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2.2.7. Securing Postfix

Postfix is a Mail Transfer Agent (MTA) that uses the Simple Mail Transfer 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.
It is recommended that anyone planning to implement a Postfix server address the following issues.

2.2.7.1. 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. The effectiveness of such attacks can be limited by setting limits of the directives in the /etc/postfix/main.cf file. You can change the value of the directives which are already there or you can add the directives you need with the value you want in the following format: 
<directive> = <value>
The following is a list of directives that can be used for limiting a denial of service attack:
  • smtpd_client_connection_rate_limit — The maximum number of connection attempts any client is allowed to make to this service per time unit (described below). The default value is 0, which means a client can make as many connections per time unit as Postfix can accept. By default, clients in trusted networks are excluded.
  • anvil_rate_time_unit — This time unit is used for rate limit calculations. The default value is 60 seconds.
  • smtpd_client_event_limit_exceptions — Clients that are excluded from the connection and rate limit commands. By default, clients in trusted networks are excluded.
  • smtpd_client_message_rate_limit — The maximum number of message deliveries a client is allowed to request per time unit (regardless of whether or not Postfix actually accepts those messages).
  • default_process_limit — The default maximum number of Postfix child processes that provide a given service. This limit can be overruled for specific services in the master.cf file. By default the value is 100.
  • queue_minfree — The minimum amount of free space in bytes in the queue file system that is needed to receive mail. This is currently used by the Postfix SMTP server to decide if it will accept any mail at all. By default, the Postfix SMTP server rejects MAIL FROM commands when the amount of free space is less than 1.5 times the message_size_limit. To specify a higher minimum free space limit, specify a queue_minfree value that is at least 1.5 times the message_size_limit. By default the queue_minfree value is 0.
  • header_size_limit — The maximum amount of memory in bytes for storing a message header. If a header is larger, the excess is discarded. By default the value is 102400.
  • message_size_limit — The maximum size in bytes of a message, including envelope information. By default the value is 10240000.