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Binding a UDP socket in a multi-homed host

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• Multi-homed hosts
  • Recall that:

    A multi-homed host has multiple IP addresses

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• The association of a socket to a network address
  • Recall the structure of a network address:

    struct sockaddr_in { sa_family_t sin_family; /* Address family - Must contain the value AF_INET */ in_port_t sin_port; /* Internet Port # - a 2 bytes port number */ struct in_addr sin_addr; /* Internet Address (IP-address) - a 4 bytes IP address */ };

    Facts:

    An network address contains one IP address value A socket that is bound to an IP address X will: Only receive IP messages with the destination IP address X

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  • Situation:

    A multi-homed node can receive messages with different IP addresses: IP packets with destination IP address: X or Y or Z         will be delivered to the multi-homed host !!! A socket bound to a specific IP address will only receive IP packets destined for the specific IP address:

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• Wild Card IP address
  • Wild card IP address

    INADDR_ANY = the wild card IP address used in network programming

    A wild card IP address will match to any IP address

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  • Fact:

    A socket that is bound to the INADDR_ANY is able to: Receive IP packets containing any IP address !!!!

    Example:

    in_addr.sin_family = AF_INET; /* Protocol domain */ in_addr.sin_addr.s_addr = htonl(INADDR_ANY); /* Use wild card IP address */ in_addr.sin_port = htons(PortNumber); /* UDP port # of socket */ /* -------------------------------- Bind socket to socket address -------------------------------- */ if ( bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) != 0 ) perror("Bind error: ");

    Result:

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• Common practice of network programming
  • Common practice in network programming:

    Network programmers always use the INADDR_ANY value in an network address Example: in_addr.sin_family = AF_INET; in_addr.sin_addr.s_addr = htonl(INADDR_ANY); /* Always use wild card IP address */ in_addr.sin_port = htons(PortNumber); /* -------------------------------- Bind socket to socket address -------------------------------- */ if ( bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) != 0 ) perror("Bind error: "); It is a general practice to use INADDR_ANY even for single-homed computers... (Because it's easier)

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  • Example Program: (Bind to INADDR_ANY)                                                
    • Prog file: click here

    Notes:

    Compile it with: gcc -o udp2 udp2.c           Run it with: udp2   Port#

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    Example:

    cheung@compute(1011)> udp2 8000 OK: bind to port = 9000 SUCCESS **** Bind was successful.... Socket is bind to: addr = 0 port = 8000