2.2.8.4. Disable Sendmail Network Listening

By default, Sendmail is set up to only listen to the local loopback address. You can verify this by viewing the file /etc/mail/sendmail.mc to ensure that the following line appears:

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

This ensures that Sendmail only accepts mail messages (such as cron job reports) from the local system and not from the network. This is the default setting and protects Sendmail from a network attack.

For removal of the localhost restriction, the Addr=127.0.0.1 string needs to be removed. Changing Sendmail's configuration requires installing the sendmail-cf package, then editing the .mc file, running /etc/mail/make and finally restarting sendmail. The .cf configuration file will be regenerated. Note that the system clock must be correct and working and that there must not be any system clock time shifts between these actions in order for the configuration file to be automatically regenerated.