5.7. Securing Sendmail

Sendmail is a Mail Transport Agent (MTA) that uses the Simple Mail Transport Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although many MTAs are capable of encrypting traffic between one another, most do not, so sending email over any public networks is considered an inherently insecure form of communication.

For more information about how email works and an overview of common configuration settings, refer to the chapter titled Email in the Reference Guide. This section assumes a basic knowledge of how to generate a valid /etc/mail/sendmail.cf by editing the /etc/mail/sendmail.mc and running the m4 command as explained in the Reference Guide.

It is recommended that anyone planning to implement a Sendmail server address the following issues.

5.7.1. Limiting a Denial of Service Attack

Because of the nature of email, a determined attacker can flood the server with mail fairly easily and cause a denial of service. By setting limits to the following directives in /etc/mail/sendmail.mc, the effectiveness of such attacks are limited.

confCONNECTION_RATE_THROTTLE — The number of connections the server can receive per second. By default, Sendmail does not limit the number of connections. If a limit is set and reached, further connections are delayed.
confMAX_DAEMON_CHILDREN — The maximum number of child processes that can be spawned by the server. By default, Sendmail does not assign a limit to the number of child processes. If a limit is set and reached, further connections are delayed.
confMIN_FREE_BLOCKS — The minimum number of free blocks which must be available for the server to accept mail. The default is 100 blocks.
confMAX_HEADERS_LENGTH — The maximum acceptable size (in bytes) for a message header.
confMAX_MESSAGE_SIZE — The maximum acceptable size (in bytes) for any one message.