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Network issues - old DHCP server might be causing DNS issue RRS feed

  • Question

  • I have 4 DC's, each running DNS.

    1 DC was running DNS and DHCP.  DHCP was moved to another DC.

    DHCP on the new server has the IP for each of the 4 DC's as a DNS server.

    However, it seems when the old DHCP server goes offline, users lose internet.

    This has happened twice.  I rebooted the router and brought the old server back online (faulty NIC - disable/enable).  Internet back up.  I replace router to be safe.

    The second time it happened, I rebooted the router and brought the old server back online. Internet was back up.

    When disabling DNS on the old server, a workstation lost network connectivity after a ipconfig /flushdns.  I re-enabled DNS and it came back up - either coincidence or I didn't wait long enough.

    What am I missing before I decommission the old server?

    Tuesday, July 16, 2019 3:42 AM

All replies

  • Hi,

    Do you mean that clients lost the internet because DNS doesn't work?

    To check if DNS works, please use command nslookup to resolve public domain names.

    Meanwhile, please run ipconfig/all to check the DNS server of clients.

    Best regards,

    Travis


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    Tuesday, July 16, 2019 5:44 AM
    Moderator
  • Not 100% sure it's DNS but fairly sure it is.

    When doing ipconfig /all, it shows all 4 DNS servers.

    Tuesday, July 16, 2019 4:00 PM
  • Hi,

    On these DNS servers, did you configure DNS forwarder?

    Please run command nslookup www.google.com, and post the result.

    Best regards,

    Travis


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    Wednesday, July 17, 2019 6:04 AM
    Moderator
  • DNS forwarders same on each server.

    DC1 - old DHCP server, NIC DNS is own IP, IP of DC2 - set to be decommissioned soon.

    DC2 - new DHCP server, NIC DNS is loopback IP, IP of DC4

    DC3 - NIC DNS is loopback IP, IP of DC2

    DC4 - NIC DNS is loopback IP, IP of DC2

    nslookup

    DC1 - shows itself.  If DNS service disabled, nslookup fails.

    DC2 - shows DC4, no change when DC1 DNS disabled

    DC3 - Shows DC2, no change when DC1 DNS disabled

    DC4 - Shows DC2, no change when DC1 DNS disabled

    Internet on DC2 stayed up when DNS on DC1 was turned off but test was only for a couple of minutes.  I don't have access to DHCP workstation at this time.

    I did have a DC5 that was decommissioned.  It was originally DC1 and DC5 and the others were added later.  DC5 was an old exchange server (I know, not recommended). This server ended up corrupted, had to manually remove from AD users and computers, and an exchange recovery install was performed and then uninstalled, adpromo demotion.  After the recovery install was performed, the remaining exchange server (one of the remaining DC's - the next exchange server wont' be a DC) has had DNS issues on the local outlook clients.




    • Edited by Susan_773 Wednesday, July 17, 2019 3:22 PM
    Wednesday, July 17, 2019 3:15 PM
  • Hi,

    I mean you need a public DNS forwarder, such as 8.8.8.8. 

    Do not use loopback IP, please use the real IP address.

    Best regards,

    Travis


    Please remember to mark the replies as an answers if they help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com

    Thursday, July 18, 2019 8:19 AM
    Moderator
  • Hi,

    I mean you need a public DNS forwarder, such as 8.8.8.8. 

    Do not use loopback IP, please use the real IP address.

    Best regards,

    Travis


    Please remember to mark the replies as an answers if they help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com

    I already have an 8.8.8.8 forwarder.

    Will changing the loopback IP to the DNS server's actual IP solve the problem I'm having?

    Thursday, July 18, 2019 6:20 PM
  • Hi,

    Using loopback IP may affect DC/DNS queries.


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    Friday, July 19, 2019 9:27 AM
    Moderator
  • Hi,

    Just checking in to see if the information provided was helpful.

    Please let us know if you would like further assistance.

    Best Regards,

    Travis


    Please remember to mark the replies as an answers if they help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com

    Tuesday, July 23, 2019 6:21 AM
    Moderator