2.3 Invoking SpamAssassin with procmail

Running spamassassin from a shell is a handy way to test the system, but for daily use you'd like to have it automatically run on every incoming email message that's being delivered to your system's mailboxes. One easy way to do this is to have your system's MDA program filter all messages through SpamAssassin as part of the delivery process.

procmail is a mail-processing program that accepts messages on standard input and applies a set of rules or actions (a "recipe") for the disposition of the message. Because the default message disposition is "append to the user 's mailbox," and because procmail is written to be very safe in its handling of messages, it makes an excellent MDA. Indeed, many Unix systems use the procmail program as their default local MDA. If procmail is available and isn't the system MDA, it's usually easy for users to configure the message-forwarding feature of the system's MTA to filter messages through procmail. In either environment, procmail can be a good place to pass messages through SpamAssassin. Figure 2-1 illustrates this configuration.

Figure 2-1. Invoking SpamAssassin with procmail

The easiest way to use SpamAssassin with procmail is to call it in the systemwide procmail recipe file, which is usually /etc/procmailrc . Example 2-6 shows a complete /etc/procmailrc .

Example 2-6. A complete /etc/procmailrc

 DROPPRIVS=yes PATH=/bin:/usr/bin:/usr/local/bin SHELL=/bin/sh # Spamassassin :0fw * <300000 /usr/bin/spamassassin 

In this example, the SpamAssassin recipe comprises the three lines beneath the comment # Spamassassin . The first line tells procmail that the message should be filtered ( f ) and that procmail should wait ( w ) for the filter's successful exit before considering the message filtered. The second line indicates that this recipe should be applied to messages less than 300,000 bytes in length and serves to prevent a lengthy SpamAssassin invocation on a long message that is unlikely to be spam. The third line directs procmail to pipe the message to spamassassin . (For more information about procmail recipes, see the man pages for procmail, procmailrc, and procmailex.)

By placing this recipe in the systemwide procmail configuration file, it will be activated every time procmail is invoked, either as the default MDA or by a user. If you don't have access to the systemwide procmail configuration file, you can still invoke SpamAssassin for your own messages in your account's per-user procmail recipe file, which is usually ~/.procmailrc . This might also be useful if you wish to run SpamAssassin a second time with a different set of command-line arguments.

If your system doesn't provide procmail, it may provide another mail- filtering system. Any mail filter that can pass a message to a program on standard input and read back the (modified) message from the program's standard output can use SpamAssassin in this way.