2 nics, 1 network, puzzle?

[Shawn Yarbrough <shawn@nailstorm.com>]

I have an x86 server computer containing two network cards:

eth0 --> 192.168.1.130
eth1 --> 192.168.1.131

Both cards work fine alone.  As you can see, both cards are on the same
network.  The subnet mask is 255.255.255.0.  I can disable either card
with ifdown and sucessfully ping the other card from elsewhere on the
network.

The puzzle comes when both cards are up at the same time.

The kernel seems to pick one of the cards (somewhat arbitrarily) and
starts answering pings to BOTH addresses on the ONE card!  Could this be
some kind of ARP bug?  I can physically pull the plug out of the other
card (the one that the kernel appears to not be using anymore) and the
system still answers pings to both addresses!


I first noticed this strange behavior by watching the indicator lights
during pings.  Pings to both addresses are clearly going thru only one of
the cards.  The other card remains idle.

This is a Debian Woody system (freshly installed and not currently in use
for anything).  The behavior is identical running either kernel 2.2.20 or
kernel 2.4.18.  In /etc/network/options the ip_forward setting is no.

This is NOT a router-type computer.  It's just a server that I really want
to have on the same network, twice.  And I'd rather not use IP aliasing +
one card.  I really want two cards for convienence and redundancy.  (ex:
in theory if one card failed or was misconfigured I should still be able
to reach the machine through the other card).


Anyone got a clue what's going on here?  Thanks,

Shawn Yarbrough
shawn@nailstorm.com


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