Thread: Reading from UDP socket on specific interface

  1. #1
    Registered User
    Join Date
    Nov 2007

    Reading from UDP socket on specific interface


    I'm currently trying to write software to test the routing functionality of a router, but whilst I can send UDP data through the router and back to my testing machine I can't read the data at the destination interface.

    My UDP packets are traveling from (Test Machine Eth0) -> Router -> (Test Machine Eth1), verified by Wireshark - so my concern is the listening UDP socket. Note that this is only one possible route, so it's important that I can be certain data is arriving on the correct interface (of several) on the test machine.

    My development environment:
    Linux kernel : 2.6
    GCC : 3.4.6

    Some of my code:

    #include <stdio.h>
    #include <string.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #define PORT  55443
    #define BSIZE 256
    void main()
        struct sockaddr_in  sin;
        char   *device_ip[]      = { "" };
        char   *buffer[ BSIZE ];
        int    sock;
        int    bytes_read;
        int    sin_len;
        /* Setup address */
        memset( &sin, 0, sizeof( sin ) );
        sin.sin_family = AF_INET;
        sin.sin_port = htons( PORT );
        sin.sin_addr.s_addr = inet_addr( device_ip );
        sin_len = sizeof( sin );
        /* Create socket */
        sock = socket( AF_INET, SOCK_DGRAM, 0 );
        if ( sock > 0 )
            /* Display any data found */
            while( 1 )
                bytes_read = recvfrom( sock, buffer, sizeof( buffer ), 0, ( struct sockaddr * ) &sin, &sin_len );
                if ( bytes_read > 0 )
                    printf( "&#37;s\n", buffer );
    As you can see the code is very simple, so that why I'm so confused by this. I've also tried using bind to lock the UDP socket to the specific interface and using the sock option SO_BINDTODEVICE, both with no luck.

    There must be a way of forcing a UDP socket to read from a specific interface, without this I'm unable to automate the testing.

    Does anyone have any ideas on how to solve this?

    Thanks in advance,

    - U
    Last edited by uriel; 11-29-2007 at 03:07 PM. Reason: Woops, typo...

  2. #2
    Registered User Codeplug's Avatar
    Join Date
    Mar 2003
        char   *device_ip[]      = { "" };
        char   *buffer[ BSIZE ];
    You've got an extra level of indirection. Remove stuff in red.


  3. #3
    Registered User ssharish2005's Avatar
    Join Date
    Sep 2005
    Cambridge, UK
    perhaps you should hae got an error at this stage, since inet_addr can just take char * but not char*[].


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