Update the details on AF_UNSPEC and circumstances in which socket can be reconnected. From a mail conversation with Eric Dumazet: > connect() man page seems obsolete or confusing : > > Generally, connection-based protocol sockets may successfully > connect() only once; connectionless protocol sockets may use > connect() multiple times to change their association. > Connectionless sockets may dissolve the association by connecting to > an address with the sa_family member of sockaddr set to AF_UNSPEC > (supported on Linux since kernel 2.2).> > > 1) At least TCP has supported AF_UNSPEC thing forever. > 2) By definition connectionless sockets do not have an association, > why would they call connect(AF_UNSPEC) to remove a connection > which does not exist ... Calling connect() on a connectionless socket serves two purposes: a) Assigns a default outgoing address for datagrams (sent using write(2)). b) Causes datagrams sent from sources other than the peer address to be discarded. Both of these things are true in AF_UNIX and the Internet domains. Using connect(AF_UNSPEC) allows the local datagram socket to clear this association (without having to connect() to a *different* peer), so that now it can send datagrams to any peer and receive datagrams for any peer, (I've just retested all of this.) > > Maybe we should rewrite this paragraph to match reality, since > this causes confusion. > > > Some protocol sockets may successfully connect() only once. > Some protocol sockets may use connect() multiple times to change > their association. > Some protocol sockets may dissolve the association by connecting to > an address with the sa_family member of sockaddr set to AF_UNSPEC > (supported on Linux since kernel 2.2). When I first saw your note, I was afraid that I had written the offending text. But, I see it has been there since the manual page was first added in 1992 (other than the piece "(supported since on Linux since kernel 2.2)", which I added in 2007). Perhaps it was true in 1992. Anyway, I confirm your statement about TCP sockets. The connect(AF_UNSPEC) thing works; thereafter, the socket may be connected to another socket. Interestingly, connect(AF_UNSPEC) does not seem to work for UNIX domain stream sockets. (My light testing gives an EINVAL error on connect(AF_UNSPEC) of an already connected UNIX stream socket. I could not easily spot where this error was being generated in the kernel though.) Reported-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>