Linux Server Security, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Sendmail

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Sendmail Architecture

As I 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 daemon.

Daemon mode is required only when Sendmail’s role is to receive mail from external

hosts; if you just use Sendmail to send mail, you shouldn’t run sendmail as a dae-

mon. In fact, you can probably stop reading now since sendmail doesn’t really need

any customization to do this, unless you wish to run it chrooted (see the section

“Configuring Sendmail to Run Semichrooted”).

The way sendmail works, then, depends on how it’s being run. If it’s running as a

daemon (i.e., with the

-bd flag), it listens for incoming SMTP connections on TCP

port 25 and periodically tries to send out any outbound messages in its queue direc-

tory, /var/spool/mqueue. If it’s being invoked on the fly, it attempts to deliver what-

ever outbound message it’s been invoked to send and/or checks /var/spool/mqueue

for other pending outbound messages.

A Disclaimer

I’m a Postfix fan myself. I run Postfix as my domain’s public SMTP gateway (though I

do use Sendmail on my private network for local mail delivery). ...