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Old 10-29-2003, 02:27 PM   #1
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Registered: Jul 2003
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Virtual IP weirdness

Hi, I am having some really strange weirdness with a virtual IP. I have a RHAS box with iptables on it, eth0 being external interface bound on .100, eth1 is internal bound to .101, and a virtual IP on eth0 which is .102. I am running DNS/BIND on it and it's bound on all 3 IPs (waiting for my utility machine to show up, hence why we're using .102 so that clients don't have to change their registrar information). Everything worked fine for months, until yesterday, and there have been no known changes.

From the box itself, dns answers on .100, .101, and .102 no problem. Internally, .101 answers fine. Externally, only .100 answers, and .102 gives no answer. I set iptables to log queries, and nothing is going to .102 (but .101 and .100 are fine). I turned off iptables just to verify, and as far as I have been able to determine, it's not any firewall problem. I added a new virtual IP, .123, and it was having the same problem as .102. I even cannibalized a working IP from another machine to test and make sure it wasn't a particular IP. And the gateway router routes traffic fine.

It appears as if the virtual IP only works if you're looking at it from the host machine itself. There aren't any errors. Unfortunately, I can't really take the firewall down to test traffic easily lest I get pissed-off clients. A quick test with tcpdump didn't show any traffic (tcp or udp) for .102 when dns was requested. The machine has been rebooted, still no-go.

Might it be the kernel?

Thanks for any help!
Old 10-31-2003, 02:53 PM   #2
Registered: Sep 2003
Location: Dallas, Tx, USA
Distribution: Red Hat, Gentoo, Libranet
Posts: 98

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I don't know that I'll be much help, but since nobody else tried to tackle it...

First thing that occurs to me (since you've rebooted the box to no effect) is to try to pin down whether this is a bind issue or a network issue.

Can you ping 102 from outside? If so, how about things like telnet, ssh, http?

If you can't even ping it, it's likely a network issue - a switch that's had a configuration change (or even a port gone bad), something like that.

Set up a virtual IP on 101. If that works, it's that much less likely that it's a problem with Linux's networking.

If it DOESN'T work, that's a pretty strong indicator there IS a problem with Linux's networking. If you have a vendor and support contract for RHAS, I'd use it at that point.

If the internal virtual address works, and especially if the external ping works, I'm inclined to say it's a problem with BIND.

I'll wait for feedback from you before I say any more.

P.S. What version of bind are you running?
Old 10-31-2003, 06:44 PM   #3
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Registered: Jul 2003
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I found something really interesting this morning, a feature with the linux 2.2-2.4 kernels. Basically, if you have more than one NIC on the same physical network, such as eth0 and eth1 on the same switch, then eth1 can answer arp for eth0 and vice versa. There is a patch for it, which I haven't done yet, since the overall plan was to move eth1 on a separate physical network than eth0 (but I couldn't due to not having equipment at the time).

(go down to section 4.5.4)

This is what I was seeing on tcpdump... eth1 was answering arp to eth0 and the router would be confused and use the wrong MAC address.

I believe the reason I hadn't seen this behavior before was that I didn't have much traffic on that network at the time. We moved a few webservers with high traffic to that network (changing datacenters) and suddenly this behavior shows up.

If you'd like, I'll post my results after I'm able to co-ordinate with the router guys some static routes...
Old 11-01-2003, 12:02 AM   #4
Registered: Sep 2003
Location: Dallas, Tx, USA
Distribution: Red Hat, Gentoo, Libranet
Posts: 98

Rep: Reputation: 16
Kudos to you for wading through that stuff...

You don't need to post it for my sake (although I will definitely remember that this situation is possible -- we use multiple NICs in most of our machines, and they all go pretty much to the same switch.)

If you think it will help out somebody else, please do post it. I hate Googling for a problem and finding nothing but somebody else 6 months ago who had the same problem (and no answer!).

Good luck,



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