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Binding to a specific (kernel) UDP port#

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• Binding a UDP socket
  • Associating (= Binding) a socket variable to a given network address:

    int bind(int s, (struct sockaddr *)addr, int length) s = socket variable addr = a socket IP address structure length = length of the socket IP address structure Return value: 0 = success Non-zero value = error code

    Example:

    int IPAddress; (a 32 bit IP address) short PortNumber; (a 16 bit port number) int s; struct sockaddr_in in_addr; s = socket(PF_INET, SOCK_DGRAM, 0); // Create UDP socket /* -------------------------------- Set up Internet network address -------------------------------- */ in_addr.sin_family = AF_INET; /* IP Protocol domain */ in_addr.sin_addr.s_addr = htonl(IPAddress); /* IP address */ in_addr.sin_port = htons(PortNumber); /* UDP port # */ /* -------------------------------- Bind socket to network address -------------------------------- */ if ( bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) != 0 ) perror("Bind error: ");

    What does the program achieve:

    The following diagram shows you what the program statements has achieved: Explanation: The socket variable s is now related to the UDP port #8000 of the computer That means: IP messages that have destination address: IP address = the IP address of this computer port # = 8000 will be delivered to the socket s !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    Notes:

    Compile it with: cc -o udp1 udp1.c Run it with: udp1   IP-address   Port# Experiment: Try run udp1 using an IP-address that is NOT assigned to the host and see what happens Use the "host" command to find the IP-address of the host and run udp1 with a legal IP-address Also, try to bind to port# lower than 1000 (system reserved ports)

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    Example: (on W301 computer):

    /* ------------------------------------------------------ Find W301's IP address in dotted decimal notation ------------------------------------------------------ */ cheung@W301(1007)> ifconfig eth0 Link encap:Ethernet HWaddr 3C:D9:2B:76:D7:09 inet addr:170.140.151.25 Bcast:170.140.151.255 Mask:255.255.254.0 inet6 addr: fe80::3ed9:2bff:fe76:d709/64 Scope:Link /* -------------------------------------- Convert IP address in binary -------------------------------------- */ cheung@W301(1008)> bc 170*256*256*256 + 140*256*256 + 151*256 + 25 2861340441 /* -------------------------------------- Bind call with W301's IP address -------------------------------------- */ cheung@W301(1011)> udp1 2861340441 8000 **** Bind was successful.... Socket is bind to: addr = 2861340441 port = 8000 /* ---------------------------------------------------- ERROR test: Bind call with not-your-own IP address ---------------------------------------------------- */ cheung@W301(1010)> udp1 2861340442 8000 Error: bind to addr = 2861340442,, port = 8000 FAILED Bind ?: Cannot assign requested address

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• Binding to a used port #
  • Warning:

    When a port # is used (= bound) by some other network program, then: The bind( ) call to the used port # will fail (and returns an error code)

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• Comment
  • Fact:

    There is no need to bind to a specific port# if: just want to transmit messages You only need to bind to a specific port#   if: Some other process needs to contact you I.e.: When you are writing a server !!!