Securing Sendmail - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

It is also possible to disable user accounts within each service directly.
To disable specific user accounts in vsftpd, add the username to /etc/vsftpd.ftpusers
43.2.6.4. Use TCP Wrappers To Control Access
Use TCP Wrappers to control access to either FTP daemon as outlined in
"Enhancing Security With TCP

43.2.7. 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.
Chapter 24, Email
Refer to
configuration settings. This section assumes a basic knowledge of how to generate a valid /etc/
mail/sendmail.cf by editing the /etc/mail/sendmail.mc and using the m4 command.
It is recommended that anyone planning to implement a Sendmail server address the following issues.
43.2.7.1. 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 is 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 a single message.
43.2.7.2. 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.
Wrappers".
for more information about how email works and an overview of common
Securing Sendmail
Section 43.2.1.1,
633