Socket bind with wildcard ip

Question

I have a created a UDP socket and bind that socket to inaddr_any (0.0.0.0) and and some well known port number. As per my understanding this socket can receive data over all the interfaces of the machine over the specified port number. But When i will call send() it will use the default IP address as the source address.

• How is the default IP address chosen?
• If I want to use some other interface (other than the default) for sending the data, how can this be done?

Context of the problem:

I am implementing LDP protocol. It can have many hello adjacencies. Thus i am creating a server to recv data from the other interfaces of the router. Once the hello adjacency is formed, then hello messages are to sent be on the specific interface over UDP over which the hello adjacency is created.

Answer 1

The default IP address is chosen based on the network the packet is sent to. For example if you have two interfaces, one connected to network A and the other connected to network B, if you send a packet to network B the packet will be sent with the IP address of the second interface. For this reason, most of the time you don't have to worry about it .

If you have two network interfaces connected to the same network, you can bind the socket to the address of one of them, and the packet will go out with that address. For example, this will bind an IP socket to 192.168.122.1 , if allowed by the network stack:

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr("192.168.122.1");
addr.sin_port = 0;
if (bind(s, (struct sockaddr*) &addr, sizeof addr) == -1) {
    perror("bind");
}