[COLUG] routing question: Selective SNAT (two external interfaces)

[Nathan Eady]

I'm working on an IP tables firewall ruleset for a change in our
network infrastructure at the library where I work, but I've run into
a logic snag, and I'm not sure I can implement what we want just with
IP tables, and I don't know exactly what I need to do.

The problem is that we really need to do SNAT selectively, using one
public IP address for some traffic, and a different public IP address
for other traffic.  I can handle the IP tables rules to *allow* the
various traffics to go out on the appropriate interface (and thus have
the appropriate public IP), and replies back, and so on, but it occurs
to me that I don't know how to direct the routing so that it actually
happens that way.

The system this needs to happen on is a special-purpose distro based
on Slackware, and I'm told the current version is based on Slackware
version 8.1.  Slackware is just about the only major distro I've not
previously worked with, so while I don't know much that's
Slackware-specific, I'm comfortable with Linux generally.  Except,
I've never had to configure routing before, other than setting up a
single default route occasionally.

Is my approach at all reasonable?  Where should I look for the
documentation I need?

TIA,
Nathan

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