What does the Recv-Q values in a Listen socket mean?

Xiaofeng Wang

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Aug 25, 2016, 9:49:19 AM8/25/16

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Hi,
My program runs in trouble with a netstat output like bellow. It cannot receive a packet. What does the Recv-Q value in the first line mean? I see the man page, and do some googling, but no result found.

```
[root@(none) /data]# netstat -ntap | grep 8000
tcp 129 0 0.0.0.0:8000 0.0.0.0:* LISTEN 1526/XXXXX-
tcp 0 0 9.11.6.36:8000 9.11.6.37:48306 SYN_RECV -
tcp 0 0 9.11.6.36:8000 9.11.6.34:44936 SYN_RECV -
tcp 365 0 9.11.6.36:8000 9.11.6.37:58446 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:55018 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:42830 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:56344 CLOSE_WAIT -
tcp 0 364 9.11.6.34:38947 9.11.6.36:8000 FIN_WAIT1 -
tcp 364 0 9.11.6.36:8000 9.11.6.37:52406 CLOSE_WAIT -
tcp 365 0 9.11.6.36:8000 9.11.6.37:53603 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:47522 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.34:48191 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:51813 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.34:57789 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.37:34252 CLOSE_WAIT -
tcp 364 0 9.11.6.36:8000 9.11.6.34:38930 CLOSE_WAIT -
tcp 365 0 9.11.6.36:8000 9.11.6.37:44121 CLOSE_WAIT -
tcp 365 0 9.11.6.36:8000 9.11.6.37:60465 CLOSE_WAIT -
tcp 365 0 9.11.6.36:8000 9.11.6.37:37461 CLOSE_WAIT -
tcp 0 362 9.11.6.34:35954 9.11.6.36:8000 FIN_WAIT1 -
tcp 364 0 9.11.6.36:8000 9.11.6.37:55241 CLOSE_WAIT -
```

Jorgen Grahn

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Aug 25, 2016, 3:00:38 PM8/25/16

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On Thu, 2016-08-25, Xiaofeng Wang wrote:
> Hi, My program runs in trouble with a netstat output like bellow. It
> cannot receive a packet. What does the Recv-Q value in the first
> line mean?
> I see the man page, and do some googling, but no result
> found.
>
> ```
> [root@(none) /data]# netstat -ntap | grep 8000
> tcp 129 0 0.0.0.0:8000 0.0.0.0:* LISTEN 1526/XXXXX-

The IP stack has read 129 bytes ok from the peer; they are all
accounted for and ready for the application ... except the application
hasn't read it from the IP stack yet.

It's often a sign that the application cannot keep up with the load --
or that it has gotten stuck, doing something else when it should
instead act upon the data from the peer.

But now I see it's on a LISTEN socket. I'm not sure what a Recv-Q
there means, but I suspect it means some new clients are trying to
connect, but the server hasn't called accept(2) yet.

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .

Rick Jones

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Aug 25, 2016, 6:51:50 PM8/25/16

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Jorgen Grahn <grahn...@snipabacken.se> wrote:
> But now I see it's on a LISTEN socket. I'm not sure what a Recv-Q
> there means, but I suspect it means some new clients are trying to
> connect, but the server hasn't called accept(2) yet.

I believe you are correct, but it may depend on how new the
kernel/netstat happens to be.

rick jones
--
Don't anthropomorphize computers. They hate that. - Anonymous
these opinions are mine, all mine; HPE might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hpe.com but NOT BOTH...

Xiaofeng Wang

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Aug 25, 2016, 9:02:50 PM8/25/16

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> > But now I see it's on a LISTEN socket. I'm not sure what a Recv-Q
> > there means, but I suspect it means some new clients are trying to
> > connect, but the server hasn't called accept(2) yet.
>
> I believe you are correct, but it may depend on how new the
> kernel/netstat happens to be.

Sorry, but that's not the situation. I copy&paste a TCP example, see
https://gist.github.com/was4444/e0ed6b852dc45588e5ba5e31fb724459#file-tcp_server-c-L42, I do not accept a connection, the netstat output like bellow:

$ netstat -ntap |grep 8888
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp 0 0 0.0.0.0:8888 0.0.0.0:* LISTEN 20013/./server
tcp 0 0 127.0.0.1:55566 127.0.0.1:8888 ESTABLISHED 20034/./client
tcp 24 0 127.0.0.1:8888 127.0.0.1:55566 ESTABLISHED -

Xiaofeng Wang

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Aug 25, 2016, 9:14:39 PM8/25/16

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not true with kernel version 2.6
$ uname -r
2.6.32-642.3.1.el6.x86_64

but with kernel version 3.10, it is true.

Xiaofeng Wang

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Aug 25, 2016, 9:19:06 PM8/25/16

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The Recv-Q in a LISTEN state means the count of un-accepted connections in kernel version 3.10

Xiaofeng Wang

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Aug 25, 2016, 10:01:38 PM8/25/16

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> > [root@(none) /data]# netstat -ntap | grep 8000
> > tcp 129 0 0.0.0.0:8000 0.0.0.0:* LISTEN 1526/XXXXX-

the default listen's backlog is 128 according the man page:
http://linux.die.net/man/2/listen
and the value of /proc/sys/net/core/somaxconn is truly 128.