Bind a socket to a specific interface with UDP? - C

I'd like to know how I can bind a socket to a specific interface in C.

My @IP is X.Y.Z.3, the gateway is X.Y.Z.1 on eth1 But if I send my packet, it'll be send on the loopback interface.

The strange thing is, if I make my packet with X.Y.Z.9 (for example) as the IP SOURCE (instead of mine), it works.

Any clue?

#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <pthread.h> #include <sys/ioctl.h> #include <net/if.h> #include <string.h> #include <netdb.h> #include <sys/socket.h> #include <arpa/inet.h> #include <sys/time.h> #define PCK_MAX_LEN 1024 #define IP_NAMESERV "172.20.10.1" #define IP_ATTACKER "172.20.10.3" #define PORT_QUERY 5555 pthread_cond_t ans_listen = PTHREAD_COND_INITIALIZER; pthread_cond_t ans_receiv = PTHREAD_COND_INITIALIZER; pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; void *fctThreadSendQuery (void *arg); // Send 1 query to random.example.com void *fctThreadListenResponse (void *arg); // Listen for response to that query int main (void) { pthread_t threadSendQuery; pthread_t threadListenResponse; pthread_create (&threadSendQuery, NULL, fctThreadSendQuery, NULL); pthread_create (&threadListenResponse, NULL, fctThreadListenResponse, NULL); pthread_join (threadListenResponse, NULL); pthread_join (threadSendQuery, NULL); return 0; } void *fctThreadSendQuery(void *arg) { unsigned int nQuery = 1; struct sockaddr_in *sin_attacker, *sin_resolver; sin_attacker = calloc(1, sizeof(struct sockaddr_in)); sin_resolver = calloc(1, sizeof(struct sockaddr_in)); sin_attacker->sin_family = AF_INET; sin_attacker->sin_addr.s_addr = inet_addr(IP_ATTACKER); sin_attacker->sin_port = htons(PORT_QUERY); int fd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW); bind(fd, sin_attacker, sizeof(struct sockaddr_in)); sin_resolver->sin_family = AF_INET; sin_resolver->sin_addr.s_addr = inet_addr(IP_RESOLVER); sin_resolver->sin_port = htons(53); while (1) { // Now, we can build and send the query char *packet = calloc(PCK_MAX_LEN, sizeof(char)); int pck_len = 0; int id = 0; char *target = calloc(16, sizeof(char)); strcpy(target, randomTarget(nQuery-1)); build_packet(IP_SRC, IP_DST, PORT_QUERY, 53, packet, &pck_len, target, NAME_LEN, id, QUERY); // Before sending the packet, we want to be sure that fctThreadListenResponse is listening pthread_mutex_lock (&mutex); puts("SEND: wait for RECV to LISTEN"); pthread_cond_wait (&ans_listen, &mutex); puts("SEND: wait for RECV to LISTEN - OK"); pthread_mutex_unlock(&mutex); sendto (fd, packet, pck_len, 0, sin_attacker, sizeof(struct sockaddr_in)); puts("SEND: PCK SENT"); pthread_mutex_lock (&mutex); puts("SEND: wait for RECV to RECV"); pthread_cond_wait (&ans_receiv, &mutex); puts("SEND: wait for RECV to RECV - OK"); pthread_mutex_unlock(&mutex); nQuery++; free(target); free(packet); } free(sin_resolver); free(sin_attacker); pthread_exit(NULL); } void *fctThreadListenResponse (void *arg) { usleep(100); struct sockaddr_in *sin_attacker, *sin_resolver; sin_attacker = calloc(1, sizeof(struct sockaddr_in)); sin_resolver = calloc(1, sizeof(struct sockaddr_in)); sin_attacker->sin_family = AF_INET; sin_attacker->sin_addr.s_addr = inet_addr(IP_ATTACKER); sin_attacker->sin_port = htons(PORT_QUERY); int fd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW); bind(fd, sin_attacker, sizeof(struct sockaddr_in)); while (1) { char *packet = calloc(PCK_MAX_LEN, sizeof(char)); unsigned int pck_len; pthread_mutex_lock (&mutex); pthread_cond_signal (&ans_listen); puts("RECV: LISTENING"); pthread_mutex_unlock(&mutex); pck_len = recvfrom(fd, packet, PCK_MAX_LEN, 0, NULL, sin_resolver); puts("RECV: PCK RECEIVED"); if (pck_len > 0) { pthread_mutex_lock (&mutex); pthread_cond_signal (&ans_receiv); pthread_mutex_unlock (&mutex); } free(packet); } pthread_exit(NULL); }