4.6 Procmail and Qmail

If you do any sorting or filtering of incoming mail, you should install the popular procmail mail filtering package. Although procmail's filter definition language is terse to the point of obscurity, it's very powerful and easy to use once you get the hang of it. In the past, procmail's default mailbox location was in /var/mail, and it didn't support Maildirs. Recent versions of procmail work well with qmail. Version 3.14 added support for Maildirs, and it's now easy to compile procmail to put the default mailbox in qmail's preferred place.

The source for procmail is available at http://www.procmail.org. Download it to a local work directory and unpack it. To make its default delivery be to Mailbox, edit the file src/authenticate.c. Around line 47 find the definition of MAILSPOOLHOME, remove the comment characters at the start of the line, and change the file name to Mailbox:

#define MAILSPOOLHOME "/Mailbox"

Or to make the default delivery to a user's Maildir, type:

#define MAILSPOOLHOME "/Maildir/"

(Note the slash after the directory name, which tells procmail that it's a Maildir rather than an mbox.)

Then make and install procmail as described in its INSTALL file. The procmail installation recommends that you install procmail as set-uid to root. When working with qmail, it does not need set-uid to work correctly, and I recommend that you don't do this. When used as the mail delivery agent for sendmail, procmail needs set-uid to run as the id of the delivered-to user. Qmail switches to the correct user ID before running procmail, as it does for any delivery agent, so procmail doesn't need to do so. Installing as set-uid won't cause any immediate problems, but it will pose a possible security problem should there turn out to be lurking bugs in procmail.

To use procmail as your default delivery agent, use this in your qmail run file:

exec env - PATH="/var/qmail/bin:$PATH" \         qmail-start '|preline procmail'

(The preline command is a qmail component that inserts a From line that procmail needs at the front of the message.) Alternatively, to make procmail the delivery agent for an individual user, put the procmail command into the user's .qmail file:

|preline procmail

Sendmail systems often pass the address extension as an argument to procmail so it can be used as $1 in scripts. That's easy enough to do in .qmail-default:

|preline procmail -a "$EXT"

Procmail makes most environment variables available in its rule files anyway, so if you're not converting from sendmail, just use $EXT in your scripts.

It's frequently advantageous to use different procmail filter definitions for different qmail subaddresses. For example, if you are user fred and use the address fred-lists for your mailing list mail, .qmail-lists could contain this:

|preline procmail procmaillists

to use procmaillists to sort list mail.