Делаем зеркало сервера на udp-proxy

/* http://www.brokestream.com/udp_redirect.html
  Build: gcc -o udp_redirect udp_redirect.c
  udp_redirect.c
  Version 2013-05-30
  Copyright (C) 2007 Ivan Tikhonov
  2015 added multi-hosts
*/

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>

int bindsocket( char* ip, int port );
int main( int argc, char* argv[] );

int bindsocket( char* ip, int port ) {
    int fd;
    struct sockaddr_in addr;

    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr( ip );
    addr.sin_port = htons( port );

    fd = socket( PF_INET, SOCK_DGRAM, IPPROTO_IP );
    if( -1 == bind( fd, (struct sockaddr*)&addr, sizeof( addr ) ) ) {
        fprintf( stderr, "Cannot bind address (%s:%d)\n", ip, port );
        exit( 1 );
    }

    return fd;
}

int main( int argc, char* argv[] ) {
    int i, listen, output;
    char *inip, *inpt, *srcip, *dstip, *dstpt;
    struct sockaddr_in src;
    struct sockaddr_in dst;
    struct sockaddr_in ret;

    if( 3 != argc && 5 != argc && 6 != argc ) {
        fprintf( stderr, "Usage: %s <listen-ip> <listen-port> [[source-ip] <destination-ip> <destination-port>]\n", argv[ 0 ] );
        exit( 1 );
    }

    i = 1;
    inip = argv[ i++ ];        /* 1 */
    inpt = argv[ i++ ];        /* 2 */
    if( 6 == argc )
        srcip = argv[ i++ ];    /* 3 */
    if( 3 != argc ) {
        dstip = argv[ i++ ];    /* 3 or 4 */
        dstpt = argv[ i++ ];    /* 4 or 5 */
    }
    
    listen = bindsocket( inip, atoi( inpt ) );
    if( 6 == argc ) {
        output = bindsocket( srcip, atoi( inpt ) );
    } else {
        output = listen;
    }

    if( 3 != argc ) {
        dst.sin_family = AF_INET;
        dst.sin_addr.s_addr = inet_addr( dstip );
        dst.sin_port = htons( atoi( dstpt ) );
    }
    ret.sin_addr.s_addr = 0;

    while( 1 ) {
        char buffer[65535];
        unsigned int size = sizeof( src );
        int length = recvfrom( listen, buffer, sizeof( buffer ), 0, (struct sockaddr*)&src, &size );
        if( length <= 0 )
            continue;

        if( 3 == argc ) {
            sendto( listen, buffer, length, 0, (struct sockaddr*)&src, size );
        } else if( ( src.sin_addr.s_addr == dst.sin_addr.s_addr ) && ( src.sin_port == dst.sin_port ) ) {
            if( ret.sin_addr.s_addr )
                   then return them to the original sender */
                sendto( output, buffer, length, 0, (struct sockaddr*)&ret, sizeof( ret ) );
        } else {
            sendto( output, buffer, length, 0, (struct sockaddr*)&dst, sizeof( dst ) );
            ret = src;
        }
    }
}