server 2008 R2 DNS round robin

Posted on 2014-09-02
Last Modified: 2014-09-02
Hi all,

we are using a 2 server 2013 exchange system with 4 CAS servers. Each CAS server has a DNS host record resolving the same URL. We have no load balancing over than DNS round robin to the CAS servers for clients accessing Exchange. One of our CAS servers is heavily utilised and so I wondered if there is a way of identifying whether the DNS round robin is actually working?
Question by:Jason Thomas
    LVL 25

    Accepted Solution

    You can use nslookup to perform a simple test of round-robin functionality:


    Type nslookup at a command prompt.


    Type server <dns_server> at the nslookup prompt, where <dns_server> is the name or IP address of the DNS server you wish to test.


    Type the FQDN that your clients use to access the CAS servers. The output should show all of the addresses associated with that FQDN.


    Repeat step 3. If round robin DNS is working correctly, you should see the same addresses in a different order. Keep repeating until you are satisfied that it is or isn't working.
    Here's an idea of what it looks like when it works:
    Nslookup output showing round-robin DNS working as advertisedRemember that round robin DNS provides very basic load balancing at best. When it doesn't work as expected, this is typically due to the fact that the client (and possibly the app as well) caches the first valid DNS response and continues to use that cached address for subsequent access attempts until the cached record expires.
    LVL 1

    Author Closing Comment

    by:Jason Thomas
    Spot on! Thank you very much. PS - DNS RR is working ;)
    LVL 9

    Expert Comment

    No points necessary for me, but think you should consider load balancing

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