Using a box with two interfaces (namely, one of them being a wireless interface) with each one connected to a different physical network.

Identifying applications locally running on the box by their source port, I want to route the packets from specific applications through one or the other interface.

The idea is that
- using a ruleset, each (or some) applications are routed through the non-default interface from the moment they start
- when the default (eth0) interface goes down, the specific applications are routed through the other interface

While, hopefully, keeping the applications' connection live.

yeah, I know things look grim.

My best guess is that it's gonna have to be a new custom filter for netfilter, but I dont know what I *could* do with that.

Any help is appreciated

Just some quick thoughts:

Use a third IP (virtual) address on both interfaces, if one goes down, packets can go on the other device (assuming your other device has a route). This needs some work.
iptables can already match on source port and user, wouldn't that be enough?
You could create a tunnel between the two devices.
Try searching for "TCP session mobility" (which would allow IP addresses to change, hopefully). I do remember seeing a program that proxied all the connections, so you could do this. It was a while ago, and can't quickly find it on google or in my bookmarks.
http://www.hpl.hp.com/personal/Jean_...ileIP/mip.html for "mobile IP" never tried it myself, but maybe you'll find something.

Anyways, post back with what you find out.

Good Luck,
chris

thanx, I'm looking into the material...

one of the things I'm pretty certain that need to be done so far is that I need a module that has knowledge of the ruleset, makes a Netfilter hook so as to receive packets (which one of the 5?) , checks for specific source port(s) and then forwards it to the appropriate interface.

a) the 5 hooks are:

NF_IP_PRE_ROUTING Called after sanity checks, before routing decisions.
NF_IP_LOCAL_IN Called after routing decisions if packet is for this host.
NF_IP_FORWARD Called if the packet is destined for another interface.
NF_IP_LOCAL_OUT Called for packets coming from local processes on their way out.
NF_IP_POST_ROUTING Called just before outbound packets "hit the wire".

which one should I be using? The NF_IP_FORWARD looks mighty hopeful, but I'm still looking at where *exactly* in the stack it takes place.

This method goes for "tunneling" between the two devices...

b) Am I forgetting/ommiting something?