and the receiver has the IDENT protocol turned on, it will include the user name
on that host.
The ${client_name}, ${client_addr}, and ${client_port} macros are set to the name, ad-
dress, and port number of the SMTP client who is invoking sendmail as a server. These can be used
in the check_* rulesets (using the $& deferred evaluation form, of course!).
5.3. C and F — Define Classes
Classes of phrases may be defined to match on the left hand side of rewriting rules, where a
“phrase” is a sequence of characters that does not contain space characters. For example a class of
all local names for this site might be created so that attempts to send to oneself can be eliminated.
17For example, on some systems gethostname might return “foo” which would be mapped to “foo.bar.com” by gethostbyname.
18Older versions of sendmail didn’t pre-define $j at all, so up until 8.6, config files always had to define $j.
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These can either be defined directly in the configuration file or read in from another file. Classes are
named as a single letter or a word in {braces}. Class names beginning with lower case letters and
special characters are reserved for system use. Classes defined in config files may be given names
from the set of upper case letters for short names or beginning with an upper case letter for long
names.
The syntax is:
Cc phrase1 phrase2...
Fc file
Fc |program
Fc [mapkey]@mapclass:mapspec
The first form defines the class c to match any of the named words. If phrase1 or phrase2 is another
class, e.g., $=S, the contents of class S are added to class c. It is permissible to split them among
multiple lines; for example, the two forms:
CHmonet ucbmonet
and
CHmonet
CHucbmonet
are equivalent. The ‘‘F’’ forms read the elements of the class c from the named file, program, or
map specification. Each element should be listed on a separate line. To specify an optional file, use
‘‘−o’’ between the class name and the file name, e.g.,
Fc −o /path/to/file
If the file can’t be used, sendmail will not complain but silently ignore it. The map form should be
an optional map key, an at sign, and a map class followed by the specification for that map. Exam-
ples include:
F{VirtHosts}@ldap:−k (&(objectClass=virtHosts)(host=*)) −v host
F{MyClass}foo@hash:/etc/mail/classes
will fill the class $={VirtHosts} from an LDAP map lookup and $={MyClass} from a hash data-
base map lookup of the key foo. There is also a built-in schema that can be accessed by only speci-
fying:
F{ClassName}@LDAP
This will tell sendmail to use the default schema:
−k (&(objectClass=sendmailMTAClass)
(sendmailMTAClassName=ClassName)
(|(sendmailMTACluster=${sendmailMTACluster})
(sendmailMTAHost=$j)))
−v sendmailMTAClassValue
Note that the lookup is only done when sendmail is initially started.
Elements of classes can be accessed in rules using $= or $˜. The $˜ (match entries not in
class) only matches a single word; multi-word entries in the class are ignored in this context.
Some classes have internal meaning to sendmail:
$=e
contains the Content-Transfer-Encodings that can be 8→7 bit encoded. It is predefined to
contain “7bit”, “8bit”, and “binary”.
$=k
set to be the same as $k, that is, the UUCP node name.
$=m
set to the set of domains by which this host is known, initially just $m.
$=n
can be set to the set of MIME body types that can never be eight to seven bit encoded. It
defaults to “multipart/signed”. Message types “message/*” and “multipart/*” are never
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Sendmail Installation and Operation Guide
encoded directly. Multipart messages are always handled recursively. The handling of
message/* messages are controlled by class $=s.
$=q
A set of Content-Types that will never be encoded as base64 (if they hav e to be encoded,
they will be encoded as quoted-printable). It can have primary types (e.g., “text”) or full
types (such as “text/plain”).
$=s
contains the set of subtypes of message that can be treated recursively. By default it con-
tains only “rfc822”. Other “message/*” types cannot be 8→7 bit encoded. If a message
containing eight bit data is sent to a seven bit host, and that message cannot be encoded
into seven bits, it will be stripped to 7 bits.
$=t
set to the set of trusted users by the T configuration line. If you want to read trusted users
from a file, use Ft/file/name.
$=w
set to be the set of all names this host is known by. This can be used to match local host-
names.
