Problems with SO_BINDTODEVICE Linux socket option

I have been looking into this for a while after seeing conflicting answers to how SO_BINDTODEVICE is actually used. Some sources claim that the correct usage is to pass in a struct ifreq pointer, which has the device name and index obtained via an ioctl. For example:

struct ifreq ifr;
memset(&ifr, 0, sizeof(struct ifreq));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "eth0");
ioctl(fd, SIOCGIFINDEX, &ifr);
setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE,  (void*)&ifr, sizeof(struct ifreq));

Where as Beej's networking tutorial says to pass the device name as a char pointer. For example:

char *devname = "eth0";
setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, devname, strlen(devname));

I have tried both of these methods and they both do what is required, but I wanted to note that the device index obtained in the first method is superfluous. If you look at the kernel code in net/core/sock.c, sock_bindtodevice just copies the device name string, calls dev_get_by_name_rcu to get the device and binds to it.

The reason that the first approach works is that the device name is the first element in the ifreq structure, see http://linux.die.net/man/7/netdevice.

NOTE: SO_BINDTODEVICE requires elevated permissions:

• run the executable with full root permission
• after building the executable you can use sudo setcap to grant the executable permission to use this specific socket option then you can run the executable without root permission and the executable has permission to use the SO_BINDTODEVICE feature (via earlier call to setcap).

OK, I've looked into it a little more. SO_BINDTODEVICE was considered "near obsolete" back in 1999, and is root-only due to some unspecified "security implications" (I couldn't find out exactly what).

However, you should be able to get the behaviour you want by binding to INADDR_ANY and setting the IP_PKTINFO socketopt. This will pass an extra message on the socket that contains a pktinfo structure describing the incoming packet. This structure includes the index of the interface that the packet came in on:

struct in_pktinfo {
    unsigned int   ipi_ifindex;  /* Interface index */
    struct in_addr ipi_spec_dst; /* Local address */
    struct in_addr ipi_addr;     /* Header Destination address */
};

The ipi_ifindex matches with the ifr_ifindex from the struct ifreq returned by the netdevice ioctls like SIOCGIFCONF. So you should be able to use that to ignore packets received on interfaces other than the one you're interested in.

Doco for IP_PKTINFO is in ip(7) and for the interface ioctls in netdevice(7).