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Wireless Networks Thread, IP Address Conflicts in Technical; Right this problem started the first day back of the new academic year and is starting to get annoying as ...
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    AngryITGuy's Avatar
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    IP Address Conflicts

    Right this problem started the first day back of the new academic year and is starting to get annoying as all attempts to resolve this have failed.

    The school DHCP server is running 2003 R2 with service pack 1 and has never given us any problems at all. The same server is also set up as one of the DNS servers for the site.

    Over the summer all workstations had XP SP3 installed, I held off for as long as possible whilst it was being tested on the network. Also over the summer we had a managed wireless network installed using Cisco hardware.

    One the first day back it was staff laptops that started to get an ip address conflict error when logging on to the domain using wireless connectivity.

    We thought this was down to the fact that our Cisco Ap's were given DHCP addresses, these were all changed to static addresses but did not resolve the issue.

    Later in the week it started to affect workstations in the ICT Suites, so the theory of the wireless causing the issues has been thrown out the window.

    A script was written to release the assigned ip address and restart the workstation to resolve the issue but this only works for a day when the laptop logs on the following day the ip address conflict message appears again.

    We have also changed the ip address range used for distribution and bizarrely this has not fixed the issue as some of the clients are still connecting with their old dhcp addresses assigned last week. However I think this maybe down to the fact that the lease for last weeks addresses expires on Wednesday, so the clients should pick up an address from the new distributed range then.

    The only thing I can think of now is that the DHCP database is corrupt as thatís the only possible cause of the problems.

    I would really appreciate some advice on this as its driving me nuts and I have almost gone through all the possible causes of the problem and nothing has resolved the issue.

    HELP PLEASE!

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    We had similar and wiping the entire DHCP leases list helped us out.

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    Have you tried pinging the IP address that is in conflict from the server? I had a similar problem (albeit 1 client) where the server was holding an IP address for the client,but when you pinged it it came back with a different name. It turned out that another laptop had taken the IP address and the server had not released it properly. So if I pinged by name I got 1 result, and by IP address I got a different result.

    You could try that to see if the server is holding 2 values for the same IP address. If it is happening with multiple clients it sounds more like the server end than the clients.

    I had to delete the reference for the 2 clients before I could clear the conflict.

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    KMOUNT do you mean literally just deleting them all?? Does that not mess things up or do they all pick up again once the machines start up again?

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    Silly question: Have you checked that ALL the new kit have their DHCP servers disabled?

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    Quote Originally Posted by treetops View Post
    KMOUNT do you mean literally just deleting them all?? Does that not mess things up or do they all pick up again once the machines start up again?
    Your machines will try to request the last ip address they had. If it is available they will assigned that address again. If not another will be assigned

    Do all the machines with an IP address conflict have the same address ?
    Last edited by tosh74; 15th September 2008 at 10:48 AM.

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    AngryITGuy's Avatar
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    @ kmount have tried that and still no joy!!!

    @ penfold we did have that issue where pinging by name and ip gave two different results. Found that to be down to DNS pointer files where the same ip address was referenced to by two machines. There are no duplicate entries for ip address in dhcp just bad addresses. Pinging a bad address from within dhcp gets a reply back from a functioning machine which does not display the ip address conflict message.

    @ rhyds there are no other working DHCP servers on site, this has been checked a few times to make sure nothing was missed out.

    @ tosh74 the machines with the ip address conflict do not have the same address. I have even checked the registry on two laptops with the problem to see if there are any references to a common ip addresses there weren't the only ip address in the registry were the ones assigned to the NIC and WIFI card and all were different.

    I am starting to think the problem is down to the server rather than clients buu there is still no light at the end of this tunnel for ME!

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    budgester's Avatar
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    This really sounds like you have another DHCP server on your network that you don't know about.

    Could something connecting wirelessly have it own DHCP server ?

    Have you tried wireshark ?

    Run wireshark and do a DHCP relase and renew on a workstation, then see which IP addresses are offering up IP addresses.

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    AngryITGuy's Avatar
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    @ budgester thanks for that I will give it ago. I know all signs point to a rogue dhcp server. However when I run ipconfig /all on a laptop that displays the conflict message the dhcp server is set to the site dhco server and nothing else. I may even stop the dhcp servce later and see if a ip renewal picks up anything but in the meantime I will give wireshark a go.

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    We had similar issues when we put a new server in. This took over the DHCP and the DNS, but for some reason the IP addresses were not being register in the DNS.

    Looking at the Log file for the DHCP server showed errors when trying to register.

    Firstly, we disabled the DHCP server to ensure that no other DHCP Server was on the network giving out addresses. If a client failed to obtain an IP address after a IPCONFIG /renew then I was happy no other Server existed.

    Then, bravely I deleted pretty much all the entries in the DNS Server and the DHCP Leases (except for the static items eg, servers). And forced a replication to the secondary DNS servers. Did an IPCONFIG /FLUSHDNS on the server also.

    And, eventually all the problems went away. The DHCP log was showing successes on all DNS rewrites and all has been well.

    One command which did prove useful, which I have to confess to not know about until I was getting these issues is IPCONFIG /REGISTERDNS which should force a write of the assigned IP address to the DNS Server. This also gave me a chance to look at the DHCP log and confirm any problems or successes.

    HTH

    Pete

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    I had this problem recently... I put it down to migrating DHCP to a new server and then buggering up the time on the DCs

    Have you turned on IP conflict detection on the DHCP server? This will be a quick fix if it is a lease expiry problem like mine.

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    AngryITGuy's Avatar
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    @ FragglePete thanks for the advice I tried everything you suggested and we still kept getting the ip address conflicts.

    @ Ric_ the server has ip conflict detection turned on.

    After checkong through the event logs of some of the machines with the I found the MAC Addresses of the two workstations causing the problem.

    Turns out its NICs on two servers we setup at the beginning of the year. I'm surprised to say the least that they have just decided to act up considering there have been no changes made to the servers since they were installed.

    Both the servers are Dells fitted with 2 Broadcom NetXtreme Gigabit cards with the latest drivers The cards are confgured to work as a 802.3ad Link Aggregation team with a static address.

    I removed the team to allow the cards to pick up a DHCP address this was then released, i flushed the DNS and cleared the ARP cache and then reconfigured the team.

    However workstations are still coming up with the ip address conflict message and the ip address in conflict is different each time!

    I am going to delete the teams and use the network card on each server that is not causing the conflict to see if that helps.

    Has anyone come accross any issues like this before?

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    I've seen malgunctioning kit do a simlar thing before.

    We had 1 small belkin wireless AP, that decided it was gonna have EVERY ip address on the network - causing conflicts everywhere.

    Eventually I just threw it in the bin as a bad lot

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