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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

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 Red Hat Enterprise Linux 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 Red Hat Enterprise Linux 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.

5.7.2. NFS and Sendmail

Never put the mail spool directory, /var/spool/mail/, on an NFS shared volume.

Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail. With NFSv4 using Kerberos, this is not the case, since the SECRPC_GSS kernel module does not utilize UID-based authentication.

5.7.3. Mail-only Users

To help prevent local user exploits on the Sendmail server, it is best for mail users to only access the Sendmail server using an email program. Shell accounts on the mail server should not be allowed and all user shells in the /etc/passwd file should be set to /sbin/nologin (with the possible exception of the root user).

 
 
  Published under the terms of the GNU General Public License Design by Interspire