route set : SIOCADDRT failed: Inappropriate ioctl for device

Question

This typical "route add default gw IP" program gives me error :

SIOCADDRT failed: Inappropriate ioctl for device

I did ifconfig to verify the interface name. Is there a way to list all the approprieate ioctl for my device ?

int main(char** args) {
        int sockfd;
        struct rtentry route;
        struct sockaddr_in *addr;
        int err = 0;

        // create the socket
        if(sockfd = socket(AF_INET, SOCK_DGRAM, 0)<0){
                perror("socket");
                exit(1);
        }

        memset(&route, 0, sizeof(route));
        addr = (struct sockaddr_in*) &route.rt_gateway;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = inet_addr("192.168.10.1");
        addr = (struct sockaddr_in*) &route.rt_dst;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = INADDR_ANY;
        addr = (struct sockaddr_in*) &route.rt_genmask;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = INADDR_ANY;

        route.rt_dev = "eth0";

        route.rt_flags = RTF_UP | RTF_GATEWAY;
        route.rt_metric = 0;
        if ((err = ioctl(sockfd, SIOCADDRT, &route)) != 0) {
                perror("SIOCADDRT failed");
                exit(1);
        }
}

New error after changing the code to:

 addr = (struct sockaddr_in*) &route.rt_gateway;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = inet_addr("192.168.10.1");
        addr = (struct sockaddr_in*) &route.rt_dst;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = inet_addr("192.168.10.3");
        addr = (struct sockaddr_in*) &route.rt_genmask;
        addr->sin_family = AF_INET;
        addr->sin_addr.s_addr = inet_addr("255.255.255.0");

gdb output :

37      if ((err = ioctl(sockfd, SIOCADDRT, &route)) != 0) {
(gdb) p route
$1 = {rt_pad1 = 0, rt_dst = {sa_family = 2, sa_data = "\000\000\300\250\n\003\000\000\000\000\000\000\000"}, 
  rt_gateway = {sa_family = 2, sa_data = "\000\000\300\250\n\001\000\000\000\000\000\000\000"}, 
  rt_genmask = {sa_family = 2, sa_data = "\000\000\377\377\377\000\000\000\000\000\000\000\000"}, 
  rt_flags = 3, rt_pad2 = 0, rt_pad3 = 0, rt_tos = 0 '\000', rt_class = 0 '\000', rt_pad4 = {0, 0, 0}, 
  rt_metric = 0, rt_dev = 0x4008af "em1", rt_mtu = 0, rt_window = 0, rt_irtt = 0}
(gdb) n
38          perror("SIOCADDRT failed");
(gdb) 
SIOCADDRT failed: Invalid argument

Answer 1

I guess the mistake is here:

    // create the socket
    if(sockfd = socket(AF_INET, SOCK_DGRAM, 0)<0)

The expression socket(AF_INET, SOCK_DGRAM, 0)<0 is evaluated before assignment. That happens because assignment operators have lower precedence than function calls and comparisons. You can read more about it in this wikipedia article:

• Operators in C and C++ - Operator_Operator precedence

So what you really have is:

    // create the socket
    if(sockfd = (socket(AF_INET, SOCK_DGRAM, 0)<0))

That evaluates to:

    // create the socket
    if(sockfd = 0)

So you are trying to perform an ioctl() on the invalid file descriptor.

Wrap the assignment into parenthesis or move the expression out of the condition, like this:

    // create the socket
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if(sockfd < 0)

By the way, GCC or Clang should detect it if you pass -Wall option.