Host interface or route for remote address

Question

How do I know which interface will be used for a specific remote address?

Say, I have two interfaces. One of them is 192.168.0.50/24, the other is 10.0.2.30/21 and all traffic is routed through it. So, for 192.168.0.100 I expect 192.168.0.50, for 10.0.3.40 or 8.8.8.8 I expect 10.0.2.30.

I'd like to ask the OS, as it somehow binds the address to a specific interface when I do socket.connect or socket.sendto. So how do I find out the address the OS would bind the socket on socket.connect without doing socket.connect?

There are two goals: to "test" the routing table and to bind socket to a specific port and the adapter that is accessible by the peer.

Unlike in Can Python select what network adapter when opening a socket?, I don't have adapters with overlapping addresses and I don't know the address of the adapter that socket would be bound to. My goal is to find this address.

What is your reason for not wanting to ``socket.connect``? Is your goal actually to find out which interface the socket *will* be bound to, or do you want to avoid opening a socket that *would* bind to a specific interface? — MisterMiyagi, Sep 04 '19 at 15:18
Possible duplicate of [Can Python select what network adapter when opening a socket?](https://stackoverflow.com/questions/8437726/can-python-select-what-network-adapter-when-opening-a-socket) — Pynchia, Sep 04 '19 at 15:29
@Pynchia Thank you for the pointing to that question though, I learned something new! — George Sovetov, Sep 05 '19 at 12:06

score 2 · Answer 1 · answered Sep 05 '19 at 15:03

2

There is no out of the box way to mock a routing call as such. If you do not want to connect, you must do something to parse the local routing table information.

If you are using Linux, you can do this with pyroute2 (pip install...). It is a helper library to read and modify the routing table.

Then you could do something like this - the output requires some parsing:

from pyroute2 import IPRoute

foo = IPRoute()
bar = foo.route('get', dst='192.168.42.42')
print(bar[0]['attrs'])

The result is a tuple. You will be interested in RTA_PREFSRC field as it will be the ip address of the local interface that would be used to route packets to that particular destination if they were to be sent.

answered Sep 05 '19 at 15:03

Hannu

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It's definitely worth checking out! What I write should work on both Linux and Windows, but there's something for Windows in the library. But, yes... It would be nicer if it's in the standard library. – George Sovetov Sep 06 '19 at 14:15
The package might now include Windows code as well - at least the source tree seemed to have a windows directory. Worth checking at least. There may also be something similar for windows - this is just about parsing the routing table anyway. If not, you probably need to build a parser that extracts routing information and matches target addresses to that. – Hannu Sep 06 '19 at 15:50

Host interface or route for remote address

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