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    SubjectRe: SO_REUSEADDR not allowing server and client to use same port
    [cc'd netdev]

    On Thu, Feb 28, 2008 at 12:15:06PM -0600, Nebojsa Miljanovic wrote:
    > Hello all,
    > I have seen similar complaints about this issue before in the list archive, but
    > I have not seen any resolution. So, I am posting the question again in hope of
    > getting some replies.
    > Currently, Linux does not allow reuse of same local port for both TCP server and
    > TCP client. I understand that there may be a need to prevent two servers from
    > binding to the same local port, but having 1 server and 1 outgoing client use it
    > should be allowed. Other Unix like operating systems do allow it.

    I've been encountering the need for SO_REUSEPORT as implemented on some BSD
    for instance. It allows any process to explicitly permit another one to bind
    to the same IP:port provided that it also sets its socket with SO_REUSEPORT.
    This is a real requirement when you need to restart a service without any
    service disruption.

    In the end, I've written a trivial patch for 2.4 (but most likely 2.6 would
    use nearly the same one). I noticed that the conflict detection logic already
    distinguished between 3 cases: sk->reuse==0, sk->reuse==1, sk->reuse >1.
    sk->reuse was set to 1 with SO_REUSEADDR. I simply had to make SO_REUSEPORT
    do sk->reuse |= 2 to get everything working as expected.

    I don't know yet if it is as easy to do in 2.6, but if there is a possible
    acceptance of including this useful feature into mainline, I can try to
    work a patch (possibly cleaner than my existing one for 2.4).


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