5.3.1 Mail Delivery Agents

Recall that the sendmail program does not generally deliver mail itself. Instead, it calls other programs to perform that delivery. The M command defines a mail delivery agent (a program that delivers the mail). For example, as was previously shown:

Mlocal,    P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40,

This tells sendmail that local mail is to be delivered by using the /bin/mail program. The other parameters in this line are introduced in Chapter 6, The Mail Hub and Delivery Agents, and detailed in Chapter 30, Delivery Agents.

5.3.2 Macros

The ability to define a value once and then use it in many places makes maintaining your sendmail.cf file easier. The D sendmail.cf command defines a macro. A macro's name is either a single letter or curly-brace-enclosed multiple characters. It has text as a value. Once defined, that text can be referenced symbolically elsewhere:

DRmail.us.edu            a single letter
D{REMOTE}mail.us.edu     multiple characters (beginning with V8.7)

Here, R and {REMOTE} are macro names that have the string mail.us.edu as their values. Those values are accessed elsewhere in the sendmail.cf file with expressions such as $R and ${REMOTE}. Macros are introduced in Chapter 7, Macros, and detailed in Chapter 31, Defined Macros.

5.3.3 Rules

At the heart of the sendmail.cf file are sequences of rules that rewrite (transform) mail addresses from one form to another. This is necessary chiefly because addresses must conform to many differing standards. The R command is used to define a rewriting rule:

R$-      $@ $1 @ $R     user -> user @ remote

Mail addresses are compared to the rule on the left ($-). If they match that rule, they are rewritten on the basis of the rule on the right ($@ $1 $@ $R). The text at the far right is a comment (that doesn't require a leading #).

Use of multicharacter macros and # comments can make rules appear a bit less cryptic:

R$-                        # If a plain user name
      $@ $1 @ ${REMOTE}    #    append "@" remote host

The details of rules like this are more fully explained beginning in Chapter 8, Addresses and Rules, and detailed in Chapter 28, Rules.

5.3.4 Rule Sets

Because rewriting may require several steps, rules are organized into sets, which can be thought of as subroutines. The S command begins a rule set:

S3

This particular S command begins rule set 3. Beginning with V8.7 sendmail, rule sets can be given symbolic names as well as numbers:

SHubset

This particular S command begins a rule set named Hubset. [3] Named rule sets are automatically assigned numbers by sendmail.

[3] This is an actual symbolic name that we will use when developing the client.cf file (specifically in Chapter 11, Rule Sets 1 and S=).

All the R commands (rules) that follow an S command belong to that rule set. A rule set ends when another S command appears to define another rule set. Rule sets are introduced in Chapter 8 and detailed in Chapter 29, Rule Sets.

5.3.5 Class Macros

There are times when the single text value of a D command (macro definition) is not sufficient. Often, you will want to define a macro to have multiple values and view those values as elements in an array. The C command defines a class macro. A class macro is like an array in that it can hold many items. The name of a class is either a single letter or, beginning with V8.7, a curly-brace-enclosed multicharacter name:

CW localhost fontserver              a single letter
C{MY_NAMES} localhost fontserver     multiple characters (beginning with V8.7)

Here, each contains two items: localhost and fontserver. The value of a class macro is accessed with an expression such as $=W or $={MY_NAMES}. Class macros are introduced in Chapter 12, Class, and detailed in Chapter 32, Class Macros.

5.3.6 File Class Macros

To make administration easier, it is often convenient to store long or volatile lists of values in a file. The F sendmail.cf command defines a file class macro. It is just like the C command above, except that the array values are taken from a file:

FW/etc/mynames
F{MY_NAMES}/etc/mynames           multiple characters (beginning with V8.7)

Here, the file class macros W and {MY_NAMES} obtain their values from the file /etc/mynames.

