In all these examples, note that sendmail sends email over other networks by running
programs that are tailored specifically for that use. Remember that the only
network sendmail can use directly is a TCP/IP-based network.*
1.6.11 Role As a Daemon
Just as sendmail can transport mail messages over a TCP/IP-based network, it can
also receive mail that is sent to it over the network. To do this, it must be run in daemon
mode. A daemon is a program that runs in the background independent of
terminal control.
As a daemon, sendmail is started once, usually when your machine is booted. Whenever
an email message is sent to your machine, the sending machine talks to the sendmail
daemon that is listening on your machine.
* Actually, we??™re fudging for simplicity. V8 sendmail can also send messages over an ISO network.
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
18 | Chapter 1: Some Basics
% grep sendmail /etc/rc* ?†? BSD-based systems
% grep sendmail /etc/init.d/* ?†? SysV-based systems
% grep sendmail /etc/*rc ?†? HP-UX systems (prior to HP-UX 10.0)
One typical example of what you will find is:
/etc/rc.local:if [ -f /usr/lib/sendmail -a -f /etc/mail/sendmail.cf ]; then
/etc/rc.local: /usr/lib/sendmail -bd -q1h; echo -n ' sendmail'
The second line in this example shows that sendmail is run at boot time witha command
line of:
/usr/lib/sendmail -bd -q1h
The -bd command-line switchtells sendmail to run in daemon mode.
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