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Binding to an available (kernel) UDP port

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• "System Selected" (= arbitrary selected) Port Numbers
  • Fact:

    Most network applications can use an arbitrary (available) port #

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  • System selected port number:

    System selected port number = an (unused) port number that is picked (selected) by the Operating System A system selected port number is just an arbitrary (unused) port #

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• Binding to an arbitrary available (kernel) port

  • How to bind to a System Selected (kernel) port

    Specify 0 (zero) for the sin_port variable in the network address

    Example:

    in_addr.sin_family = AF_INET; in_addr.sin_addr.s_addr = htonl(INADDR_ANY); in_addr.sin_port = htons( 0 ); /* Let system assign port # */ ^^^ if ( bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) != 0 ) perror("Bind: ");

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs455001/demo/netw-prog-ipv4/udp3.c

      Use port = 0 in the arguments