Specify interface when adding a default gateway via SIOCADDRT
I’M TRYING TO SET MY MACHINE’S DEFAULT GATEWAY VIA SIOCADDRT
AND IOCTL().
I found the following code snippet online and it works great:
int fd = socket( PF_INET, SOCK_DGRAM, IPPROTO_IP );
struct rtentry route;
memset( &route, 0, sizeof( route ) );
struct sockaddr_in *addr = (struct sockaddr_in *)&route.rt_gateway;
addr->sin_family = AF_INET;
addr->sin_addr.s_addr = inet_addr( "10.0.2.2" );
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;
TODO Add the interface name to the request
route.rt_flags = RTF_UP | RTF_GATEWAY;
route.rt_metric = 0;
int rc = ioctl( fd, SIOCADDRT, &route );
// [...]
Now TODO...
How do you actually specify the interface to use? When I execute this code, it seems to default to eth0
on my machine (which is my only interface). The rt_entry
structure in the route.h
header is defined as follows (also here.) ):
/* This structure gets passed by the SIOCADDRT and SIOCDELRT calls. */
struct rtentry {
unsigned long rt_pad1;
struct sockaddr rt_dst; /* target address */
struct sockaddr rt_gateway; /* gateway addr (RTF_GATEWAY) */
struct sockaddr rt_genmask; /* target network mask (IP) */
unsigned short rt_flags;
short rt_pad2;
unsigned long rt_pad3;
void *rt_pad4;
short rt_metric; /* +1 for binary compatibility! */
char *rt_dev; /* forcing the device at add */
unsigned long rt_mtu; /* per route MTU/Window */
#define rt_mss rt_mtu /* Compatibility :-( */
unsigned long rt_window; /* Window clamping */
unsigned short rt_irtt; /* Initial RTT */
};
Thanks for any help.
Solution
Obviously, you can use two fields to specify the interface:
struct ifnet *rt_ifp;
struct ifaddr *rt_ifa;
These two fields represent the “answer”, as it were, to the question posed by a route
lookup; that is, they name the interface and interface address to be
used in sending a packet to the destination or set of destinations
which this route represents.
Update:
There is a rt_dev
field in the structure, which I think you can set to interface name:
char ifname[] = "eth1";
rtentry.rt_dev = ifname;