Re: two nics to one lan

From: P Gentry (rdgentry1_at_cablelynx.com)
Date: 05/21/04 

Date: 21 May 2004 08:53:25 -0700

Hans Fugal <hfugal@wencor.com> wrote in message news:<c8jc71$hbi$1@terabinaries.xmission.com>...
> I have two NICs, one LAN. eth0: 172.16.59.1, eth1: 172.16.59.17. This
> afternoon something odd happened (I'm not sure what), I see in the logs:
>
> May 20 13:08:37 x440 kernel: NETDEV WATCHDOG: eth1: transmit timed out
> May 20 13:08:40 x440 kernel: e1000: eth1 NIC Link is Up 1000 Mbps Full
> Duplex
>
> The link may have come back up, but all connectivity was hosed. No ping
> response on 172.16.59.17 _or_ 172.16.59.1. ifconfig claimed things were
> normal, and the routing table was also normal, which I believe is the
> problem. Whatever happened caused the eth1 route to be non-functional,
> but all traffic bound to 172.16.59.0/24 was being routed through eth1.
>
> Unfortunately I don't have the routing table from then, because I was
> frantically bringing interfaces down and up in hopes that connectivity
> would be restored (it was). But here is my current route config:
>
> # ip route show
> 172.16.59.0/24 dev eth1 scope link
> 172.16.59.0/24 dev eth1 proto kernel scope link src 172.16.59.17
> 127.0.0.0/8 dev lo scope link
> default via 172.16.59.15 dev eth0
>
> # tcpdump -n -i eth0 src 172.16.59.1 and dst net 172.16.59.0/24
>
> That command gives no output at all, but s/eth0/eth1/ gives lots. All
> the traffic is being routed through eth1, as you would expect from that
> routing configuration. But I didn't ask for two 172.16.59.0/24 routes
> via eth1, and before I did ifup eth1 (after doing some forensics and it
> was down) there was one well-configured route via eth0 for that subnet.
> (This is RHEL AS 2.1)
>
> So the crux of my question is, is there a way to configure the routing
> such that the preferred interface for frames from 172.16.59.1 is eth0,
> and the preferred interface for frames from 172.16.59.17 is eth1?
> Preferably one that wouldn't preclude the alternate route if one route
> goes down, but the route isn't likely to go down unless the whole
> interface does and there's not likely going to be traffic to
> 172.16.59.17 if eth1 is down...

Linux with two nics on same subnet on one host is not intuitive ;-)

> At least I'd like to have a route over eth0 if eth1 goes away, and the
> routing config above doesn't look like there is (although I could add
> one, right?) Note I'm not asking about load balancing, although that
> wouldn't be a bad side effect either.

Not clear to me what you want to accomplish here -- a backup link?

> I'm just starting to feel very comfortable with routing stuff, so it's
> possible (likely) that I've made some bad assumptions in the above
> dialog; I'd appreciate it if you'd point them out to me!

Don't feel too bad about your assumptions -- Linux has a quite unusal
and very flexible stack implementation.

Start with these:
http://lartc.org/howto/
http://linux-ip.net/html/
http://www.policyrouting.org/iproute2-toc.html
http://www.policyrouting.org/PolicyRoutingBook/ONLINE/TOC.html

You should already have:
/usr/share/doc/iproute?X???/ip?cref.ps

It will also be handy to have all the howtos from tldp for background
and to answer specifics that will likely crop up.
http://www.tldp.org/docs.html#howto

As is, Linux uses a single, main routing table -- and any nic will
answer/accept frames for any IP used by the host. You will need
multiple routing tables -- ie., basic policy routing -- configured to
get firmer control.

I think you'll find all your answers in the above docs -- the online
book is a very good background/primer on policy routing. You will
likely need to adjust your FW rules if you want each nic to "see"
different traffic.

hth,
prg
email above disabled

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