Using Postfix for Secure SMTP Gateways

Junk mail is one of the most common and annoying types of e-mail abuse. Postfix offers protection against UCE (Unsolicited Commercial E-mail) via a couple of settings in main.cf. Some caution is in order, however: there's a fine line between spam and legitimate dissemination, and it's entirely possible that even modest UCE controls will cause some legitimate (i.e., desired) mail to be dropped.

Having said that, for most sites this is an acceptable risk (avoidable, too, through end-user education), and we recommend that at a minimum, you set the following in main.cf:

In order to prevent giving out information that serves no purpose to legitimate external parties, it is wise to set in the main.cf file the parameter masquerade_domains = $mydomain (remember, “$mydomain” refers to a variable). If you wish to make an exception for mail sent by “root” (probably a good idea), you can set the parameter masquerade_exceptions = root. This will cause internal host names to be stripped from FQDSes in “From” addresses of outbound messages.

Now we come to one of the groovier things we can do to secure postfix: running it in a “chroot jail”. chroot is a UNIX command that confines the “chrooted” process to a specified directory; that directory becomes “/” for that process. This usually requires you to first create copies of things needed by the process but normally kept elsewhere. For example, if the process looks for “/etc/mydaemon.conf” upon startup but is being chrooted to “/var/mydaemon”, the process will actually look for “/var/mydaemon/etc/mydaemon.conf”.

The advantage to chrooting should be obvious: should a chrooted-postfix process become hijacked somehow, the attacker will find himself in a “padded cell” from which (hopefully) no sensitive or important system files or data can be accessed. This isn't a panacea, but it significantly. increases the difficulty of exploiting postfix.

Happily, the preparations required to chroot postfix are provided in a subdirectory of the postfix documentation called “examples”. These files aren't really shell scripts: they're suggested sequences of commands.

Better still, some binary distributions of postfix have installation scripts that automatically make these preparations for you after installing postfix. In SuSE, for example, the postfix RPM package runs a script that creates a complete directory tree for postfix to use when chrooted (etc, usr, lib, and so forth) in /var/spool/postfix, with the appropriate ownerships and permissions.

In addition to “provisioning” postfix's chroot jail, you need to edit /etc/postfix/master.cf to toggle the postfix dæmons you wish to run chrooted (i.e., put a “y” in the “chroot” column of each dæmon to be chrooted). Do not, however, do this for dæmons whose “command” column indicates that they are of type “pipe” or “local”. Some binary-package distributions toggle the appropriate dæmons to chroot automatically during postfix installation (again, SuSE does).

After configuring the chroot jail and editing master.cf, all you need to do is start postfix the way you normally would: postfix start. Postfix's master process handles the actual chroot-ing.