windows - Can Python select what network adapter when opening a socket?

Question

The target machine running the python application will have three network interfaces available to it. In general all three networks will be vastly different, however there is a possibility that two of the three could be on similar networks.

In the example below I do not have control over the destination address on ETH 2 (as it a pre-configured system), so I forced into selecting what adapter to use programmaticly.

I am fairly sure that this will fall on how the OS works with routing the connections. My hope is that there will be a platform independent way to solve the issue using python, because there is a possibility that this application will need to run on Windows 7 as well as a Linux machine.

Example Code

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('192.168.0.2', 8000)) # Which device will this connect to??

Normal Case

• ETH 0 Source: 192.168.0.1
• ETH 0 Destination: 192.168.0.2
• ETH 1 Source: 10.20.30.1
• ETH 1 Destination: 10.20.30.2
• ETH 2 Source: 60.50.40.1
• ETH 2 Destination: 60.50.40.1

Possible Trouble Case

• ETH 0 Source: 192.168.0.1
• ETH 0 Destination: 192.168.0.2
• ETH 1 Source: 10.20.30.1
• ETH 1 Destination: 10.20.30.2
• ETH 2 Source: 192.168.0.3
• ETH 2 Destination: 192.168.0.2

Additional Information
Adapters ETH0,1,and 2 are all connected to different physical netoworks

Answer 1

I can't speak much for Windows, but on Linux the interface is normally not chosen until a routing decision is made, therefore you usually don't have a say on which interface your packets leave.

You do have the option though, of using SO_BINDTODEVICE (see man 7 socket) on Linux. This binds a socket to a device, however, only root can set this option on a socket.

---

Just checked, and the python socket library doesn't have SO_BINDTODEVICE defined, but you get it from socket.h:

# from socket.h
# define SO_BINDTODEVICE 25

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, 25, 'eth0')

See also:

• Linux: How do I force a specific network interface to be used?
• How do I make an outgoing socket to a SPECIFIC network interface?