Outgoing TCP port matches listening port

Question

I've encountered a weird happenstance where the results of my

lsof | grep 40006

produced

java      29722     appsrv   54u     IPv6           71135755        0t0      TCP localhost:40006->localhost:40006 (ESTABLISHED)

Generally I see

java      30916     appsrv   57u     IPv6           71143812        0t0      TCP localhost:43017->localhost:40006 (ESTABLISHED)

where the ports do not match on either side of the arrow. While lsof was producing the former result, I could not start an application which attempts to listen on the port 40006 even though the socket is configured as SO_REUSEADDR.

Can this happen? Should it?

uname gives: Linux femputer 2.6.32-24-server #39-Ubuntu SMP Wed Jul 28 06:21:40 UTC 2010 x86_64 GNU/Linux

Answer 1

It is possible to arrange such a connection by creating a socket, binding it to 127.0.0.1:40006, then connect() it to 127.0.0.1:40006. (Note: no listen()). I believe this is called an "active-active open".

The following program demonstrates this:

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>

int main()
{
    int s;
    struct sockaddr_in sa = {
        .sin_family = PF_INET,
        .sin_port = htons(40006),
        .sin_addr.s_addr = htonl(INADDR_LOOPBACK) };

    s = socket(PF_INET, SOCK_STREAM, 0);

    if (s < 0) {
        perror("socket");
        return 1;
    }

    if (bind(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
        perror("bind");
        return 1;
    }

    if (connect(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
        perror("connect");
        return 1;
    }

    pause();

    return 0;
}

The reason that the port cannot be re-used is because the port is not listening - it is an outgoing port.