Sendmail: Listen on Loopback Only
It is almost always necessary to run the sendmail daemon, even on systems that don't act as public SMTP servers. Without sendmail, output from cron and at jobs won't be delivered. However, unnecessarily exposing your SMTP ports to the Internet or even to a large LAN is poor security practice. This article will explain how to configure sendmail to listen only on the local loopback interface (127.0.0.1).
You must have the sendmail-cf package installed, which in turn requires the m4 package. These packages are available as part of the official Slackware distribution.
Edit the Configuration File
Change into the directory /usr/share/sendmail/cf/cf. This directory is installed by the sendmail-cf package and contains a wide variety of sample sendmail configuration files. The default configuration on Slackware is the sendmail-slackware.mc file. Edit this file, and make the following additions:
- Along with the other FEATURE options, add this line:
- Right before the MAILER lines, add this line:
Install the Configuration File
m4 sendmail-slackware.mc >/etc/mail/sendmail.cf
Warning: If you had previously hard-coded any configuration options in sendmail.cf, they will be overwritten. It is recommended (and generally easier) to make changes only to sendmail-slackware.mc and then rebuild sendmail.cf with the command above.