DNS A update

Discussion in 'DNS Server' started by ed, Jan 20, 2010.

  1. ed

    ed Guest

    Hi all,

    windows 2003 DNs servers and allow dynamic updates through DHCP
    if a computer has two connections: wireless and LAN, wireless has
    faster connection than LAN, How would DNS A record update with which
    IP address?

    Thank you.
    ed, Jan 20, 2010
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  2. They both get dynamically registered under the zone for the same host name.
    If a client were to query the record, DNS will Round Robin (toggle through
    the list of possible answers) where the client will pick the top one on the
    list. Query again, the answers are randomized to be picked from. You can
    test it using nslookup. Type in:

    (it's IP address)

    Then hit the Up arrow to repeat the command, hit enter, and you will see it
    switch. Repeat, and you will watch it toggle back and forth. With a larger
    example, you can lookup Hotmail's MX records. The last time I checked, there
    were four groups of 4 MX records that *each* have 12 different IP addresses,
    totalling 48 possible MX IPs. When you run nslookup, you will find that it
    will show you the four MX records, but underneath, you only see the IP
    addresses for the top two. Hit up arrow and run it again, you will see the
    same but with the MX records randomizing.


    This posting is provided "AS-IS" with no warranties or guarantees and
    confers no rights.

    Please reply back to the newsgroup or forum for collaboration benefit among
    responding engineers, and to help others benefit from your resolution.

    Ace Fekay, MVP, MCT, MCITP EA, MCTS Windows 2008 & Exchange 2007, MCSE &
    MCSA 2003/2000, MCSA Messaging 2003
    Microsoft Certified Trainer
    Microsoft MVP - Directory Services

    If you feel this is an urgent issue and require immediate assistance, please
    contact Microsoft PSS directly. Please check http://support.microsoft.com
    for regional support phone numbers.
    Ace Fekay [MVP-DS, MCT], Jan 21, 2010
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  3. Oops, forgot to add, that the returned number of records wtih a large
    response, such as Hotmail's MX records, will be limited to your firewall's
    capabilities whether it supports EDNS0 or not. If not, you will see a max of
    512 bytes. If it does, the limit is whatever you set EDNS0 limits to. I have
    my Pix set using the " fixup protocol dns 1280" command, which sets my max
    to 1280 bytes. No use setting it higher, because the answer will be below
    that, at least for A, MX and other records. However, TXT records, such as
    SPF records, can be much larger, depending on how much info is in the SPF.

    Ace Fekay [MVP-DS, MCT], Jan 21, 2010
  4. ed

    ed Guest


    thanks for your help and time.
    one interesting I found out, if I disconnect the wireless connection and
    connect the LAN connection, the DNS should update A record with LAN IP. But,
    somehow, when I check the DNS servr, the A record still has disconnecte
    wireless IP. Is this supposed to be?
    ed, Jan 21, 2010

  5. If you do not have scavenging enabled, or have configured the DHCP server to
    own the record to update it, yes, it will remain. Even with Scavenging,
    there's a time-based process involved to scavenge (remove) old records.

    If the DHCP server providing configuration is different than the DHCP server
    providing configs for wireless clients, I can see why you may be seeing
    this. In most cases, it's suggested to set the APs in 'corporate' mode so
    the AP literally acts as a switch to the main subnet and uses the same IP
    range as wired clients. If DHCP is configured to own the records it provides
    to clients, it simply updates the current IP address. However, if both
    interfaces are connected simultaneously to the network, then you will see
    two different IP addresses in DNS, because the two interfaces can't share a
    common IP. The one way to control this is to disable dynamic registration on
    the wireless interface. However the caveat is you won't be able to resolve
    the laptop if it is only connected via wireless. I'm not if your
    infrastructure requires the ability to resolve all clients, but in many
    cases I've seen this is not necessary so disbling the wireless interface
    from registering is not a big deal, unless of course it is a server, but I
    can't see a server being on wireless.

    As for how to configure DHCP to own records, and how to configure
    scavenging, please read my blog on this subject. I hope you find it useful.

    DHCP, Dynamic DNS Updates, Scavenging, static entries & timestamps, and the
    DnsProxyUpdate Group (How to remove duplicate DNS host records)

    Ace Fekay [MVP-DS, MCT], Jan 21, 2010
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