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Nic
Asked: 2010-02-27 15:56:38 +0800 CST

Some DHCP clients end up with wrong DNS server

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The scenario:

  • DC running Windows Server 2008 R2 providing DNS + DHCP
  • Cisco 1811 Router as the gateway
  • 30 Windows XP DHCP clients on the LAN

The problem:

  • Some workstations are spontaneously switching to an incorrect DNS server. Specifically, ipconfig /all shows that they start using the gateway as a DNS server.
  • This happens about 5-10 times a day to various computers, sometimes more than once per day.

The workaround:

  • Repairing the connection on the XP client always fixes the problem, and the correct DNS server address is obtained.

We lost our main DNS/DHCP machine a week ago, and had to bring this one online as a spare. We've been having this issue since then. DHCP leases on the old and new servers are configured for "wired" (8 day) duration. There are definitely no other DHCP servers active on the LAN. So far there is no discernible pattern about which clients will show this problem, or when.

When I ran DCDIAG /test:DNS it came back clean. Manual inspection of the DNS zone shows that all the records are appearing as expected, with no traces of the previous machine in there.

Update Feb 27: Added screenshots.

Here is a screenshot of the DHCP scope options on the 2008 R2 server. http://nicwaller.com/screens/dhcpscope.png

And here is a screenshot of ipconfig /all running on a healthy host. I don't have any ailing hosts at the moment, but will grab a screencap next time it happens. http://nicwaller.com/screens/ipconfigall.png

Update Feb 28: More screenshots.

Here's a screenshot of DHCP and DNS traffic from a healthy client when repairing the local area connection. There's definitely only one server responding, but it does seem strange that the negotiation takes place twice. I'll try to get a similar capture from a sick machine this coming week. http://nicwaller.com/screens/dhcprenew_screen.png

Update Mar 01: Caught a bad ipconfig.

Here's a screenshot of ipconfig /all from a client that had this issue. It says the lease was issued this morning, but it doesn't even have an entry for the secondary DNS I set up yesterday. Both DNS servers were discovered properly when repairing the connection. http://nicwaller.com/screens/bad_dns.png

Update Mar 01: It even got the sysadmin!

This issue finally affected my personal workstation this morning. Unfortunately I had just rebooted and wasn't running a packet dump at the time. I set up a secondary server yesterday, and was logging all DNS traffic to it. My machine had not contacted the secondary DNS in over half an hour, so that says to me that it's just spontaneously reverting to the gateway without even failing over to secondary DNS first.

Today I swapped the order of the DNS servers in DHCP, so the secondary is primary and vice versa. I will update again once I know how that goes.

domain-name-system dhcp windows-server-2008-r2

7 Answers

  1. Check your router to make sure that it isn't providing any sort of DHCP service. If you telnet into the router and it has lines in the configuration that start with "ip dhcp", then it is providing some sort of DHCP response.

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  2. Clients will change from Primary to Secondary DNS if the primary DNS doesn't respond in a timely manner and they won't switch back until the lease is renewed. I think DNS will just fail if the secondary doesn't respond, ie. I don't think they then switch to the Gateway but it's possible. This could be tested pretty easily.

    The options that I can think of:
    -The scope actually has the incorrect IP as a DNS option. Corrupt DHCP scope, delete and recreate or post a screen shot or export of the DHCP scope options.
    -There is another DHCP server running. Ipconfig /all lists the IP of the DHCP server the client obtained the lease from & the timestamp when it was obtained.
    -On the clients there is a DNS server(s) setup under the Static or Alternative configurations.
    -The clients are changing between wired and wireless and getting a different lease on the wireless network.

    If IPConfig shows the DNS server listed and the IP of the DHCP server is also as expected then something on that IP is giving out the bad/unexpected leases.

    As Kyle suggested, wireshark/netmon on the server will confirm if the lease is actually coming from that server with the bad info.

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  3. Best Answer

    I would run a packet dump on a few of these boxes until it happens. See if you can find anything network related. Maybe you will see some packets that give you an idea if it is not that.

    Can a group policy in Windows set the DNS server. Maybe somehow there has been a strange GP applied on the domain?

    Update:
    I have never done this, but since it seems like you are getting a little desperate, what about blowing away the current DHCP database. These instructions say how to back up the mdb file, so maybe moving it somehwere else will make it so DHCP creates a new one after restarting. That might fix the problem...

    The thing that doesn't jive in my mind, :-), is why clients would be getting new information if their lease hasn't expired yet and they haven't rebooted... is this what is happening?

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  4. I know this is an old thread, but i experienced this same issue, and maybe it can contribute to someon else.

    In my work network we had a service that included a modem that a provider used to connect to a piece of equipment they provided. I didn't know at the time, but it seems this modem was also a router with DHCP Enabled, and started causing this same issue.

    What was happening was that after a client sent a DHCP Request, both the main server and this router answered. The client got the IP from the main server but the DNS from the router, wich caused the conflict. After get the modem/rotuer offline, the services were normalized.

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  5. I know this is a an old post but I thought I would send an update since this just happened to our latest migration to 2012. The root cause was the fact that we imported the dhcp database from the old 2003 server. All was fine initially but once we needed to make scope change, specifically for dns option, the clients started experiencing the issue described. They would initially have the correct dns setting but would all of a sudden revert back to the old dns settings. A renew or reboot would always fix the issue but only temporary.

    Hope this helps

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  6. I'm sure you've already checked this, but if not - check the global options. The DNS server might be set incorrectly in there, and for some unknown reason be overriding some of your clients.

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  7. Maybe it's the Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP) or Enhanced Interior Gateway Routing Protocol (EIGRP) (depending what routing protocol your router uses) that is telling your clients "Hey over here I'm the shortest route not that DNS server he sux" hence why your clients are switching over to the Routers Gateway to get their DNS! I could be wrong on my understanding of Routing Protocols but that is my 2 cents worth!

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