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  • Dns / dhcp

    Wasnít sure where to stick this one as its kinda related to both.

    Iíve noticed I have a load of DNS entryís which have a different IP from the DHCP Records.

    As an example in the DNS I might have

    Forward Lookup - TestPC1 - 192.168.0.1
    Reverse Lookup - TestPC1 - 192.168.0.1

    But in the DHCP I have:

    Address Leases - TestPC2 - 192.168.0.1

    Iíve been manually checking laptops & PC DNS entryís against the what the DHCP entry is. If an entry is present in DHCP with the same IP as in the DNS Iíve been deleting the DNS entry.

    Is this the best thing to do or Am I going to cause issues?

    I need the DNS to sync up with the DHCP in the easiest possible way without having to manually update everything.

  • #2
    Re: Dns / dhcp

    Couple of questions for you:

    1: How long is your lease time in DHCP?
    2: Is this something that happens to everything that your DHCP is handing out to or just a few boxes?
    3: Is the computer showing a correlating IP address with DHCP or with DNS? i.e. according to the computer is DHCP correct or is DNS right?
    Two things:
    1) If I wrote something wrong please please please let me know. I want to know ESPECIALLY if I am wrong.
    2) I have a tendency to write things that are misconstrued as being agressive or not so pleasant. That is not my intent.

    Comment


    • #3
      Re: Dns / dhcp

      Lease: 20 days, 0 hours, 0 mins

      From what I can see its only with a few boxes

      It also looks like its the DNS side thatís not updating from DHCP.

      If an old PC took an IP address from DHCP & then wasnít on the network for over 20 days, then the same IP was given to another PC should this delete the DNS record of the first PC?

      Comment


      • #4
        Re: Dns / dhcp

        20 days? That was not the answer I was expecting

        Hmm so what happens if you go to a box with an incorrect address in DNS and preform a "ipconfig /registerdns" ?

        Also how long is your scavenging time set at in DNS? If at all.

        "If an old PC took an IP address from DHCP & then wasnít on the network for over 20 days, then the same IP was given to another PC should this delete the DNS record of the first PC?"

        The obvious answer to this is "yes" however that answer really does neither of us any good because I can not tell you the mechanism as to why the answer is yes. If that makes sense. I was under the assumption (and we both know about those now don't we?) that when an IP address is handed out it is registers with dns as part of the process. This may not be the case by default or if it is then you have the problem that think you have.
        Two things:
        1) If I wrote something wrong please please please let me know. I want to know ESPECIALLY if I am wrong.
        2) I have a tendency to write things that are misconstrued as being agressive or not so pleasant. That is not my intent.

        Comment


        • #5
          Re: Dns / dhcp

          Ok a little research into this (ok yeah a lot of research later) and now I have some answers. DHCP does not automatically change the A record with DNS when it is handed out. Option 81 in DHCP will make this happen. The changing of these types of records in DNS is handed by the "norefresh" and "refresh" settings (also the scavenging setting is thrown in there for good measure.

          So you might want to take a look at your DNS norefresh setting and see what that is set to. Default is 7 days (with the DCHP default being 8 days).

          Post any more questions you have, this was an interesting little bit of stuff to work out and I learned a LOT! Thanks for posting it.

          http://support.microsoft.com/kb/932464 - explanation of DHCP and DNS

          http://technet.microsoft.com/en-us/l...34(WS.10).aspx -briefly touches on Option 81 and what it does.
          Last edited by Draenok; 29th July 2010, 20:51. Reason: Added links to explain better than I did what MIGHT be the problem.
          Two things:
          1) If I wrote something wrong please please please let me know. I want to know ESPECIALLY if I am wrong.
          2) I have a tendency to write things that are misconstrued as being agressive or not so pleasant. That is not my intent.

          Comment


          • #6
            Re: Dns / dhcp

            Option 81 does ring a bell from somewhere in the past, I think I may have been told I needed this before but I never got time to set it up.

            Many thanks, I'll give this a good read.

            Comment


            • #7
              Re: Dns / dhcp

              After reading through the links I’ve had a look on the DHCP server & made the following changes:

              Scope [192.168.0.0] CCM Scope - Right mouse> Properties> DNS Tab

              Old Settings:
              Radio dot next to - Dynamically update DNS A and PTR records only if requested by the DHCP clients


              Changed to
              Unchecked - Dynamically update DNS A and PTR records only if requested by the DHCP clients
              Radio dot next to - Always dynamically update DNS A and PTR records



              I have another question, The dynamic updates of DNS are listed in 2 places. See the attached for my DHCP setup

              Sh-dmc1.ccm-ca.local [192.168.2.4] - Right mouse> Properties> DNS Tab
              Scope [192.168.0.0] CCM Scope - Right mouse> Properties> DNS Tab

              I changed the settings in the Scope 192.168.0.0 so they are different form the settings in the parent (sh-dmc1.ccm-ca.local…..) which settings will win over? I can’t see anything to say "don’t inherit settings"

              Also we have a number of old PC's that were destroyed in a flood some time ago, I’ve run a query on Active Directory for boxes which haven’t been logged on for in the last 90 days so I have a good idea which these are. The A-Records & PTR’s aren’t being deleted even though I have a tick next to “Discard A and PTR records when lease is deleted” on the DHCP server" - would I have to remove these manually & would you have any reasons why they were'nt removed?


              Would I cause any issues if I cleared out all the A-Records / PTR's? would DHCP just fill them back up again or meass things up?



              Attached Files
              Last edited by Deland01; 30th July 2010, 10:53.

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