V8 sendmail provides an easy way to create a custom configuration file for your site. In the cf subdirectory of the V8 sendmail source distribution you will find a file named README. It contains easy-to-understand, step-by-step instructions that allow you to create a custom configuration file for your site. This chapter supplements that file.
Creating a configuration file with m4(1) is simplicity itself. The m4(1) program is a macro preprocessor that produces a sendmail configuration file by processing a file whose name ends in .mc (for macro configuration). That is, it processes (reads) its input and gathers definitions of macros, then replaces those macros with their values and outputs the result.
This use of macros is much the same as that described in Section 7.1, "Overview", except that m4's rules are different. With m4, macros are defined (given values) like this:
macro is a symbolic name that you will use later.
Legal names must begin with an underscore or letter and may contain
letters, digits, and underscores. The
value can be any
arbitrary text. A comma separates the two, and that comma can be
surrounded with optional whitespace.
There must be no space between the
define and the left parenthesis.
The definition ends with the right parenthesis.
To illustrate, consider this one-line m4 source file named /tmp/x:
input text to be converted define(A,B)A the m4 definition
When m4 is run to process this file, the output produced shows
A (the input) is redefined to become
define(A,B) define(A,C) A B
Here, the first line assigns the value
B to the macro named
A. The second line notices that
A is a define macro,
so m4 replaces that
B and then
B as having the value
C. The output of this file,
after processing with m4, will be:
To prevent this kind of greedy behavior (and to prevent the confusion it can create), you may quote an item to prevent m4 from interpreting it. zzz You quote with m4 by surrounding each item with left and right single quotes:
define(A,B) define(`A',C) A B
Here, the first line defines
B as before. But
the second line no longer sees
A as a macro.
Instead, the single quotes allow
A to be redefined as
So the output is now:
Although it is not strictly necessary,
we recommend that all
pairs be quoted. The above should generally
be expressed like this:
define(`A',`B') define(`A',`C') A B
This is the form that we use when illustrating m4 throughout this book.
Another problem with m4 is that it replaces its commands
with empty lines. The above
define commands, for example,
will actually print like this:
a blank line a blank line A B
You can use
dnl to remove blank lines where they might
prove inconvenient or unsightly in a configuration file.
When an m4 macro name is immediately followed by a right parenthesis,
it is treated like a function call. Arguments given to it in that role
are used to replace
digit expressions in the original definition.
For example, suppose the macro CONCAT is defined like this:
and then later used like this:
CONCAT(`host', `.', `domain')
The result will be that
host will replace
dot will replace
$2, and the
domain will replace
jammed tightly together just as
Macro arguments are used to create such techniques as FEATURE() and OSTYPE(), which are described later in this chapter.
One of the m4 program's strengths is its ability to divide its input
into different parts and to later reassemble them in a more logical
fashion. Consider, for example, the desire to output all options together.
One way to do this is with the m4 program's
undivert commands, for example,
divert(1)dnl O ALIASFILE=/etc/aliases divert(2)dnl Pfirst-class=0 divert(1)dnl O OperatorChars=.:%@!^/+ undivert(1)dnl undivert(2)dnl
divert(1) causes all subsequent lines (up to the next
divert or next
undivert) to be held in a buffer numbered
one. Buffer one will hold all the options. The second
switches to buffer two, which is used to hold priorities. The third
divert switches back to buffer one.
undivert(1) causes all the options gathered in buffer one
to be output, and the
undivert(2) causes the priorities to
be output. The result looks like this:
O ALIASFILE=/etc/aliases O OperatorChars=.:%@!^/+ Pfirst-class=0
The diversions used by sendmail's m4 technique are listed in Table 19.1. In general, the macros listed should be used in place of diversion numbers because the meaning of those numbers may be changed in future versions of sendmail.
|(-1)||Internal to m4(1), tells it ignore all lines that follow|
|(0)||Internal to m4(1), tells it to stop diverting and to output immediately|
|(1)||Local host detection and resolution with LOCAL_NET_CONFIG (see Section 19.6.37, LOCAL-NET-CONFIG)|
|(2)||Rule set 3 (via 96) additions with LOCAL_RULE_3 (see Section 19.6.35, LOCAL-RULE-3)|
|(3)||Rule set 0 (via 98) additions with LOCAL_RULE_0 (see Section 19.6.32, LOCAL-RULE-0)|
|(4)||Rule set 0 UUCP additions (Section 19.4.6, "UUCP")|
|(5)||Locally interpreted names (overrides $R) with LOCAL_USER (Section 19.6.38, LOCAL-USER)|
|(6)||Local configuration (at top of file) with LOCAL_CONFIG (see Section 19.6.30, LOCAL-CONFIG)|
|(7)||Delivery agent definitions with MAILER (see Section 19.3.2, "MAILER()") and MAILER_DEFINITIONS (see Section 19.6.40, MAILER-DEFINITIONS)|
|(9)||Rule sets 1 and 2 with LOCAL_RULE_1 and LOCAL_RULE_2 (see Section 19.6.34, LOCAL-RULE-2), rule set 5, and LOCAL_RULESETS (see Section 19.6.36, LOCAL-RULESETS)|