Linux : how to set default route from C?

Question

How can I set (and replace the existing) default network route from a C program? I'd like to do it without shell commands if possible (this is a low memory embedded system). Also can you set the default route without specifying the gateway IP address? In my application I want to make either ppp0 or eth0 the default route, depending on whether the cable is plugged into eth0 or not.

Thanks, Fred

Answer 1

You can make IOCTL calls to set the default route from a C program.

void main()
{
   int sockfd;
   struct rtentry rt;

   sockfd = socket(AF_INET, SOCK_DGRAM, 0);
   if (sockfd == -1)
   {
      perror("socket creation failed\n");
      return;
   }

   struct sockaddr_in *sockinfo = (struct sockaddr_in *)&rt.rt_gateway;
   sockinfo->sin_family = AF_INET;
   sockinfo->sin_addr.s_addr = inet_addr("Your Address");

   sockinfo = (struct sockaddr_in *)&rt.rt_dst;
   sockinfo->sin_family = AF_INET;
   sockinfo->sin_addr.s_addr = INADDR_ANY;

   sockinfo = (struct sockaddr_in *)&rt.rt_genmask;
   sockinfo->sin_family = AF_INET;
   sockinfo->sin_addr.s_addr = INADDR_ANY;

   rt.rt_flags = RTF_UP | RTF_GATEWAY;
   rt.rt_dev = "eth0";

   if(ioctl(sockfd, SIOCADDRT, &rt) < 0 )
       perror("ioctl");

   return;
}

Answer 2

You could strace the route command you are wanting to mimic. This gives you the relevant syscalls useful to change routing.

You may be interested by the proc(5) interface, e.g. its /proc/net/route pseudo-file.

See also ip(7).