External DNS Redundancy

JMD5
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We have 4 external DNS servers for A record lookups for www and mx record lookups for email..  2 DNS servers are in one City 2 are in another.

If 2 go down, the other 2 should be sufficient to handle queries.

I am looking for supporting documentation which shows how a resolver will be able to go through the list of DNS servers on behalf of the client and not timeout before finding a server that is not down.

Please comment and provide a link to how a resolver goes thru a list of Name Servers if if does not receive an answer from the first Name Server.

Thanks

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Amit BhatnagarSystems Development Principal - Security and Infrastructure

Commented:
Please refer to the link given below. It contains the process on how clients resolve DNS names when multiple DNS Servers are available for Name Resolution Process.

http://technet.microsoft.com/en-us/library/cc772774(WS.10).aspx#w2k3tr_dns_how_gaxc

1.The DNS Client service sends the name query to the first DNS server on the preferred adapters list of DNS servers and waits one second for a response.
2.If the DNS Client service does not receive a response from the first DNS server within one second, it sends the name query to the first DNS servers on all adapters that are still under consideration and waits two seconds for a response.
3.If the DNS Client service does not receive a response from any DNS server within two seconds, the DNS Client service sends the query to all DNS servers on all adapters that are still under consideration and waits another two seconds for a response.
4.If the DNS Client service still does not receive a response from any DNS server, it sends the name query to all DNS servers on all adapters that are still under consideration and waits four seconds for a response.
5.If it the DNS Client service does not receive a response from any DNS server, the DNS client sends the query to all DNS servers on all adapters that are still under consideration and waits eight seconds for a response.

Author

Commented:
Thanks but your example is taken from the perspective of the client.

I am looking for documentation taken from the perspective of the resolver.

That is, assuming that the client's DNS server is up and working properly,
when this DNS server acts as a resolver on behalf of the client, it should pull
a list of NS records for my domain.  What if only 2 of the 4 Name Servers are up and
running and this DNS (resolver) attempts to query a downed server.   How long before it
times-out and moves on to the next in the list?

Amit BhatnagarSystems Development Principal - Security and Infrastructure
Commented:
:) When the DNS Server of other domain is sending request to your DNS Servers, don't you think it is acting as a client as well.
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Author

Commented:
I am looking for something quite different.
I am trying to find documentation on how the resolver will use the list in step 2 below.
As you can see, adapters do not come into play here.

****using jd.com as an example ****

Web Client querries its local dns server for www.jd.com

1. Resolver queries root servers > root returns .com name server list
2. Resolver queries .com servers > .com servers return name server list for jd.com
* this list contains all 4 of my name servers
ns1.jd.com
ns2.jd.com
ns3.jd.com
ns4.jd.com

3. Resolver checks ns1.jd.com for SOA of jd.com
* ns1.jd.com does not respond (it is down)

4. Resolver checks ns2.jd.com for SOA of jd.com
5. ns2.jd.com responds as SOA
6. Resolver requests A record (www).
7. ns2.jd.com responds with A record address

Thanks


Commented:
Still no takers?
Any External DNS Experts out there?

Thanks

Author

Commented:
Thanks.

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