$={persistentMacros}
set to the macros that should be saved across queue runs. Care should be taken when
adding macro names to this class.
Sendmail can be compiled to allow a scanf(3) string on the F line. This lets you do simplistic
parsing of text files. For example, to read all the user names in your system /etc/passwd file into a
class, use
FL/etc/passwd %[ˆ:]
which reads every line up to the first colon.
5.4. M — Define Mailer
Programs and interfaces to mailers are defined in this line. The format is:
Mname, {field=value }*
where name is the name of the mailer (used internally only) and the “field=name” pairs define at-
tributes of the mailer. Fields are:
Path
The pathname of the mailer
Flags
Special flags for this mailer
Sender
Rewriting set(s) for sender addresses
Recipient
Rewriting set(s) for recipient addresses
recipients
Maximum number of recipients per envelope
Argv
An argument vector to pass to this mailer
Eol
The end-of-line string for this mailer
Maxsize
The maximum message length to this mailer
maxmessages
The maximum message deliveries per connection
Linelimit
The maximum line length in the message body
Directory
The working directory for the mailer
Userid
The default user and group id to run as
Nice
The nice(2) increment for the mailer
Charset
The default character set for 8-bit characters
Type
Type information for DSN diagnostics
Wait
The maximum time to wait for the mailer
Queuegroup
The default queue group for the mailer
/
The root directory for the mailer
Only the first character of the field name is checked (it’s case-sensitive).
The following flags may be set in the mailer description. Any other flags may be used freely
to conditionally assign headers to messages destined for particular mailers. Flags marked with † are
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not interpreted by the sendmail binary; these are the conventionally used to correlate to the flags
portion of the H line. Flags marked with ‡ apply to the mailers for the sender address rather than
the usual recipient mailers.
a
Run Extended SMTP (ESMTP) protocol (defined in RFCs 1869, 1652, and 1870). This flag
defaults on if the SMTP greeting message includes the word “ESMTP”.
A
Look up the user (address) part of the resolved mailer triple, in the alias database. Normally
this is only set for local mailers.
b
Force a blank line on the end of a message. This is intended to work around some stupid ver-
sions of /bin/mail that require a blank line, but do not provide it themselves. It would not nor-
mally be used on network mail.
B
Strip leading backslashes (\) off of the address; this is a subset of the functionality of the s flag.
c
Do not include comments in addresses. This should only be used if you have to work around a
remote mailer that gets confused by comments. This strips addresses of the form “Phrase <ad-
dress>” or “address (Comment)” down to just “address”.
C‡ If mail is received from a mailer with this flag set, any addresses in the header that do not have
an at sign (“@”) after being rewritten by ruleset three will have the “@domain” clause from
the sender envelope address tacked on. This allows mail with headers of the form:
From: usera@hosta
To: userb@hostb, userc
to be rewritten as:
From: usera@hosta
To: userb@hostb, userc@hosta
automatically. Howev er, it doesn’t really work reliably.
d
Do not include angle brackets around route-address syntax addresses. This is useful on mailers
that are going to pass addresses to a shell that might interpret angle brackets as I/O redirection.
However, it does not protect against other shell metacharacters. Therefore, passing addresses
to a shell should not be considered secure.
D† This mailer wants a “Date:” header line.
e
This mailer is expensive to connect to, so try to avoid connecting normally; any necessary con-
nection will occur during a queue run. See also option HoldExpensive.
E
Escape lines beginning with “From ” in the message with a ‘>’ sign.
f
The mailer wants a −f from flag, but only if this is a network forward operation (i.e., the mailer
will give an error if the executing user does not have special permissions).
F† This mailer wants a “From:” header line.
g
Normally, sendmail sends internally generated email (e.g., error messages) using the null re-
turn address as required by RFC 1123. However, some mailers don’t accept a null return ad-
dress. If necessary, you can set the g flag to prevent sendmail from obeying the standards; er-
ror messages will be sent as from the MAILER-DAEMON (actually, the value of the $n
macro).
h
Upper case should be preserved in host names (the $@ portion of the mailer triplet resolved
from ruleset 0) for this mailer.
i
Do User Database rewriting on envelope sender address.
