Second NIC Breaks Network Connectivity

I've got a 486 with dual NICs I'm trying to use as a firewall / router / proxy / DNS server. The first NIC (eth0) works fine, but when I try to bring up eth1 on another IP, I lose all network connectivity (on either interface). Good thing it's on my desk and not somewhere I don't have console access. Second NIC Breaks Network Connectivity

It's possible that eth1 is bad (hardware problem), or that Linux doesn't know what it is -- I'm not sure I actually configured it. But when I run
ifconfig eth1 up 192.168.1.58
I get no errors. I just lose network connectivity, even on eth0 (LAN).

It does this whether I do it in webmin or in the console. Routing looks like:
192.168.1.0/24 eth0
192.168.1.0/24 eth1
0.0.0.0 eth0 [through the 'real' router]

I thought the duplicate entry might be a problem, but on second thought, won't it just use the first one, completely ignoring the eth1 entry? (I'm connecting over eth0, so I shouldn't lose that connection!) I couldn't figure out how to delete the route (route del what?), so I just brough eth1 down, but still couldn't connect (via eth0) until I restarted eth0.

Does anyone get why both go down? Am I correct in assuming that Linux would give me some error if I had no module for eth1 (or the wrong one)? I don't get errors. Second NIC Breaks Network Connectivity

 

 

 

 

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