How to use different network interfaces for different processes?

Question

I have two network interfaces on a Linux PC, and I need to manually set the interface that a given process will use.

The program (Twinkle softphone) does not have a similar option, so I believe that it must be set externally.

How can I do it?

Edit: I'm not trying to make a server process bind to a specific interface, but rather to make a client program contact a server using a specific interface.

Answer 1

you can replace code at runtime by the use of LD_PRELOAD (@windows you can use a similar technique call detours, quite fancy). what this does is to inform the dynamic linker to first load all libs into the process you want to run and then add some more ontop of it. you normally use it like this:

% LD_PRELOAD=./mylib.so ls

and by that you change what ls does.

for your problem i would try http://www.ryde.net/code/bind.c.txt, which you can use like:

% BIND_ADDR="ip_of_ethX" LD_PRELOAD=./bind.so twinkle

here is how you build it:

% wget http://www.ryde.net/code/bind.c.txt -O bind.c
% gcc -nostartfiles -fpic -shared bind.c -o bind.so -ldl -D_GNU_SOURCE

a longer howto is http://daniel-lange.com/archives/53-Binding-applications-to-a-specific-IP.html

similar hacks and tools:

• bindhack
• liboverride
• fixsrcip
• netjail

Answer 2

I don't think it is possible to force a process to use a certain interface.

However, I think you might be able to play with ipchain/iptables and force that a certain port your process is listening at will only get packets coming through a particular interface.

Useful HOWTO: http://tldp.org/HOWTO/IPCHAINS-HOWTO.html

Answer 3

Usually if a program has no option for setting listening interface, it's listening on ALL interfaces. (You can verify this with lsof -i).

Creating iptables firewall rules that drop incoming traffic pointed towards its ports on interfaces you don't want it to be visible on is the easiest thing to do.

Answer 4

Why would you want a program to use an interface other than the one connected to the server to talk to that server? And if the system isn't using the interface connected to a server to talk to that server, it's a system-level (routing table) issue and has nothing to do with which process happens to want to talk to that server.

Different servers on IP networks have different IP addresses. The kernel should know which interface to use to reach a particular IP address based on the routing table. If you're trying to talk to two different servers that have the same IP address, the system will get confused (because, among other things, it only indexes the connections internally by destination address). You can make that work, but it's a system-level fix involving putting one server in a separate logical network that's only connected to the machine through software NAT.

So if they have different IP addresses, use routes to select the correct interface. If they have the same IP address, you need to use NAT so that they appear to have different IP addresses to the system.