Re: Linux and Routing

["Matthew Walker" <mwalker@silug.org>]

----------
> From: KoReE <koree@ameth.org>
> To: SILUG <silug-discuss@silug.org>
> Subject: Linux and Routing
> Date: Wednesday, April 05, 2000 7:12 PM
> 
> 
> Alrighty, this is what I'd like to do, and I'm about halfway there, but
> I've run into a snag.  I'm wanting to load a Linux box up with Ethernet
> cards, to separate out a new Ethernet segment, but not have to go through
> my Cisco router to do so.  So, here's what I've done.  I have two
Ethernet
> cards in a Linux machine.  One card has been given an IP# on our main
> network - 206.152.121.0/42.  The second, has not yet been given an IP#,

42 bit subnet mask?  

> but, what I would like, is to give it an IP#, and then route a network of
> IPs to that address, and then have the Linux box route those addresses
> onto the new Ethernet segment.  Am I making any sense at all?  So, the
> outcome would be, I'd hook an Ethernet hub up to the card, and be able to
> plug devices into the hub and give them addresses on the routed network.
> Does anyone know what direction I need to look to do this?  I've tried
> routing a /28 network to the machine, then giving the second card an
> address on that network, and then routed that network to that card.  But,
> I can only access the machine itself through the card, and the other
> machines I put on the hub and give secondary addys to, can't get out, nor
> can I get into them.  I know this is probably horribly confusing...the
way
> I've put this...but, I'm working totally theoretically here, anyway.  So,
> any pointers would be greatly appreciated.
> 

It sounds like one set or another of the machines don't know how to get
back to the other machines... for instance, say the a.a.b.x network is
connected by a router a.a.b.1 to the internet and your machine is a.a.b.254
on that network.  The second network is a.a.c.x and your machine is a.a.c.1
on that network.  The machines on a.a.c.x network would have a.a.c.1 as
their default gateway.  The machines on the a.a.b.x network, however, will
need to have a route to the a.a.c.x network via a.a.b.254 in addition to
their default gateway a.a.b.1

Also, I have used tcpdump in an instance like this to help me understand
where the traffic is going or not going...  you would probably see the
inside machines successfully send a ping packet to the middle network, but
the machines in the middle network probably are sending their info out to
the default router.

Another way to do this is put a static route on your cisco router... the
middle network machines will get an ICMP redirect from the router and will
"learn" the route to the innermost network as well as propogate out the
information to other routers via whatever routing protocol (if necessary)
> Thanks,
> 
> Koree
> 
> ______________________________________________
> Koree A. Smith  | Co-Administrator, ameth.org
> koree@Ameth.org | http://www.koree.net/
> koree@koree.net | Linux Rules!
> NT < *IX        | I Corinthians 2:1-5
> 
> Nous pensons que Linux représente à terme une menace pour Windows.
> 
> 
> 
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