Packet router for C Linux. Setting up sendto() - c

Question

I am trying to make a packet router that receives a packet, reads its destination, and sends it on its way. It receives packets just fine, the problem is that when I attempt to do a sendto() I always get the error
sendto() failed: No such device or address
I am grabbing the destination MAC address from the ethernet header of the received packet, loading it into a struct sockaddr_ll and passing that to sendto() but it isn't working.
Also when I ping one host to another the output of my print statements has the source ip as 40.41.42.43 and the destination ip as 44.45.46.47, and neither of those devices exist. Am I reading the ethernet and IP headers correctly? Or maybe the socket is getting messed up somehow?
Anyway here is the code.
int main(int argc, char *argv[])
{
int sock1, sock2, sock3;
int status = 0;
fd_set readfds;
int buffer[256];
int socknum = 0;
// Create 3 sockets
if ((sock1 = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))) < 0){
perror("socket() failed");
exit(1);
}
if ((sock2 = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))) < 0){
perror("socket() failed");
exit(1);
}
if ((sock3 = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))) < 0){
perror("socket() failed");
exit(1);
}
// Bind sockets to interfaces
struct ifreq ifr;
memset(&ifr, 0, sizeof(ifr));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "r0-eth1");
if (setsockopt(sock1, SOL_SOCKET, SO_BINDTODEVICE, (void *)&ifr, sizeof(ifr)) < 0) {
perror("setsockopt() inf config failed");
exit(1);
}
memset(&ifr, 0, sizeof(ifr));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "r0-eth2");
if (setsockopt(sock2, SOL_SOCKET, SO_BINDTODEVICE, (void *)&ifr, sizeof(ifr)) < 0) {
perror("setsockopt() inf config failed");
exit(1);
}
memset(&ifr, 0, sizeof(ifr));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "r0-eth3");
if (setsockopt(sock3, SOL_SOCKET, SO_BINDTODEVICE, (void *)&ifr, sizeof(ifr)) < 0) {
perror("setsockopt() inf config failed");
exit(1);
}
printf("All sockets bound to interfaces.\n");
while(1)
{
//Setup select
FD_ZERO(&readfds);
FD_SET(sock1, &readfds);
FD_SET(sock2, &readfds);
FD_SET(sock3, &readfds);
printf("Sockets set to read.\n");
struct timeval tv = {2, 0};
printf("Starting select.\n");
status = select( sock3 + 1, &readfds , NULL , NULL , &tv );
printf("status = %d\n", status);
struct sockaddr_in sockAddr;
socklen_t sockLen = sizeof(sockAddr);
memset(&sockAddr, 0, sockLen);
sockAddr.sin_family = AF_INET;
sockAddr.sin_addr.s_addr = htonl(INADDR_ANY);
int bufLen = 0;
// Check select
if (status > 0) {
printf("status = %d, preparing to read.\n", status);
// Read IP datagram (d)
if (FD_ISSET(sock1, &readfds)) {
printf("Socket 1 ready to receive.\n");
printf("Attempting to receive packet.\n");
bufLen = recvfrom(sock1, buffer, 65535, 0, (struct sockaddr *) &sockAddr, (socklen_t *)&sockLen);
printf("Received packet.\n");
socknum = 1;
}
else if (FD_ISSET(sock2, &readfds)) {
printf("Socket 2 ready to receive.\n");
printf("Attempting to receive packet.\n");
bufLen = recvfrom(sock2, buffer, 65535, 0, (struct sockaddr *) &sockAddr, (socklen_t *)&sockLen);
printf("Received packet.\n");
socknum = 2;
}
else if (FD_ISSET(sock3, &readfds)) {
printf("Socket 3 ready to receive.\n");
printf("Attempting to receive packet.\n");
bufLen = recvfrom(sock3, buffer, 65535, 0, (struct sockaddr *) &sockAddr, (socklen_t *)&sockLen);
printf("Received packet.\n");
socknum = 3;
}
printf("Testing Packet.\n");
struct ethhdr *eth = (struct ethhdr *)(buffer);
printf("\nEthernet Header\n");
printf("\t|-Source Address : %.2X-%.2X-%.2X-%.2X-%.2X-%.2X\n",eth->h_source[0],eth->h_source[1],eth->h_source[2],eth->h_source[3],eth->h_source[4],eth->h_source[5]);
printf("\t|-Destination Address : %.2X-%.2X-%.2X-%.2X-%.2X-%.2X\n",eth->h_dest[0],eth->h_dest[1],eth->h_dest[2],eth->h_dest[3],eth->h_dest[4],eth->h_dest[5]);
printf("\t|-Protocol : %d\n",eth->h_proto);
// Inspect IP (e)
unsigned short iphdrlen;
struct sockaddr_in source;
struct sockaddr_in dest;
struct iphdr *ip = (struct iphdr*)(buffer + sizeof(struct ethhdr));
memset(&source, 0, sizeof(source));
source.sin_addr.s_addr = ip->saddr;
memset(&dest, 0, sizeof(dest));
dest.sin_addr.s_addr = ip->daddr;
printf("\t|-Version : %d\n",(unsigned int)ip->version);
printf("\t|-Internet Header Length : %d DWORDS or %d Bytes\n",(unsigned int)ip->ihl,((unsigned int)(ip->ihl))*4);
printf("\t|-Type Of Service : %d\n",(unsigned int)ip->tos);
printf("\t|-Total Length : %d Bytes\n",ntohs(ip->tot_len));
printf("\t|-Identification : %d\n",ntohs(ip->id));
printf("\t|-Time To Live : %d\n",(unsigned int)ip->ttl);
printf("\t|-Protocol : %d\n",(unsigned int)ip->protocol);
printf("\t|-Header Checksum : %d\n",ntohs(ip->check));
printf("\t|-Source IP : %s\n", inet_ntoa(source.sin_addr));
printf("\t|-Destination IP : %s\n",inet_ntoa(dest.sin_addr));
printf("index=%d\n",ifreq_i.ifr_ifindex);
// Pull the address from the ethernet header.
struct sockaddr_ll addr;
memset(&addr, 0, sizeof(struct sockaddr_ll));
addr.sll_family = AF_PACKET;
addr.sll_ifindex = ifr.ifr_ifindex;
addr.sll_halen = ETHER_ADDR_LEN;
addr.sll_protocol = htons(0x0800);
addr.sll_addr[0] = eth->h_dest[0];
addr.sll_addr[1] = eth->h_dest[1];
addr.sll_addr[2] = eth->h_dest[2];
addr.sll_addr[3] = eth->h_dest[3];
addr.sll_addr[4] = eth->h_dest[4];
addr.sll_addr[5] = eth->h_dest[5];
if(sendto(sock1, buffer, bufLen, 0, (struct sockaddr *) &addr, sizeof(addr)) < bufLen){
perror("sendto() failed");
exit(1);
}
}
Any help is appreciated.

Answer 1

I immediately found the answer, but I'm gonna leave this up just in case somebody else has a similar problem. At the bottom part when I am setting up the struct sockaddr_ll addr;. This line:
addr.sll_ifindex = ifr.ifr_ifindex;
should be this instead:
addr.sll_ifindex = ifreq_i.ifr_ifindex;
The socket and the index of the socket were different so sendto() couldn't find a device that fit those parameters.