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Multiple A record question RRS feed

  • Question

  • Dear all:

         Windows 2012 server。 I created four HOST AA record (e.g. mail.contoso.com) pointing to 4 different IPs(TWO site A, TWO site B) , run nslookup finding it is round robin.

         Q1: I have a lab with same configuration, run nslookup on a Site A client and finding it always get TWO site A IPs on the top, same as site B client ,does it mean my lab is in Subnet Priortization ?

         Q2:  If I disconnect one of 4 IPs, run nslookup still gets 4 IPs, does client know not using the disconnected IP? how about ping, http, outlook?

         Q3:  I installed 2 WSUS servers and publish it as http://mail.contoso.com, will site A clients only resolver SITE A IPs and connect to it? Both in  round robin and Subnet Priortization situation ? 

         Q4:  I remember outlook is site aware software, it means outlook will always connects to it's own site server IP, right?

         Thanks in advance.

    Thursday, September 7, 2017 3:09 AM

All replies

  • Hi emman.yoyo,

    1. If we have multiple A records for one FQDN in DNS, then when we do nslookup on the client to query the DQDN, it will follow round robin;

    2. If we also enable netmask ordering, then it will follow subnet prefer rules. For example:

    Client in subnet 192.168.2.0/24 with IP address 192.168.2.3;

    In DNS, we have four A records for www.test.com :

    www.test.com A 192.168.2.5;

    www.test.com A 192.168.3.5;

    www.test.com A 192.168.4.5;

    www.test.com A 192.168.5.5;

    When we run nslookup www.test.com on the client 192.168.2.3, the IP 192.168.2.5 for www.test.com will always in the first order, so the client will visit the nearest www host;

    You may check if you enable "round robin" and "netmask ordering" in DNS properties:

    3. If we delete one of the A record in the DNS server, while still get the same IP addresses on the client, this may due to DNS cache on the client, please run ipconfig /flushdns on client to clear DNS cache;

    4. For the WSUS server settings, it's not recommended to make clients change WSUS server frequently. If you have several WSUS servers and have the same FQDN and A records in DNS server, please use the IP address of the WSUS server in clients URL, for example: http://x.x.x.x:8530 (for wsus 4.0, we need to append the port number);

    5. For outlook, you'd turn to outlook community for more information about how it choose the IP address for same FQDN.

    Best Regards,

    Anne


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    Thursday, September 7, 2017 6:31 AM
    Moderator
  • Thanks.

    1&2: Both round robin and netmask ordering are being selected in my Product and Lab, but nslookup gets different result, why?

    3: Disconnect means shutdown one node or network issue, not delete.

    Monday, September 11, 2017 1:24 PM
  • Hi emma,yoyo,

    >1&2: Both round robin and netmask ordering are being selected in my Product and Lab, but nslookup gets different result, why?

    Could you provide the result of the nslookup just like the example in my last reply?

    Best Regards,

    Anne


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

    Tuesday, September 26, 2017 5:52 AM
    Moderator