sendmail, 4th Edition

MaxDaemonChildren

Maximum forked daemon children V8.8 and later

The sendmail program fork(3)s often. It forks to process each incoming connection, and it forks to process its queue.

You can limit the number of forked children that the listening sendmail daemon produces by defining the MaxDaemonChildren option, the forms of which are as follows:

O MaxDaemonChildren=num                     ← configuration file (V8.8 and later)
-OMaxDaemonChildren=num                     ← command line (V8.8 and later)
define(`confMAX_DAEMON_CHILDREN',`num')     ← mc configuration (V8.8 and later)

The num is of type numeric and specifies the maximum number of forked children that are allowed to exist at any one time. If num is less than or equal to zero, if it is missing, or if this entire option is missing, no limit is imposed. If num is greater than zero, connections that cause more than that number of forked children to be created will be rejected. While rejecting more connections, sendmail will change its process title to read:

rejecting connections: maximum children: num

If num is greater than zero, sendmail will also limit the number of forked daemon children it creates to handle queue runs.

If the daemon handling incoming mail has this option set, a denial-of-service attack can easily be launched against your machine. Beginning with V8.8, the ConnectionRateThrottle option (ConnectionRateThrottle on page 988) can be used to slow rapid incoming connections and can be used with the incoming daemon.

The MaxDaemonChildren option is appropriate for ...