Ephemeral port range free#
This means that the kernel is free to reuse an ephemeral port of any opened UDP socket (because listen is not used for datagram sockets) and any opened TCP socket for which listen was not called yet. The important point here is that the kernel doesn’t check whether there is an opened socket for an ephemeral port, it only checks whether there is a socket in the listening state for that port. Hence, when an ephemeral port is allocated, SO_REUSEADDR enables the kernel to reuse any other non-listening ephemeral port. There is an active listening socket bound to the address. ForĪF_INET sockets this means that a socket may bind, except when In a bind(2) call should allow reuse of local addresses. Indicates that the rules used in validating addresses supplied The socket(7) man page states the following: SO_REUSEADDR Reusing addressīe careful when using SO_REUSEADDR and the port is allowed to be ephemeral. This feature is not specified in POSIX but is available in many operating systems that implement BSD sockets, including Linux. This behavior is described in the ip_local_port_range section of the ip(7) man page.
In particular, bind, listen, connect, and sendto may automatically allocate an ephemeral port for AF_INET and AF_INET6 sockets. The ephemeral port range is a range of ports used by the kernel when the user wants the socket to be bound to a random unused port.
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