How do I increase sendmail's listen() backlog from 10 to 100?

Solution Unverified - Updated -

Issue

  • Sendmail sometimes fails to send emails getting the following errors:
Jan 27 06:00:06 mailsrv kernel: possible SYN flooding on port 25. Sending cookies.
Jan 31 06:00:05 mailsrv kernel: possible SYN flooding on port 25. Sending cookies.
Feb  1 06:00:05 mailsrv kernel: possible SYN flooding on port 25. Sending cookies.
Feb  3 06:00:04 mailsrv kernel: possible SYN flooding on port 25. Sending cookies.
Feb  4 06:00:05 mailsrv kernel: possible SYN flooding on port 25. Sending cookies.

  • According to the existing KCS doc#30453, we should Increase the application's listen() backlog from a smaller value, to the new maximum value.

  • How do I increase sendmail's listen() backlog from 10 (default) to 100?

Environment

  • sendmail-8.14.4-8.el6.x86_64

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