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
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) can be used to slow rapid
incoming connections and can be used with the incoming daemon.
The MaxDaemonChildren option is appropriate for
use in certain queue-processing situations. For example, consider a
special queue that exclusively holds mail for a popular host (say,
/var/spool/bigqueue). To handle the outgoing
mail, you could run sendmail in queue-processing
mode like this:
/usr/sbin/sendmail -q5m -OMaxDaemonChildren=2 -OQueueDirectory=/var/spool/bigqueue
Here, the queue is processed once every five minutes. If the number
of children were not limited and if the queue were large or the
destination host slow, too many parallel invocations of
sendmail could be spawned, thus causing
excessive connections to the destination host. By limiting the number
of children with the MaxDaemonChildren option, you
allow a small, polite amount of parallelism. (See also the
MaxQueueRunSize option, MaxQueueRunSize.)
The MaxDaemonChildren option is not safe. If
specified from the command line, it can cause
sendmail to relinquish its special privileges.