How can I get the interface name/index associated with a TCP socket?

Question

I\'m writing a TCP server that needs to know which interface each connection arrived from. I cannot use the address/subnet to deduce which interface was used, since there mi

Answer 1

In general, you shouldn't need to know what interface the packets are going to be sent/received on; that's the kernel's routing table's job. It's difficult to find out the interface for a socket because there really is no direct association. The routing of packets can change within the socket's lifetime based on routing information.

For datagram (UDP) sockets, you may be able to use getsockopt(s, IPPROTO_IP, IP_PKTINFO, ...); see getsockopt(2) and ip(7).

For stream (TCP) sockets, one option might be to open multiple listening sockets, one for each interface on the system, and use setsockopt(s, SOL_SOCKET, SO_BINDTODEVICE, ...) to bind each to one interface; see setsockopt(2) and socket(7).