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2 cnames, round robin configuration

Discussion in 'Windows Server 2003 / 2008 / 2012 Exams' started by bruun963, Jul 14, 2012.

  1. bruun963

    bruun963 New Member

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    Recently started studying for the 70-643 exam. According to the screenshot below the servers are configured to use DNS round robin to distribute the load among the servers. One thing i don't understand that i see 2 cname records in the same zone? This isnt possible as far as i know (tested it)

    This is how i see it :

    192.168.3.11 --> a-record : websrv1 | cname record : web(.contoso.com)
    192.168.3.12 --> a-record : websrv2 | cname record : web(.contoso.com)

    Configuring Server Clusters_1.gif
     
  2. demarrer

    demarrer Byte Poster

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    Hi,

    I've recently been working with DNS at work (not windows but IPM efficient IP which is a DNS appliance which manages the DNS on BIND servers) anyhow, I'll give your question my best shot.

    A cname is an alias for an Aname. CNames can also be chained together i.E a cname that points to another cname. From the example if you did an NSlookup for web.contoso.com (the CNAME) the DNS server will respond in a round robin fashion once with the Aname websrv1.contoso.com (ip 192.168.3.11) then the next time websrv1.contoso.com (IP 192.168.3.12). In effect the DNS server is performing round robin loadbalancing using DNS.

    Your questions about 2 Cnames in the same zone. What I have seen from experience is your can have a many CNAMES as you like in the same zone. What counts is what Aname the Cname is associated with.

    Does that help?

    P.S from what I've seen it's better to use a real Loadbalancer like F5 BIG-IP/Cisco ACE than do DNS loadbalancing if you have the cash that is :) Check out Cisco GSS which is able to do global DNS loadbalancing i.e the GSS is able to select the best server to respond to the request located in multiple datacentres spanning physical locations across the globe. Pretty cool stuff.
     
    Certifications: A+, Security +, CCNA, CCSA
    WIP: music, (dreaming of) CCIE Security :D
  3. bruun963

    bruun963 New Member

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    Its an clear explanation, but i just dont get it somehow. Mind going blanc at the moment. And i stated it wrong in the question, what i ment is : it is not possible to create two cname records with the same name right?

    They way that i see it is that both servers have a cname record with the name : web

    Another to way to clarify it :
    Suppose i install two servers. For both servers i and wanted to create two exact same CNAMEs named : web.contoso.com pointing to both servers. So i create the first cname pointing to websrv1, but then i create the same cname again and pointing it to websrv2. If i do that the following error appears : there can only be one record with this name. Would you like to remove the previous record with this name and replace it with the new one?

    192.168.3.11 --> a-record : websrv1 | cname record : web(.contoso.com)
    192.168.3.12 --> a-record : websrv2 | cname record : web(.contoso.com)

    I just dont see it how it can distribute the load in a simple way since i cant create two cname name records with the same name. If i cannot create the two cname, then i would have to create to seperate cname records, for example web.contoso.com and web2.contoso.com. But then the traffic would only be directed to web.contoso.com (aka websrv1)

    Pretty sure im confused with setting up the appropiate records. If someone could post some configuration steps according to the image in my first post, which a-record, which cname record, and where the cname records are pointing to, then i would make some sense to me.
     
    Last edited: Jul 14, 2012
  4. demarrer

    demarrer Byte Poster

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    You only need to create one cname. Remember that one cname can be associated with as many anames as you like.
    So web.contoso.com (cname) points to both srv1.contoso.com (aname) and websrv2.contoso.com.
     
    Certifications: A+, Security +, CCNA, CCSA
    WIP: music, (dreaming of) CCIE Security :D
  5. bruun963

    bruun963 New Member

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    Thank you for the reply, that has put me in the right direction. I always thought that a cname record could only point to one arecord. Was pretty confused because on the image it appears there are 2 cname records with the same name pointing each to a different server. Did a short ping after the right configuration and it works perfectly.

    Steps :

    1. Create multiple a-records all with the same name : web pointing to the different servers. The FQDN of the arecords : web.contoso.com
    2. Create one cname with the name : www and point it to one of the previous created a-record.

    dns.gif
     
  6. demarrer

    demarrer Byte Poster

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    Glad it works. For me DNS is not as simple as it first looks. To get a good understanding keep working with it, read the rfc's about it to gain more depth in your understanding. Good luck wih the study.
     
    Certifications: A+, Security +, CCNA, CCSA
    WIP: music, (dreaming of) CCIE Security :D

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