How do I make an outgoing socket to a SPECIFIC network interface?

Question

I have a server with two different network interfaces, each with a different IP address. How can I create a socket so it'll go out a specific IP address?

I'd prefer a python example, but the question is language agnostic, so shoot away.

EDIT: Please don't give me "You can't" as an answer. I mean, it is a computer. I can do anything I like to it, for example - I can problematically disable the one interface I don't want on the fly. I'm looking for something prettier.

Answer 1

You can certainly bind a socket to a specific device.

I don't know how to do it in python, but using the berkeley socket api (in C) you need to call setsockopt(), using the option SO_BINDTODEVICE.

You pass in an interface descriptor, which is of type struct ifreq. Ideally you would get the contents of the interface descriptor by using ioctl(), and requesting SIOCGIFINDEX - passing the name of the interface (eg. eth0) as an argument.

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edit: Just did a quick search and found this documentation of the socket methods in python. setsockopt() is amongst them.

Answer 2

Just a little note - what I really needed is to bind to a specific IP, and just for the sake of completeness, the solution is to bind the socket after creation. Source in python:

import socket
s = socket.socket()
s.bind(("127.0.0.1", 0))
s.connect(("321.12.131.432", 80))