If the system uses sendmail, locate the sendmail.cf file.
Procedure:
# find / -name sendmail.cf
Determine if sendmail only binds to loopback addresses by examining the "DaemonPortOptions" configuration options.
Procedure:
# grep -i "O DaemonPortOptions" </path/to/sendmail.cf>
If there are uncommented DaemonPortOptions lines, and all such lines specify system loopback addresses, this is not a finding.
Otherwise, determine if sendmail is configured to allow open relay operation.
Procedure:
# find / -name sendmail.mc
# grep -i promiscuous_relay </path/to/sendmail.mc>
If the promiscuous relay feature is enabled, this is a finding.
If the system uses Postfix, locate the main.cf file.
Procedure:
# find / -name main.cf
Determine if Postfix only binds to loopback addresses by examining the "inet_interfaces" line.
Procedure:
# grep inet_interfaces </path/to/main.cf>
If "inet_interfaces" is set to "loopback-only" or contains only loopback addresses such as 127.0.0.1 and [::1], Postfix is not listening on external network interfaces, and this is not a finding.
Otherwise, determine if Postfix is configured to restrict clients permitted to relay mail by examining the "smtpd_client_restrictions" line.
Procedure:
# grep smtpd_client_restrictions </path/to/main.cf>
If the "smtpd_client_restrictions" line is missing, or does not contain "reject", this is a finding. If the line contains "permit" before "reject", this is a finding.
If the system is using other SMTP software, consult the software’s documentation for procedures to verify mail relaying is restricted.