from a command line type arp -a (after pinging the address in question) sorry didn't really specify where and when to run arp -a.
Cheers,
Shooter
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Question
DNS Issue?
Obviously, there must be a physical computer plugged into my network but how can I find it? Here's the scenario: a new server was built with a static IP address, the server worked for 4 days with no problem. On this past weekend, we shut the server down and put it on our rack. This morning, we see an IP conflict with the server's static IP. We unplugged the network cable and pinghed the IP address. It answered. We pingged with the command ping -a XXX.XXX.XXX.XXX, it answers with the server name. We pingged the server by name and it answers but like I said before, the server is now unplugged from the network. Where should I look to resolve this problem?
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Answers
If you look at the ARP table on a switch, you can get the machine's MAC address and perhaps track it down from there.
I would also consider hard coding the IP<->MAC address on the switch for the legitimate server if that switch configuration allows it.
If you want to see the traffic from that [mis]configured server and your switch does port mirroring, you can monitor the type of traffic that's being sent from that server.
How large is the network, are the ethernet cables into the switch labeled and can you trace the cables from to the switch to account for the machines?
You said you brought a new server on line:
First off, (and most will disagree with me on this), It can take a couple days to replicate the DNS settings of a new server to other servers within the Domain. I say, most will disagree, because they will say it takes up to 8 hours to replicate that DNS out.
This is just a thought. If you took an old server off line with the same static IP. It may have taken a couple days to replicate out to the other server. By bringing an old server off line, there may be DNS metadata lef over from the old server left on some of the other Domain controllers. When you brought the DC on line, you may have replicated the DNS settings to the other servers and they are reporting back that the old server's DNS settings still had a DNS Host A left in DNS. Now they don't know what to do about it except error you out. So, you may have been doing OK for a couple days and when you restarted your machine, you also restarted the netlogon service. Now, the servers are giving you an error.
My best guess is you still have metadata pertaining to DNS on the remaining servers and that is interfering with this new server's DNS registration to the other domain controllers.
Thank you for your suggestions. We don't managed switches. We have switches from Linksys and they are plug and play. Someone suggested that our router may be calprit but I looked at the router with the show interfaces command. It shows FastEthernet0 being bind to xxx.xxx.xxx.1/24 which is our gateway IP. It shows Serial0 being bind to our external IP address xxx.xxx.xxx.xxx/30. I couldn't see how the router would take that IP since it is not included in our DHCP range and the two interfaces are bind to our internal and external IP's. And even if the router took the IP address how would it know to assume the netbios name of the server?
If you can ping that IP and get an alive response, then there is a machine that has that IP address.
For servers with static addresses, those addresses should be outside of the DHCP pool anyway. If they aren't, moving them outside of the pool would be the first step that I would take.
Is this [duplicate] IP from the same subnet that the router interface is on?
If so, on the router you should be able to look at the arp table to see what MAC address is broadcasting that IP. Then hardcode the correct MAC address to that IP to keep the rogue machine from advertising it to the router.
The problem was of our own making! A year or so ago we decided to segment our network into two subnets, XXX.XXX.100.0 and XXX.XXX.200.0. This did not work very well so we stopped using the .220.0 subnet but we left the router in place and that router had the IP address that my co-worker assigned to the new server. I am surprised that we didn't get an IP conflict until four days later. We found a tool called wireshark which found the IP and told us the source was a Cisco device. I looked at our active router but found nothing, then the thought hit me that we still had the other router on line which we called the student router. Since it was not being used, I simply unplugged it and away went our headaches. Thanks for all your help.
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by: LilshooterPosted on 2007-11-12 at 09:43:47ID: 20265723
you can always look at the arp entry for the IP address, do you have managed switches? arp -a after pinging the ip address in question, copy down the mac address, then start looking for the offending machine. If you have DHCP enabled on the network, check the DHCP server log to see if the address matches the mac address. In this case you would need to set an exclusion in your DHCP server to exclude that IP address, reboot the machine that has the IP address and all should be well.
If you need a tool to monitor the network and seek out this mac address, Ethereal is a good start.
Cheers,
Shooter