How to determine which interfaces a Linux socket is bound to in C++

Question

I have been trying to figure this out for a while now. Given a Linux socket, how can I

1. Determine if it is bound?
2. If it is bound, determine which subset of network interfaces it bound to on a multihomed system?

In short I have a machine with multiple network cards and a socket that is only bound to a subset of these cards. I want to programmatically detect and log these interfaces after they have been bound else where in our libraries meaning I don't have access to this infnormation at the time of binding.

I should also point out that I need an IPV4/6 compatible solution preferably for that works with SCTP sockets (though I'm fine with UDP or TCP solutions for now)

Answer 1

The getsockname function returns socket name ie the local socket address. Just call this function for a unbound and a bound socket to determine your socket implementations behavior.

EDIT : When you have an AF_INET (ie IPv4) socket, the member sin_addr.S_un.S_addr is identical to the appropriate member in the SOCKADDR_IN struct that you have passed to the bind function. So if you have bound to INADDR_ANY you get INADDR_ANY when the socket isn't bound to a specific network adapter.

I checked this with the socket implementation of my OS (Windows 7). If your implementation returns something else, you might experience an undefined behavior caused by a bug somewhere else. Of course you can verify the implementation of the network stack if you have the source code available.

There is no support for binding to a subset of network adapters. You can only bind to one or all.