Hardening Sendmail

Ah, sendmail. You either love it for being so versatile and ubiquitous, or you hate it for being bloated, complicated and insecure. Or perhaps you're a complete newcomer to the e-mail server game and would like to give sendmail a try (after all, sendmail is arguably the most popular open-source software package on the Internet).

Well, contrary to popular belief, sendmail isn't a total loss where security is concerned, nor does it require learning the arcane syntax of sendmail.cf (although hardcore sendmail gurus do indeed master it). This month we examine these and other sendmail security controversies, using sendmail's handy m4 macros to rapidly build a secure but functional Simple Mail Transport Protocol (SMTP) gateway to handle internet mail.

Sendmail is one of the most venerable internet software packages still in widespread use. It first appeared in 4.1c BSD UNIX (April 1983), and to this day it has remained the most relied-upon application of its kind. Among message transfer agents (MTAs), sendmail is the great workhorse of the Internet, transferring e-mail between networks dependably and (to end users) transparently. But sendmail has both advantages and disadvantages.

On the good side, sendmail has a huge user community, with the result that it's easy to find both free and commercial support for it, not to mention a wealth of electronic and print publications. It's also stable and predictable, due to its maturity.

On the negative side, sendmail has acquired a certain amount of cruft (old code) over its long history, with the result that it has a reputation of being insecure and bloated. Both charges are open to debate, however. It's true that it has had a number of significant vulnerabilities over the years. However, these have been brought to light and fixed rapidly.

As for the “bloatware” charge, it's true that sendmail has a much larger code base than other MTAs such as qmail and Postfix, and a larger RAM footprint too. This probably has at least as much to do with the fact that sendmail is monolithic (one executable provides most of sendmail's functionality) as it does with cruft. Indeed, sendmail's code has been scrutinized so closely by so many programmers over the years that it's a little hard to believe that too much cruft (i.e., code that is strictly historical and obsolete) has survived intact over the past 20 years.

It's much more useful to observe that being monolithic, sendmail must run as root if any portion of its required functionality does, i.e., writing mail to multiple users' home directories. For this reason, sendmail can run only as an unprivileged user on systems on which it's to act solely as an e-mail relay or gateway.

Sendmail also is criticized for its complexity. The syntax of its configuration file, sendmail.cf, is non-instinctive to say the least—in my opinion, its difficulty ranks somewhere between C programming and regular expressions. As with these, this is because of how powerful sendmail is (though many of us do wish sendmail used C, regular expressions or some other standard configuration language in sendmail.cf rather than its own equally complex but much more proprietary syntax). Nowadays, though, this point is largely moot. Recent versions of sendmail can be configured via m4 macros, which provide a much less user-hostile experience than editing sendmail.cf directly.

Mick's Disclaimer

Regardless of one's opinions on sendmail, it's unquestionably a powerful and well-supported piece of software. If sendmail's benefits are more compelling to you than the negatives, you're in good company. But you'll be in even better company if you learn to run sendmail securely.

As mentioned earlier, sendmail is monolithic in that it does all its real work with one executable, sendmail. Sendmail has two modes of operation: it can be invoked as needed, in which case it will process any queued mail and then quit; or it can be run as a persistent background dæmon.

Dæmon mode is required only when sendmail's role is to receive mail from external hosts; if all you use sendmail for is sending mail, you shouldn't run sendmail as a dæmon, and in fact you can probably stop reading now, because sendmail really doesn't need any customization to do this unless you wish to run it chroot-ed.

The way sendmail works, then, depends on how it's being run. If it's running as a dæmon (i.e., with the -bd flag), it listens for incoming SMTP connections on TCP port 25 and periodically tries to send any outbound messages in its queue directory /var/spool/mqueue. If it's being invoked on the fly, it attempts to deliver the outbound message it's been invoked to send and/or checks /var/spool/mqueue for other pending outbound messages.