Socket listening on an IP subnet in C / Unix

Question

I'm trying to write a server-client socket program in C. The objective is for the server to listen on a specific port, but across a range of IP addresses belonging to the same IP subnet. This IP subnet is part of the 127.x.x.x range (not 127.0.0.1 of course).

Couple of points to note:

• This is a stream-based socket, and not Datagram sockets.
• This is not a broadcast address.
• Implementation in C/C++ only on Unix/Linux platform

I do not want to open multiple sockets on the server for each IP address in the range. This is not scalable.

Any help would be ideally appreciated. Is this even feasible?

Answer 1

You can only bind to one address on a single socket. Why can't you bind to INADDR_ANY and simply reject any packets not bound for your target IPs? Alternatively, you could bind to an arbitrary port and use OS-level magic (e.g. iptables, bpf) to reroute packets destined for those IP/port combinations to your socket.

Answer 2

The socket API does not allow binding to a subnet -- you can bind to one IP or to any IP. You can listen for all inbound connections and reject those that don't apply. If you need to divvy connections out between processes on the same server, use a single listening socket, and transfer incoming connections to the worker processes.

Answer 3

You can use a firewall to prevent anyone from outside the desired subnet from connecting (that's at the o/s level). You can put the socket in promiscuous mode and accept all connections on a given interface. I don't know if you can do both (have a socket in promiscuous mode and run iptables on it). Essentially it's like building a packet sniffer that only listens on one port.