bind socket to network interface

Question

How can I bind a socket to a particular network interface? I tried using setsockopt on server side, but the clients can still access the service through both eth0 and lo interfaces.

I can achieve this by setting the particular IP address using serv_addr.sin_addr.s_addr.

But I suspect that we can bind to an interface using only setsockopt (without mentioning the IP address).

Answer 1

You can bind to a specific interface by setting SO_BINDTODEVICE socket option.

struct ifreq ifr;

memset(&ifr, 0, sizeof(ifr));
snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "eth0");
if (setsockopt(s, SOL_SOCKET, SO_BINDTODEVICE, (void *)&ifr, sizeof(ifr)) < 0) {
    ... error handling ...
}

Warning: You have to be root or have the CAP_NET_RAW capability in order to use this option.

The second method is that you can resolv IP address tied to an interface with getifaddrs().

Follow the latter link for a comprehensive example.

Answer 2

The only way you can do it is as you mention -

by setting the particular IP address using serv_addr.sin_addr.s_addr

You can't do it without knowing the address to bind to.

You can use ioctls to determine the current IP address if you need, though there may be a cleverer way to do this these days - I've not done much in modern Linux distros lately.

Answer 3

Maybe someone will find it useful, so I am sharing the solution that worked for me (Linux, C++):

uint32_t interfaceIndex = if_nametoindex(interfaceName);

Where "interfaceName" is the name of the interface we want to bind to, e.g. "eth0" (see: https://linux.die.net/man/3/if_nametoindex). Now we can specify this interface in the socket address structure via "sin6_scope_id" (in case we use IPv6):

struct sockaddr_in6 socketAddress;
socketAddress.sin6_scope_id = interfaceIndex;

Now we can bind the socket to the interface via "bind" as usual.