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Subject: Re: how to bind smtp client to specific address?
From: Bennett Todd (betrahul.net)
Date: Fri Mar 31 2000 - 16:37:24 CST

2000-03-30-15:44:20 Christopher E. Brown:
> See previous mentions of iproute2. THERE ARE NO MULTIPLE
> INTERFACES.
>
> You can use ip to assugn multiple IPs the the interface,
> and if you with you can assign a label to each addidional address
> (such as eth0:0) for backwards compat.

To expand just a little on what he said, it turns out (I had no idea
of this before), Linux kernels 2.2 and later have a fairly
dramatically revised networking stack, that has a _lot_ of sexy
stuff hidden under the hood. The standard commands for manipulating
the network --- ifconfig, netstat, route, etc. --- are just
backward-compatibility shims to keep people from going ouch too hard
the moment they have to do the first basic thing.

Unfortunately, one of the features that doesn't quite map perfectly
through the backward-compat shims is the decoupling of addresses
from interfaces. An interface can have one or more addresses
associated with it, they are all equal.

A routing table entry can have a preferred source address associated
with it. Unfortunately, the illusion that these addresses are
associated with "subinterfaces" like eth0:0 falls apart here, and
the route command doesn't provide a fix.

The "ip" command, from the iproute2 package, provides direct access
to this (and bunches o' other stuff).

So far the best intro and orientation doc I've found is the
2.4networking Howto; if you search for iproute2 on freshmeat that'll
take you there, and that howto has a link for where to collect the
latest version of the software.

Thanks for calling my attention to this. Amazing stuff. There's
policy based routing, routing based on source addr, bandwidth
management, all kinds of cool goo in here.

-Bennett

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