The file class macro can also take its list of values from the output of a program. That form looks like this:

FM|/bin/shownames
F{MY_NAMES}|/bin/shownames        multiple characters (beginning with V8.7)

Here, sendmail runs the program /bin/shownames. The output of that program is appended to the class macro. File class macros are introduced in Chapter 12, and detailed in Chapter 32.

5.3.7 Options

Options tell the sendmail program many useful and necessary things. They specify the location of key files, set timeouts, and define how sendmail will act and how it will dispose of errors. They can be used to tune sendmail to meet your particular needs.

The O command is used to set sendmail options. An example of the option command looks like this:

OQ/var/spool/mqueue
O QueueDirectory= /var/spool/mqueue       beginning with V8.7

Here, Q option (beginning with V8.7 called QueueDirectory) defines the name of the directory in which mail will be queued as /var/spool/mqueue. Options are introduced in Chapter 13, Setting Options, and detailed in Chapter 34, Options.

5.3.8 Headers

Mail messages are composed of two parts: a header followed (after a blank line) by the body. The body may contain virtually anything. [4] The header, on the other hand, contains lines of information that must strictly conform to certain standards. The H command is used to specify which mail headers to include in a mail message and how each will look:

[4] With the advent of MIME (Multipurpose Internet Mail Extensions), the message body can now be composed of many minimessages, each with its own MIME header and sub-body.

HReceived: $?sfrom $s $.by $j ($v/$Z)$?r with $r$. id $i$?u for $u$.; $b

This particular H command tells sendmail that a Received: header line must be added to the header of every mail message. Headers are introduced in Chapter 14, Headers, Precedence, and Trust, and detailed in Chapter 35, Headers.

5.3.9 Priority

Not all mail has the same priority. Mass mailings (to a mailing list, for example) should be transmitted after mail to individual users. The P command sets the beginning priority for a mail message. That priority is used to determine a message's order when the mail queue is processed.

Pjunk= -100

This particular P command tells sendmail that mail with a Precedence: header line of junk should be processed last. Priority commands are introduced in Chapter 14 and detailed in Chapter 35.

5.3.10 Trusted Users

For some software (such as UUCP) to function correctly, it must be able to tell sendmail whom a mail message is from. This is necessary when that software runs as a different user identity (uid) than that specified in the From: line in the message header. The T sendmail.cf command [5] lists those users that are trusted to override the From: address in a mail message. All other users have a warning included in the mail message header.

[5] The T command was ignored from versions V8.1 through V8.6 and restored under V8.7. With V8.7 it is actually implemented as the class $=t.

Troot daemon uucp

This particular T sendmail.cf command says that there are three users who are to be considered trusted. They are root (who is a god under UNIX), daemon (sendmail usually runs as the pseudo-user daemon), and uucp (necessary for UUCP software to work properly). Trusted users are introduced in Chapter 14 and detailed in Chapter 22, Security.

5.3.11 Keyed Databases

Certain information, such as a list of UUCP hosts, is better maintained outside of the sendmail.cf file. External databases (called keyed databases) provide faster access to such information. Keyed databases were introduced with V8.6 and come in several forms, the nature and location of which are declared with the K configuration command:

Kuucp hash /etc/mail/uucphosts

This particular K command declares a database with the symbolic name uucp, with the type hash, located in /etc/mail/uucphosts. The K command is detailed and the types of databases are explained, in Chapter 33, Database Macros.

5.3.12 Environment variables

The sendmail program is ultraparanoid about security. One way to circumvent security with suid programs like sendmail is by running them with bogus environmental variables. To prevent such an end run, V8 sendmail erases all its environment variables when it starts. It then presets the values for a small set of variables (such as TZ and SYSTYPE). This small, safe environment is then passed to its delivery agents. Beginning with V8.7 sendmail, sites that wish to augment this list may do so with the E configuration command.

EPOSTGRESHOME=/home/postgres

Here, the environment variable POSTGRESHOME is assigned the value /home/postgres. This allows programs to use the postgres(1) database to access information. The E command is detailed in Chapter 22.