Strange DHCP Issue



  • Hey there.

    I was hoping some of you would be able to help me diagnose a strange DNS issue I am having at work.

    So roughly every 2 days, typically 1 computer will have this issue, and its been going on for a few months now but havent had 30 seconds to look at it until post xmas, then higher priority projects etc.

    So to begin it will "typically" appear that external internet access is lost, the user will reboot their computer, and upon reboot will have lost internal and external access.

    Once at this state the only resolution appears to be to do an ipconfig /release /renew,

    I would say out of an office of roughly 50 people, this has only ever affected maybe...7 people, for a total of....maybe 40 times in the past 4-5 months.

    Its definitely nothing 911 critical however; it has started to affect our President a few times now so that obviously jumps it up in the priority que.

    Would anyone happen to have any suggests on where to start going down this rabbit hole.

    Thanks!


  • Service Provider

    What makes you think that it is DNS? Maybe it is DHCP related?



  • Oh shoot, sorry I meant DHCP
    (Sorry fighting a cold, not really head strong right now)


  • Service Provider

    Are you sure it is DNS?

    1. From a machine that has had the problem, right now while working, run ipconfig /all and verify the settings.
      1. Do you have a odd DNS entry
      2. Do you have a strange DHCP lease time
      3. Run nslookup and see what DNS server it is using.
      4. Etc.
    2. After it manifests, prior to a reboot, ipconfig /all
      1. Repeat 1-4 above
    3. After is manifest, after a reboot and things do not work, repeat step 2.

    Post results here.



  • I would, to start with, check if your DHCP pool has available addresses. Next time you have this situation, check IP settings on computer with problem. If your adapter has IP from 169.x.x.x range you can be pretty sure DHCP is to blame.

    Next, I would check if you have another DHCP server in network, eg some WiFi router or something like that. Maybe it hands out IP from another pool or with wrong IP settings.

    Also, I would test if I can ping some internal IP - your default gateway and if that works, try to ping 8.8.8.8. This way you can test if you have connectivity at all or DNS server doesn't work.


  • Service Provider

    @triple9 said in Strange DHCP Issue:

    I would, to start with, check if your DHCP pool has available addresses. Next time you have this situation, check IP settings on computer with problem. If your adapter has IP from 169.x.x.x range you can be pretty sure DHCP is to blame.

    Next, I would check if you have another DHCP server in network, eg some WiFi router or something like that. Maybe it hands out IP from another pool or with wrong IP settings.

    Also, I would test if I can ping some internal IP - your default gateway and if that works, try to ping 8.8.8.8. This way you can test if you have connectivity at all or DNS server doesn't work.

    I would bet on rogue DHCP server. Next time a computer won't connect, do a: ipconfig /all
    and then see what address it has for a DHCP server.



  • Awesome thanks guys, I'll wait for the next computer to go down then I'll report back with results!


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