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Multiple NIC preventing DNS server connection?

By bwiedor ·
This is an odd one, I haven't come across it before.

I just purchased a Dell Optiplex 330 for use as an admin console for my server room. It will be connected to both the external campus network (130.xx.xx.xx) and the internal server room network which does not go outside (192.xx)

The onboard NIC of the Dell is Gigabit, as is our internal network switch, and so I have that port connectd to the Internal network. I put in a D-Link 530TX PCI NIC in as well, and that connects to the External campus network.

The problem is that I cannot reach the campus DNS servers, if I try an nslookup let's say. However, if I disable the onboard NIC, (the Internal network one), then I can reach them just fine. When I re-enable the onboard NIC, then I can once again not contact my DNS servers.

It seems to me that there is some conflict between the two NICs, but I cannot determine the root cause. I would assume that it's because the onboard NIC is taking some sort of priority and it can't contact the DNS server (because the switch doesn't go to the outside).

I switched the cables and IP addresses around, effectively swapping NICs and networks, and at first it did not work that way either. Then I swapped them back and it did work. That was yesterday; when I came in this morning, the problem had re-established itself.

So now I'm not sure at all what's going on. All I am sure of is that disabling the onboard/Internal NIC allows contact with the DNS server again. Any ideas?

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by bwiedor In reply to Multiple NIC preventing D ...

Ongoing research gave me the answer, though this is not something I ever came across before.

I was not aware that if you have multiple NICs/networks on a single computer, that you should avoid having multiple Default Gateways. In this case, the Internal network made having a default gateway unnecessary (as it does not go outside the room) - and it also sparked a conflict.

With both NICs configured for a default gateway, it appeared that whichever card was configured with the 192.xxx address was then first in the bindings, and hence could never reach the DNS server.

Once I removed the 192.168.0.1 default gateway from it, the problem was gone.

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Nice to know all is working the way it should.

Please post back if you have any more problems or questions.

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