How to get BACKLOG of listening socket

Question

I have a listening socket on port 80 on ubuntu linux.

tcp    0  0 0.0.0.0:80      0.0.0.0:*   LISTEN  12248/nginx

Is there any way to get backlog value of that socket (backlog value that was sent to listen() call)?

I know that I could view the nginx configuration but configuration file could be changed without reloading nginx with new configuration, so the backlog argument in configuration and in actual LISTEN call could be different.

Solution

ss -lt gives this value in the Send-Q column.

Comments

[Chucky]

There is nothing useful to do with that information, as there was No point on having more than 640K of RAM.

[user207421]

@Chucky The fact is that in the 35-ish years since the BSD Sockets API was designed nobody has seen fit to add such a function, which indicates that no use fort has been found. If you have a counterexample please provide it. Merely juxtaposing two unrelated issues doesn't actually consisting a logical argument, but this particular juxtaposition is poorly chosen for your purpose. The 640k mistake was recognized and addressed within a few years. The lack of a listen backlog API has never been addressed at all, which shows that generations of TCP/IP implementors agree with me.

[aredridel]

Error messaging to the operator is one reason. Showing that the backlog is being hit is another, for exposing load status.

[Schneems]

This shows what the max value is, is there a way to see how many requests are currently queued in the backlog on the socket?

[user207421]

If any of these downvoters would care to tell us what they think the name of this non-existent API is, it would be most entertaining.

[user207421]

@aredridel The question is about 'the backlg value that was set in the lsten() call', not about its current occupancy, and it's hard to see what they operator can do with, or about, that information.

[Ciro Costa]

Schneems, you should be able to see that in the other column (Recv-Q). For instance, create a server that listens on a socket but never accepts. If you telnet four times and leave that running in the background, you should see Recv-Q with a value of 4.

[Ciro Costa]

Note that the value that you see in Send-Q (the effective backlog size for such socket) might be different from the value set by the user in the listen(2) syscall. That's because Linux will make use of /proc/sys/net/core/somaxconn as a ceiling and change the backlog value based on that (see net/socket.c#sys_listen: elixir.bootlin.com/linux/v4.15/source/net/socket.c#L1479)

[user207421]

@CiroCosta That's the current size, not the maximum size. It doesn't provide the 'backlog value of that socket (backlog value that was sent to listen() call)', which is what was asked for.

[user786]

Is there a way I can change the running server backlog value in c code. Some system call or kernel hook or ioctl or something. Or the only solution is to close socket and start listening again which I think probably used in server I may be wrong