I
This flag is deprecated and will be removed from a future version. This mailer will be speak-
ing SMTP to another sendmail — as such it can use special protocol features. This flag should
not be used except for debugging purposes because it uses VERB as SMTP command.
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j
Do User Database rewriting on recipients as well as senders.
k
Normally when sendmail connects to a host via SMTP, it checks to make sure that this isn’t ac-
cidentally the same host name as might happen if sendmail is misconfigured or if a long-haul
network interface is set in loopback mode. This flag disables the loopback check. It should
only be used under very unusual circumstances.
K
Currently unimplemented. Reserved for chunking.
l
This mailer is local (i.e., final delivery will be performed).
L
Limit the line lengths as specified in RFC 821. This deprecated option should be replaced by
the L= mail declaration. For historic reasons, the L flag also sets the 7 flag.
m
This mailer can send to multiple users on the same host in one transaction. When a $u macro
occurs in the argv part of the mailer definition, that field will be repeated as necessary for all
qualifying users. Removing this flag can defeat duplicate suppression on a remote site as each
recipient is sent in a separate transaction.
M† This mailer wants a “Message-Id:” header line.
n
Do not insert a UNIX-style “From” line on the front of the message.
o
Always run as the owner of the recipient mailbox. Normally sendmail runs as the sender for
locally generated mail or as “daemon” (actually, the user specified in the u option) when deliv-
ering network mail. The normal behavior is required by most local mailers, which will not al-
low the envelope sender address to be set unless the mailer is running as daemon. This flag is
ignored if the S flag is set.
p
Use the route-addr style reverse-path in the SMTP SMTP MAIL command rather than just the
return address; although this is required in RFC 821 section 3.1, many hosts do not process re-
verse-paths properly. Rev erse-paths are officially discouraged by RFC 1123.
P† This mailer wants a “Return-Path:” line.
q
When an address that resolves to this mailer is verified (SMTP VRFY command), generate 250
responses instead of 252 responses. This will imply that the address is local.
r
Same as f, but sends a −r flag.
R
Open SMTP connections from a “secure” port. Secure ports aren’t (secure, that is) except on
UNIX machines, so it is unclear that this adds anything. sendmail must be running as root to
be able to use this flag.
s
Strip quote characters (" and \) off of the address before calling the mailer.
S
Don’t reset the userid before calling the mailer. This would be used in a secure environment
where sendmail ran as root. This could be used to avoid forged addresses. If the U= field is
also specified, this flag causes the effective user id to be set to that user.
u
Upper case should be preserved in user names for this mailer. Standards require preservation
of case in the local part of addresses, except for those address for which your system accepts
responsibility. RFC 2142 provides a long list of addresses which should be case insensitive. If
you use this flag, you may be violating RFC 2142. Note that postmaster is always treated as a
case insensitive address regardless of this flag.
U
This mailer wants UUCP-style “From” lines with the ugly “remote from <host>” on the end.
w
The user must have a valid account on this machine, i.e., getpwnam must succeed. If not, the
mail is bounced. See also the MailboxDatabase option. This is required to get “.forward” ca-
pability.
W
Ignore long term host status information (see Section "Persistent Host Status Information").
x† This mailer wants a “Full-Name:” header line.
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X
This mailer wants to use the hidden dot algorithm as specified in RFC 821; basically, any line
beginning with a dot will have an extra dot prepended (to be stripped at the other end). This
insures that lines in the message containing a dot will not terminate the message prematurely.
z
Run Local Mail Transfer Protocol (LMTP) between sendmail and the local mailer. This is a
variant on SMTP defined in RFC 2033 that is specifically designed for delivery to a local mail-
box.
Z
Apply DialDelay (if set) to this mailer.
0
Don’t look up MX records for hosts sent via SMTP/LMTP. Do not apply FallbackMXhost ei-
ther.
1
Strip null characters (’ ’) when sending to this mailer.
2
Don’t use ESMTP even if offered; this is useful for broken systems that offer ESMTP but fail
on EHLO (without recovering when HELO is tried next).
