Specify interface when adding default gateway via SIOCADDRT

Question

I'm trying to set my machine's default gateway via `SIOCADDRT` and `ioctl()`. I found the following snippet online that works just fine: ``` 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 for the `TODO` in this snippet... How do you actually specify which interface to use? When I execute this code, it seems to default to `eth0` on my machine (which is my only interface). The definition of the `rt_entry…

Accepted Answer

Apparently there are two fields you can use 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's an `rt_dev` field in the sturct, I think you can set it to the interface name:

```
char ifname[] = "eth1";
rtentry.rt_dev = ifname;
```

Specify interface when adding default gateway via SIOCADDRT

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