Binding sendmail to the loopback interface


The sendmail SMTP server comes with the vast majority of UNIX Operating systems, and is configured to listen for new connections on TCP ports *.25 (SMTP) and *.587 (MSP) by default. For workstation and servers that aren’t responsible for mail delivery, this can cause chaos when a new sendmail exploit is released into the wild. This behavior can be changed by adjusting the “DaemonPortOptions” in the sendmail configuration file (usually /etc/mail/sendmail.cf). The default configuration looks similar to the following:

O DaemonPortOptions=Name=MTA-v4, Family=inet
O DaemonPortOptions=Port=587, Name=MSA, M=E

If we add “Addr=127.0.0.1” to each entry, sendmail will only listen for new connections on the loopback interface:

O DaemonPortOptions=Addr=127.0.0.1,Port=25,Name=MTA
O DaemonPortOptions=Addr=127.0.0.1,Port=587,Name=MSA, M=E

Once the changes are integrated into the sendmail.cf file ( hand editing the sendmail.cf file or using M4 macros ), sendmail needs to be restarted. Once sendmail is restarted, we can view the new behavior with the netstat command:

$ netstat -an | egrep LISTEN | egrep '(25|587)'

127.0.0.1.25 0 0 49152 0 LISTEN
127.0.0.1.587 0 0 49152 0 LISTEN

Back in the sendmail 8.108.11 days, a smart relay could be used to forward mail, alleviating the need to run sendmail as a daemon. I am still trying to find a way to revert back to the old behavior, but the MSP seems to cause some issues when smart relays are in use. More to come …

This article was posted by Matty on 2005-01-06 23:52:00 -0400 -0400