3
Extend the list of characters converted to =XX notation when converting to Quoted-Printable to
include those that don’t map cleanly between ASCII and EBCDIC. Useful if you have IBM
mainframes on site.
5
If no aliases are found for this address, pass the address through ruleset 5 for possible alternate
resolution. This is intended to forward the mail to an alternate delivery spot.
6
Strip headers to seven bits.
7
Strip all output to seven bits. This is the default if the L flag is set. Note that clearing this op-
tion is not sufficient to get full eight bit data passed through sendmail. If the 7 option is set,
this is essentially always set, since the eighth bit was stripped on input. Note that this option
will only impact messages that didn’t hav e 8→7 bit MIME conversions performed.
8
If set, it is acceptable to send eight bit data to this mailer; the usual attempt to do 8→7 bit
MIME conversions will be bypassed.
9
If set, do limited 7→8 bit MIME conversions. These conversions are limited to text/plain data.
:
Check addresses to see if they begin with “:include:”; if they do, convert them to the “*in-
clude*” mailer.
|
Check addresses to see if they begin with a ‘|’; if they do, convert them to the “prog” mailer.
/
Check addresses to see if they begin with a ‘/’; if they do, convert them to the “*file*” mailer.
@
Look up addresses in the user database.
%
Do not attempt delivery on initial receipt of a message or on queue runs unless the queued
message is selected using one of the -qI/-qR/-qS queue run modifiers or an ETRN request.
!
Disable an MH hack that drops an explicit From: header if it is the same as what sendmail
would generate.
Configuration files prior to level 6 assume the ‘A’, ‘w’, ‘5’, ‘:’, ‘|’, ‘/’, and ‘@’ options on the
mailer named “local”.
The mailer with the special name “error” can be used to generate a user error. The (optional)
host field is an exit status to be returned, and the user field is a message to be printed. The exit sta-
tus may be numeric or one of the values USAGE, NOUSER, NOHOST, UNAVAILABLE, SOFT-
WARE, TEMPFAIL, PROT OCOL, or CONFIG to return the corresponding EX_ exit code, or an
enhanced error code as described in RFC 1893, Enhanced Mail System Status Codes. For example,
the entry:
$#error $@ NOHOST $: Host unknown in this domain
on the RHS of a rule will cause the specified error to be generated and the “Host unknown” exit sta-
tus to be returned if the LHS matches. This mailer is only functional in rulesets 0, 5, or one of the
check_* rulesets. The host field can also contain the special token quarantine which instructs
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sendmail to quarantine the current message.
The mailer with the special name “discard” causes any mail sent to it to be discarded but oth-
erwise treated as though it were successfully delivered. This mailer cannot be used in ruleset 0,
only in the various address checking rulesets.
The mailer named “local” must be defined in every configuration file. This is used to deliver
local mail, and is treated specially in several ways. Additionally, three other mailers named “prog”,
“*file*”, and “*include*” may be defined to tune the delivery of messages to programs, files, and
:include: lists respectively. They default to:
Mprog, P=/bin/sh, F=lsoDq9, T=DNS/RFC822/X-Unix, A=sh −c $u
M*file*, P=[FILE], F=lsDFMPEouq9, T=DNS/RFC822/X-Unix, A=FILE $u
M*include*, P=/dev/null, F=su, A=INCLUDE $u
Builtin pathnames are [FILE] and [IPC], the former is used for delivery to files, the latter for
delivery via interprocess communication. For mailers that use [IPC] as pathname the argument vec-
tor (A=) must start with TCP or FILE for delivery via a TCP or a Unix domain socket. If TCP is
used, the second argument must be the name of the host to contact. Optionally a third argument can
be used to specify a port, the default is smtp (port 25). If FILE is used, the second argument must
be the name of the Unix domain socket.
If the argument vector does not contain $u then sendmail will speak SMTP (or LMTP if the
mailer flag z is specified) to the mailer.
If no Eol field is defined, then the default is "\r\n" for SMTP mailers and "\n" of others.
