Contents:
Role in the Filesystem
Role in Local Delivery
Role in Network Transport
Role as a Daemon
Things to Try
The sendmail program plays a variety of roles, all critical to the proper flow of electronic mail. It listens to the network for incoming mail, transports mail messages to other machines, and hands local mail to a local program for local delivery. It can append mail to files and can pipe mail through other programs. It can queue mail for later delivery and understands the aliasing of one recipient name to another.
The sendmail program's role (position) in the local file system hierarchy can be viewed as an inverted tree (see Figure 3.1).
When sendmail is run, it first reads the /etc/sendmail.cf configuration file. Among the many items contained in that file are the locations of all the other files and directories that sendmail needs.
Files and directories listed in sendmail.cf are usually specified as full pathnames for security (such as /var/spool/mqueue rather than mqueue). As the first step in our tour of those files, run the following command to gather a list of them: [1]
[1] If you are not currently running V8.7 or above sendmail, you will have to grep for "/[^0-9].*/" instead. If you're not running sendmail at all, you won't be able to do this, so for now just read along instead.
%grep =/ /etc/sendmail.cf
The output produced by the grep(1) command may look something like the following: [2]
[2] Lines that begin with
F
orK
may also appear. If so, ignore them for now.
O AliasFile=/etc/aliases #O ErrorHeader=/etc/sendmail.oE O HelpFile=/usr/lib/sendmail.hf O QueueDirectory=/var/spool/mqueue O StatusFile=/etc/sendmail.st #O UserDatabaseSpec=/etc/userdb #O ServiceSwitchFile=/etc/service.switch #O HostsFile=/etc/hosts #O SafeFileEnvironment=/arch Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10/30, R=20/40, Mprog, P=/bin/sh, F=lsDFMoeu, S=10/30, R=20/40, D=$z:/,
Notice that some lines begin with an O
character, some
with an M
, and others with a #
.
The O
marks a line as a configuration option.
The word following the O
is the name of the option.
The options in the above output show the location of the files that
sendmail uses. AliasFile
, for example, defines the
location of the aliases(5) database.
The lines that begin with M
define delivery agents.
The lines that begin with a #
are comments.
First we will examine the files in the O
option
lines. Then we will discuss local delivery and the files
in the M
delivery agent lines.
Aliasing is the process of converting one recipient name into another. One use is to convert a generic name (such as root) into a real username. Another is to convert one name into a list of many names (for mailing lists).
Take a few moments to examine your aliases file. Its
location is determined by the AliasFile
option in your sendmail.cf file. For example,
O AliasFile=/etc/aliases
Compare what you find in your aliases file to the brief example of an aliases file listed below:
# Mandatory aliases.
postmaster: root
MAILER-DAEMON: postmaster
# The five forms of aliases
John_Adams: adamj
xpres: ford,carter,bush
oldlist: :include: /usr/local/oldguys
nobody: /dev/null
ftphelp: |/usr/local/bin/sendhelp
Your aliases file is probably far more complex, but even so, note that the example shows all the possible forms of aliases.
Lines that begin with #
are comments. Empty lines
are ignored.
As the first comment indicates, there are two
aliases that are mandatory in every aliases file.
Both are the simplest form of alias: a name and what to change
that name into.
The name on the left of the :
is changed into the name on the right.
Names are not case-sensitive. For example, POSTMASTER
,
Postmaster
, and postmaster
are all the same.
[3]
[3] According to RFC822, all usernames are case-sensitive except postmaster. But sendmail, when processing its aliases file, views all names as case-insensitive.
For every envelope that lists a local user as a recipient,
sendmail looks up that recipient's name in the aliases
file. (A local user is any address that would normally be delivered
on the local machine.
That is, postmaster is local, whereas
postmaster@remote may not be.)
When sendmail matches the recipient to one of the
names on the left of the aliases file, it replaces that
recipient name with the text to the right of the :
character.
For example, the recipient postmaster
becomes the recipient root
.
After a name is substituted, the new name is then looked up,
and the process is repeated until
no more matches are found.
The name MAILER-DAEMON
is first changed
to postmaster
. Then postmaster
is looked up again and changed to root
. Since
there is no entry for root
in the aliases file,
the mail message is delivered into root's mailbox.
Every aliases file must have an alias for postmaster
that will expand to the name of a real user.
[4]
Mail about mail
problems is always sent to postmaster
both by mail-related
programs and by users who are having trouble sending mail.
[4] The name
postmaster
is required by RFC822 and RFC1648, so resist the temptation to redefine it aspostperson
orsysop
.
When mail is
bounced (returned because it could not be delivered),
it is always sent from MAILER-DAEMON
.
That alias is needed because users may reply to bounced mail.
Without it, replies to bounced mail would themselves bounce.
The five types of lines in an aliases file areas are as follows:
John_Adams: adamj
xpres: ford,carter,reagan,bush
oldlist: :include: /usr/local/oldguys
nobody: /dev/null
ftphelp: |/usr/local/bin/sendhelp
You have already seen the first (it was the form used to convert
postmaster
to root
). In the above example, mail sent
to John_Adams
is delivered to the user whose
login name is adamj
.
The xpres:
line shows how one name can
be expanded into a list of many names. Each of those new names
becomes a new name for further alias processing.
If a name can't be further expanded, a copy of the mail
message is delivered to it.
The oldlist:
line shows how a mailing list can be read
from a file. The expression
:include:
tells sendmail to read a specific file
and to use the names in that file as the list of recipients.
The nobody:
line shows how a name can be aliased
to a file.
The mail message is appended to the file.
The /dev/null file listed here is a special one. That
file is an empty hole into which the mail message simply vanishes.
The ftphelp:
line shows how a name can be replaced by
the name of a program. The |
character causes
sendmail to pipe the mail message through the program whose
full pathname follows (in this case, we specified the full pathname as
/usr/local/bin/sendhelp).
The aliases file can become very complex. It can be used to solve many special mail problems. The aliases file is covered in greater detail in Chapter 24, Aliases.
A mail message can be temporarily undeliverable for a wide variety of reasons, such as when a remote machine is down or has a temporary disk problem. To ensure that such messages are eventually delivered, sendmail stores them in its queue directory until they can be delivered successfully.
The QueueDirectory
option
in your configuration file tells sendmail
where to find its queue directory:
O QueueDirectory=/var/spool/mqueue
The location of that directory must be a full pathname. Its exact
location varies from vendor to vendor, but you
can always find it by looking for the QueueDirectory
option in your
configuration file.
If you have permission, take a look at the queue directory. It may be empty if there is no mail waiting to be sent. If it is not empty, it will contain files like these:
dfQAA07038 dfMAA08000 qfQAA07038 qfMAA08000
When a mail message is queued, it is split into two parts,
each part being saved in a separate file. The header information
is saved in a file whose name begins with the characters qf
.
The body of the mail message is saved in a file whose name begins
with the characters df
.
The example above shows two queued mail messages. One
is identified by the unique string QAA07038
and the other
by MAA08000
.
The internals of the queue files and the processing of those files are covered in Chapter 23, The Queue.