Linux Network Administrator's Guide, Second Edition

Interpreting and Writing Rewrite Rules

Arguably the most powerful feature of sendmail is the rewrite rule. Rewrite rules are used by sendmail to determine how to process a received mail message. sendmail passes the addresses from the headers of a mail message through collections of rewrite rules called rulesets The rewrite rules transform a mail address from one form to another and you can think of them as being similar to a command in your editor that replaces all text matching a specified pattern with another.

Each rule has a lefthand side and a righthand side, separated by at least one tab character. When sendmail is processing mail it scans through the rewriting rules looking for a match on the lefthand side. If an address matches the lefthand side of a rewrite rule, the address is replaced by the righthand side and processed again.

sendmail.cf R and S Commands

In the sendmail.cf file, the rulesets are defined using commands coded as S n, where n specifies the ruleset that is considered the current one.

The rules themselves appear in commands coded as R. As each R command is read, it is added to the current ruleset.

If you’re dealing only with the sendmail.mc file, you won’t need to worry about S commands at all, as the macros will build those for you. You will need to manually code your R rules.

A sendmail ruleset therefore looks like:

Sn
Rlhs 
                  rhs
Rlhs2 
                  rhs2

Some Useful Macro Definitions

sendmail uses a number of standard macro definitions internally. The most useful of these ...