The Sender and Recipient rewriting sets may either be a simple ruleset id or may be two ids
separated by a slash; if so, the first rewriting set is applied to envelope addresses and the second is
applied to headers. Setting any value to zero disables corresponding mailer-specific rewriting.
The Directory is actually a colon-separated path of directories to try. For example, the defini-
tion “D=$z:/” first tries to execute in the recipient’s home directory; if that is not available, it tries to
execute in the root of the filesystem. This is intended to be used only on the “prog” mailer, since
some shells (such as csh) refuse to execute if they cannot read the current directory. Since the queue
directory is not normally readable by unprivileged users csh scripts as recipients can fail.
The Userid specifies the default user and group id to run as, overriding the DefaultUser op-
tion (q.v.). If the S mailer flag is also specified, this user and group will be set as the effective uid
and gid for the process. This may be given as user:group to set both the user and group id; either
may be an integer or a symbolic name to be looked up in the passwd and group files respectively. If
only a symbolic user name is specified, the group id in the passwd file for that user is used as the
group id.
The Charset field is used when converting a message to MIME; this is the character set used
in the Content-Type: header. If this is not set, the DefaultCharSet option is used, and if that is not
set, the value “unknown-8bit” is used. WARNING: this field applies to the sender’s mailer, not the
recipient’s mailer. For example, if the envelope sender address lists an address on the local network
and the recipient is on an external network, the character set will be set from the Charset= field for
the local network mailer, not that of the external network mailer.
The Type= field sets the type information used in MIME error messages as defined by RFC
1894. It is actually three values separated by slashes: the MTA-type (that is, the description of how
hosts are named), the address type (the description of e-mail addresses), and the diagnostic type (the
description of error diagnostic codes). Each of these must be a registered value or begin with “X−”.
The default is “dns/rfc822/smtp”.
The m= field specifies the maximum number of messages to attempt to deliver on a single
SMTP or LMTP connection. The default is infinite.
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The r= field specifies the maximum number of recipients to attempt to deliver in a single en-
velope. It defaults to 100.
The /= field specifies a new root directory for the mailer. The path is macro expanded and
then passed to the “chroot” system call. The root directory is changed before the Directory field is
consulted or the uid is changed.
The Wait= field specifies the maximum time to wait for the mailer to return after sending all
data to it. This applies to mailers that have been forked by sendmail.
The Queuegroup= field specifies the default queue group in which received mail should be
queued. This can be overridden by other means as explained in section ‘‘Queue Groups and Queue
Directories’’.
5.5. H — Define Header
The format of the header lines that sendmail inserts into the message are defined by the H
line. The syntax of this line is one of the following:
Hhname: htemplate
H[?mflags?]hname: htemplate
H[?${macro}?]hname: htemplate
Continuation lines in this spec are reflected directly into the outgoing message. The htemplate is
macro-expanded before insertion into the message. If the mflags (surrounded by question marks)
are specified, at least one of the specified flags must be stated in the mailer definition for this header
to be automatically output. If a ${macro} (surrounded by question marks) is specified, the header
will be automatically output if the macro is set. The macro may be set using any of the normal
methods, including using the macro storage map in a ruleset. If one of these headers is in the input
it is reflected to the output regardless of these flags or macros. Notice: If a ${macro} is used to set a
header, then it is useful to add that macro to class $={persistentMacros} which consists of the
macros that should be saved across queue runs.
Some headers have special semantics that will be described later.
A secondary syntax allows validation of headers as they are being read. To enable validation,
use:
HHeader: $>Ruleset
HHeader: $>+Ruleset
The indicated Ruleset is called for the specified Header, and can return $#error to reject or quaran-
tine the message or $#discard to discard the message (as with the other check_* rulesets). The
ruleset receives the header field-body as argument, i.e., not the header field-name; see also
${hdr_name} and ${currHeader}. The header is treated as a structured field, that is, text in paren-
theses is deleted before processing, unless the second form $>+ is used. Note: only one ruleset can
be associated with a header; sendmail will silently ignore multiple entries.
