src/forcebindiface.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <sys/socket.h>
   4 #include <netinet/in.h>
   5 #include <dlfcn.h>
   6 #include <net/if.h>
   7 #include <string.h>
   8 #include <errno.h>
   9
  10 //Credits go to https://catonmat.net/simple-ld-preload-tutorial and https://catonmat.net/simple-ld-preload-tutorial-part-two
  11 //And of course to https://unix.stackexchange.com/a/648721/334883
  12
  13 //compile with gcc -nostartfiles -fpic -shared bindInterface.c -o bindInterface.so -ldl -D_GNU_SOURCE
  14 //Use with BIND_INTERFACE=<network interface> LD_PRELOAD=./bindInterface.so <your program> like curl ifconfig.me
  15
  16 int socket(int family, int type, int protocol)
  17 {
  18     //printf("MySocket\n"); //"LD_PRELOAD=./bind.so wget -O- ifconfig.me 2> /dev/null" prints two times "MySocket". First is for DNS-Lookup.
  19                             //If your first nameserver is not reachable via bound interface,
  20                             //then it will try the next nameserver until it succeeds or stops with name resolution error.
  21                             //This is why it could take significantly longer than curl --interface wlan0 ifconfig.me
  22     char *bind_addr_env;
  23     struct ifreq interface;
  24     int *(*original_socket)(int, int, int);
  25     original_socket = dlsym(RTLD_NEXT,"socket");
  26     int fd = (int)(*original_socket)(family,type,protocol);
  27     bind_addr_env = getenv("BIND_INTERFACE");
  28     int errorCode;
  29     if ( bind_addr_env!= NULL && strlen(bind_addr_env) > 0)
  30     {
  31         //printf(bind_addr_env);
  32         strcpy(interface.ifr_name,bind_addr_env);
  33         errorCode = setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, &interface, sizeof(interface));
  34         if ( errorCode < 0)
  35         {
  36             perror("setsockopt");
  37             errno = EINVAL;
  38             return -1;
  39         };
  40     }
  41     else
  42     {
  43         //printf("Warning: Programm with LD_PRELOAD started, but BIND_INTERFACE environment variable not set\n");
  44         fprintf(stderr,"Warning: Programm started with LD_PRELOAD, but BIND_INTERFACE environment variable not set\n");
  45     }
  46
  47     return fd;
  48 }