How to determine the value of socket listen() backlog parameter?

Question

How should I determine what to use for a listening socket's backlog parameter? Is it a problem to simply specify a very large number?

As a warning to anyone using boost asio, the SOMAXCONN value is used as 5 with boost. — Brian R. Bondy, Sep 22 '08 at 14:49
I don't think Boost Asio defines this value per se. It has it's own constant, `socket_base::max_listen_connections` that is assigned the system SOMAXCONN value. — selbie, May 29 '20 at 09:05

score 36 · Answer 1 · answered Sep 22 '08 at 13:56

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There's a very long answer to this in the Winsock Programmer's FAQ. It details the standard setting, and the dynamic backlog feature added in a hotfix to NT 4.0.

answered Sep 22 '08 at 13:56

Mike Dimmick

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Fantastic answer in that FAQ; thanks for sharing. Recommended. – Roman Starkov Mar 27 '12 at 17:50
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If you're on a Linux box, see the `listen` manpage: "If the backlog argument is greater than the value in `/proc/sys/net/core/somaxconn`, then it is silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128." – tonysdg May 15 '18 at 14:52

score 4 · Answer 2 · answered Sep 22 '08 at 14:03

I second using SOMAXCONN, unless you have a specific reason to use a short queue.

Keep in mind that if there is no room in the queue for a new connection, no RST will be sent, allowing the client to automatically continue trying to connect by retransmitting SYN.

Also, the backlog argument can have different meanings in different socket implementations.

In most it means the size of the half-open connection queue, in some it means the size of the completed connection queue.
In many implementations, the backlog argument will multiplied to yield a different queue length.
If a value is specified that is too large, all implementations will silently truncate the value to maximum queue length anyways.

score 1 · Accepted Answer · answered Sep 22 '08 at 13:37

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From the docs:

A value for the backlog of SOMAXCONN is a special constant that instructs the underlying service provider responsible for socket s to set the length of the queue of pending connections to a maximum reasonable value.

answered Sep 22 '08 at 13:37

Ben Hoffstein

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Terrible answer. What are such "reasonable values"? – Nyerguds Apr 02 '13 at 14:18
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I have to agree, this doesn't answer the question at all. – spuriousdata Jun 05 '13 at 11:27
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I disagree with the other comments. This is a perfectly good answer to the common question: "What the heck should I pass as the second parameter to listen()?" If you don't know, use SOMAXCONN. – Kenton Varda Jun 30 '16 at 19:43

How to determine the value of socket listen() backlog parameter?

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