For example, the configuration lines:
HMessage-Id: $>CheckMessageId
SCheckMessageId
R< $+ @ $+ >
$@ OK
R$*
$#error $: Illegal Message-Id header
would refuse any message that had a Message-Id: header of any of the following forms:
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Message-Id: <>
Message-Id: some text
Message-Id: <legal text@domain> extra crud
A default ruleset that is called for headers which don’t hav e a specific ruleset defined for them can
be specified by:
H*: $>Ruleset
or
H*: $>+Ruleset
5.6. O — Set Option
There are a number of global options that can be set from a configuration file. Options are
represented by full words; some are also representable as single characters for back compatibility.
The syntax of this line is:
O option=value
This sets option option to be value. Note that there must be a space between the letter ‘O’ and the
name of the option. An older version is:
Oo value
where the option o is a single character. Depending on the option, value may be a string, an integer,
a boolean (with legal values “t”, “T”, “f”, or “F”; the default is TRUE), or a time interval.
All filenames used in options should be absolute paths, i.e., starting with ’/’. Relative file-
names most likely cause surprises during operation (unless otherwise noted).
The options supported (with the old, one character names in brackets) are:
AliasFile=spec, spec, ...
[A] Specify possible alias file(s). Each spec should be in the format ‘‘class: info’’
where class: is optional and defaults to ‘‘implicit’’. Note that info is required for
all classes except “ldap”. For the “ldap” class, if info is not specified, a default
info value is used as follows:
−k (&(objectClass=sendmailMTAAliasObject)
(sendmailMTAAliasName=aliases)
(|(sendmailMTACluster=${sendmailMTACluster})
(sendmailMTAHost=$j))
(sendmailMTAKey=%0))
−v sendmailMTAAliasValue
Depending on how sendmail is compiled, valid classes are “implicit” (search
through a compiled-in list of alias file types, for back compatibility), “hash” (if
NEWDB is specified), “btree” (if NEWDB is specified), “dbm” (if NDBM is speci-
fied), “cdb” (if CDB is specified), “stab” (internal symbol table — not normally
used unless you have no other database lookup), “sequence” (use a sequence of
maps previously declared), “ldap” (if LDAPMAP is specified), or “nis” (if NIS is
specified). If a list of specs are provided, sendmail searches them in order.
AliasWait=timeout
[a] If set, wait up to timeout (units default to minutes) for an “@:@” entry to exist
in the alias database before starting up. If it does not appear in the timeout inter-
val issue a warning.
AllowBogusHELO
If set, allow HELO SMTP commands that don’t include a host name. Setting this
violates RFC 1123 section 5.2.5, but is necessary to interoperate with several
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SMTP clients. If there is a value, it is still checked for legitimacy.
AuthMaxBits=N Limit the maximum encryption strength for the security layer in SMTP AUTH
(SASL). Default is essentially unlimited. This allows to turn off additional en-
cryption in SASL if STARTTLS is already encrypting the communication, be-
cause the existing encryption strength is taken into account when choosing an al-
gorithm for the security layer. For example, if STARTTLS is used and the sym-
metric cipher is 3DES, then the keylength (in bits) is 168. Hence setting Auth-
MaxBits to 168 will disable any encryption in SASL.
AuthMechanisms List of authentication mechanisms for AUTH (separated by spaces). The adver-
tised list of authentication mechanisms will be the intersection of this list and the
list of available mechanisms as determined by the Cyrus SASL library. If START-
TLS is active, EXTERNAL will be added to this list. In that case, the value of
{cert_subject} is used as authentication id.
AuthOptions
List of options for SMTP AUTH consisting of single characters with intervening
white space or commas.
A
Use the AUTH= parameter for the MAIL
command only when authentication succeeded.
This can be used as a workaround for broken
MTAs that do not implement RFC 2554 correctly.
a
protection from active (non-dictionary) attacks
during authentication exchange.
c
require mechanisms which pass client credentials,
and allow mechanisms which can pass credentials
to do so.
d
don’t permit mechanisms susceptible to passive
dictionary attack.
f
require forward secrecy between sessions
(breaking one won’t help break next).
m
require mechanisms which provide mutual authentication
(only available if using Cyrus SASL v2 or later).
p
don’t permit mechanisms susceptible to simple
passive attack (e.g., PLAIN, LOGIN), unless a
security layer is active.
y
don’t permit mechanisms that allow anonymous login.
