sendmail, 4th Edition

ConnectionRateThrottle

Incoming SMTP connection rate V8.8 and later

Whenever an outside site connects to sendmail’s SMTP port, sendmail fork(2)s a copy of itself. That copy (the child) processes the incoming connection and its message. The primary load-limiting mechanisms are the QueueLA (QueueLA on page 1072), RefuseLA (RefuseLA on page 1078), and DelayLA (DelayLA on page 1002) options. However, these options rely on the system load average, which can generally be sluggish and can lag behind events. This ConnectionRateThrottle option, and similar options, exist to help flatten out the actual load until the load average can catch up. The ConnectionRateThrottle option is used like this:

O ConnectionRateThrottle=num                      ← configuration file (V8.8 and
later)
-OConnectionRateThrottle=num                      ← command line (V8.8 and later)
define(`confCONNECTION_RATE_THROTTLE', `num')     ← mc configuration (V8.8 and
later)

The num is of type numeric. If it is present and greater than zero, connections are slowed when more than that number of connections arrive within one second. If num is less than or equal to zero, or absent, no threshold is enforced. If the entire option is missing, the default becomes zero. The default for the mc technique is to omit this option.

To illustrate how the slowdown operates, consider a situation in which num is set to 3, and 12 connections come in simultaneously. The first three connections are handled immediately. The next three are handled after one second. The three after that are ...