Linux : netstat listening queue length

freebsdlinuxnetstatnetworkingsocket

Is there a way to view queue length on listening socket under Linux, the same way as netstat -L outputs for FreeBSD? I.e. you can see X/Y/Z in netstat -L output, but netstat under Linux doesn't support -L flag.

Best Answer

Let's look into source code, as it's the best documentation in the world of open source.

net/ipv4/tcp_diag.c: 

if (sk->sk_state == TCP_LISTEN) {
    r->idiag_rqueue = sk->sk_ack_backlog;
    r->idiag_wqueue = sk->sk_max_ack_backlog;
} else {
    r->idiag_rqueue = max_t(int, tp->rcv_nxt - tp->copied_seq, 0);
    r->idiag_wqueue = tp->write_seq - tp->snd_una;
}

The same thing we can see in unix domain sockets, net/unix/diag.c:

if (sk->sk_state == TCP_LISTEN) {
    rql.udiag_rqueue = sk->sk_receive_queue.qlen;
    rql.udiag_wqueue = sk->sk_max_ack_backlog;
} else {
    rql.udiag_rqueue = (u32) unix_inq_len(sk);
    rql.udiag_wqueue = (u32) unix_outq_len(sk);
}

So.

If socket is established, Recv-Q and Send-Q means bytes as it's described in documentation.
If socket is listening, Recv-Q means current queue size, and Send-Q means configured backlog.

Going deeper into mans gives us folowing in sock_diag(7):

      UDIAG_SHOW_RQLEN
             The attribute reported in answer to this request is
             UNIX_DIAG_RQLEN.  The payload associated with this
             attribute is represented in the following structure:

                 struct unix_diag_rqlen {
                     __u32 udiag_rqueue;
                     __u32 udiag_wqueue;
                 };

             The fields of this structure are as follows:

             udiag_rqueue
                    For listening sockets: the number of pending
                    connections.  The length of the array associated
                    with the UNIX_DIAG_ICONS response attribute is
                    equal to this value.

                    For established sockets: the amount of data in
                    incoming queue.

             udiag_wqueue
                    For listening sockets: the backlog length which
                    equals to the value passed as the second argu‐
                    ment to listen(2).

                    For established sockets: the amount of memory
                    available for sending.

In other words, ss -ln is the only command you need

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