The first option applies to sendmail as a client, the others to a server. Example:
O AuthOptions=p,y
would disallow ANONYMOUS as AUTH mechanism and would allow PLAIN
and LOGIN only if a security layer (e.g., provided by STARTTLS) is already ac-
tive. The options ’a’, ’c’, ’d’, ’f’, ’p’, and ’y’ refer to properties of the selected
SASL mechanisms. Explanations of these properties can be found in the Cyrus
SASL documentation.
AuthRealm
The authentication realm that is passed to the Cyrus SASL library. If no realm is
specified, $j is used. See also KNOWNBUGS.
BadRcptThrottle=N
If set and the specified number of recipients in a single SMTP transaction have
been rejected, sleep for one second after each subsequent RCPT command in that
transaction.
BlankSub=c
[B] Set the blank substitution character to c. Unquoted spaces in addresses are re-
placed by this character. Defaults to space (i.e., no change is made).
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CACertPath
Path to directory with certificates of CAs. This directory directory must contain
the hashes of each CA certificate as filenames (or as links to them).
CACertFile
File containing one or more CA certificates; see section about STARTTLS for
more information.
CertFingerprintAlgorithm
Specify the fingerprint algorithm (digest) to use for the presented cert. If the op-
tion is not set, md5 is used and the macro ${cert_md5} contains the cert finger-
print. If the option is explicitly set, the specified algorithm (e.g., sha1) is used and
the macro ${cert_fp} contains the cert fingerprint.
CipherList
Specify cipher list for STARTTLS (does not apply to TLSv1.3). See ciphers(1)
for possible values.
CheckAliases
[n] Validate the RHS of aliases when rebuilding the alias database.
CheckpointInterval=N
[C] Checkpoints the queue every N (default 10) addresses sent. If your system
crashes during delivery to a large list, this prevents retransmission to any but the
last N recipients.
ClassFactor=fact [z] The indicated factor is multiplied by the message class (determined by the
Precedence: field in the user header and the P lines in the configuration file) and
subtracted from the priority. Thus, messages with a higher Priority: will be fa-
vored. Defaults to 1800.
ClientCertFile
File containing the certificate of the client, i.e., this certificate is used when send-
mail acts as client (for STARTTLS).
ClientKeyFile
File containing the private key belonging to the client certificate (for STARTTLS
if sendmail runs as client).
ClientPortOptions=options
Set client SMTP options. The options are key=value pairs separated by commas.
Known keys are:
Port
Name/number of source port for connection (defaults to any free port)
Addr
Address mask (defaults INADDR_ANY)
Family
Address family (defaults to INET)
SndBufSize
Size of TCP send buffer
RcvBufSize
Size of TCP receive buffer
Modifier
Options (flags) for the client
The Address mask may be a numeric address in IPv4 dot notation or IPv6 colon
notation or a network name. Note that if a network name is specified, only the
first IP address returned for it will be used. This may cause indeterminate behav-
ior for network names that resolve to multiple addresses. Therefore, use of an ad-
dress is recommended. Modifier can be the following character:
h
use name of interface for HELO command
A
don’t use AUTH when sending e-mail
S
don’t use STARTTLS when sending e-mail
If ‘‘h’’ is set, the name corresponding to the outgoing interface address (whether
chosen via the Connection parameter or the default) is used for the HELO/EHLO
command. However, the name must not start with a square bracket and it must
contain at least one dot. This is a simple test whether the name is not an IP ad-
dress (in square brackets) but a qualified hostname. Note that multiple ClientPor-
tOptions settings are allowed in order to give settings for each protocol family
(e.g., one for Family=inet and one for Family=inet6). A restriction placed on one
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SMM:08-65
family only affects outgoing connections on that particular family.
