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MS DNS Round Robin:  sessions & alternating requests

Posted on 2007-03-29
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Last Modified: 2010-04-07
A couple of DNS Round Robin questions please:
I am running basic MS DNS round robin which sends requests to one of 2 identical web servers.
I understand that MS DNS server will alternate requests between the multiple IPs for same DNS records.

1. When the DNS server sends a request to box 1, will box 2 ever receive a request from that same session?
In other words, do I need to worry about session variables not persisting during a client's viewing because they have been bounced from box to box?

2. How does MS DNS alternate between boxes exactly? I host about 1500 websites, whos DNS records are identical. So say the DNS server gets a request for website1.com and sends it to box 1 will a request for website2.com be sent to box2? or will the request be sent to box1 and only the next request for website1.com be sent to box2? Is there a preference of box by default?
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Question by:juliandormon
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Chris Dent earned 250 total points
ID: 18822127

1. Depends on the clients DNS cache, which, in turn, depends on the TTL (Time To Live) you have set on the record.

The maximum cache time for an XP client is 24 hours, so even if the TTL on your record is higher the client will still ask the local DNS server for an answer again.

I couldn't really say if that means you need to worry about the session or not. It would seem (from the outside) sensible to plan for that contingency, but it does depend very much on what you need to happen, it's possible for the record to expire mid-session.

2.  On a per record basis. So for an individual record it answers DNS requests with Box1, then Box2, then Box1, then Box2 (ad infinitum).

A different record within a different zone will show the same behaviour but  doesn't depend on the answers given for website1.com.

There's no preference in Round Robin.

HTH

Chris
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