DNS on two different networks

I have 2 internet connections one is much slower than the other and I wish to have the second one as a failover for my webservers.  I have 2 dns servers both pointed to the first connection.  I was figuring that if i set up a third dns server on the second network, it could point everyone to the webservers through the slower connection.  Since dns server 1 and 2 are only accessable through the first internet connection, if that line goes down, i was thinking that it would try the third dns server and it would work because it is on the second connection.  It would then point everyone to use that network.

What I would like to know is, does it work that way, or does it work in a round robin fashion?  I dont want people to be hitting the third dns server unless the first line is down due to the fact that the first line is a DS3 and the second is a slow t-1.

publicvoidDirector of ITAsked:
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Chris DentConnect With a Mentor PowerShell DeveloperCommented:

Round Robin.

You would have an number of issues to deal with in this scenario:

 - NS3 would have to hold a Primary copy of the zone which only lists the IP for the Web Server over the third connection.
 - The TTL (Time To Live) for all records associated with the web server would have to be low. Any DNS client performing a query for the name will remember the address for the value of the TTL. Without reducing the TTL you will find clients still have the other web server IPs while the line is (potentially) down.
 - While the lines are up you would not be able to control which connection is used, you're reliant on Round Robin rotation.

You may find that is is more beneficial to set a low TTL and change the web server record. In that instance you can control exactly how the web server is accessed. The change itself can be scripted, monitoring the state of the connections is the hard part.

publicvoidDirector of ITAuthor Commented:
Thanks Chris.  Well the round robin kills it anyway.  I thought that might be the case, just hoping it wouldn't be.  I don't want the second line used at all unless the main one is down.

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