ClientSSLOptions
A space or comma separated list of SSL related options for the client side. See
SSL_CTX_set_options(3) for a list; the available values depend on the OpenSSL
version against which
sendmail
is compiled.
By default,
SSL_OP_ALL
SSL_OP_NO_SSLv2 SSL_OP_NO_TICKET -SSL_OP_TLSEXT_PADDING are
used (if those options are available). Options can be cleared by preceding them
with a minus sign. It is also possible to specify numerical values, e.g., -0x0010.
ColonOkInAddr If set, colons are acceptable in e-mail addresses (e.g., “host:user”). If not set,
colons indicate the beginning of a RFC 822 group construct (“groupname: mem-
ber1, member2, ... memberN;”). Doubled colons are always acceptable (“node-
name::user”)
and
proper
route-addr
nesting
is
understood
(“<@re-
lay:user@host>”). Furthermore, this option defaults on if the configuration ver-
sion level is less than 6 (for back compatibility). However, it must be off for full
compatibility with RFC 822.
ConnectionCacheSize=N
[k] The maximum number of open connections that will be cached at a time. The
default is one. This delays closing the current connection until either this invoca-
tion of sendmail needs to connect to another host or it terminates. Setting it to
zero defaults to the old behavior, that is, connections are closed immediately.
Since this consumes file descriptors, the connection cache should be kept small: 4
is probably a practical maximum.
ConnectionCacheTimeout=timeout
[K] The maximum amount of time a cached connection will be permitted to idle
without activity. If this time is exceeded, the connection is immediately closed.
This value should be small (on the order of ten minutes). Before sendmail uses a
cached connection, it always sends a RSET command to check the connection; if
this fails, it reopens the connection. This keeps your end from failing if the other
end times out. The point of this option is to be a good network neighbor and
avoid using up excessive resources on the other end. The default is five minutes.
ConnectOnlyTo=address
This can be used to override the connection address (for testing purposes).
ConnectionRateThrottle=N
If set to a positive value, allow no more than N incoming connections in a one sec-
ond period per daemon. This is intended to flatten out peaks and allow the load
av erage checking to cut in. Defaults to zero (no limits).
ConnectionRateWindowSize=N
Define the length of the interval for which the number of incoming connections is
maintained. The default is 60 seconds.
ControlSocketName=name
Name of the control socket for daemon management. A running sendmail dae-
mon can be controlled through this named socket. Available commands are: help,
mstat, restart, shutdown, and status. The status command returns the current
number of daemon children, the maximum number of daemon children, the free
disk space (in blocks) of the queue directory, and the load average of the machine
expressed as an integer. If not set, no control socket will be available. Solaris and
pre-4.4BSD kernel users should see the note in sendmail/README .
CRLFile=name
Name of file that contains certificate revocation status, useful for X.509v3 authen-
tication. Note: if a CRLFile is specified but the file is unusable, STARTTLS is
disabled.
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Sendmail Installation and Operation Guide
CRLPath=name Name of directory that contains hashes pointing to certificate revocation status
files. Symbolic links can be generated with the following two (Bourne) shell
commands:
C=FileName_of_CRL
ln -s $C ‘openssl crl -noout -hash < $C‘.r0
DHParameters
This option applies to the server side only. Possible values are:
5
use precomputed 512 bit prime.
1
generate 1024 bit prime
2
generate 2048 bit prime.
i
use included precomputed 2048 bit prime (default).
none
do not use Diffie-Hellman.
/path/to/file
load prime from file.
This is only required if a ciphersuite containing DSA/DH is used. The default is
‘‘i’’ which selects a precomputed, fixed 2048 bit prime. If ‘‘5’’ is selected, then
precomputed, fixed primes are used. Note: this option should not be used (unless
necessary for compatibility with old implementations). If ‘‘1’’ or ‘‘2’’ is selected,
then prime values are computed during startup. Note: this operation can take a
significant amount of time on a slow machine (several seconds), but it is only
done once at startup. If ‘‘none’’ is selected, then TLS ciphersuites containing
DSA/DH cannot be used. If a fi