True DNS Backup

[Gregory Hicks]

> To: comp-protocols-dns-bind at isc.org
> From: "Mike" <ward007 at swbell.net>
> Subject: True DNS Backup
> Date: Thu, 21 Feb 2008 14:52:32 -0600
> 
> Hello all, I have a question about how best to set up dns for fast recovery 
> in case one fails. Here is my scenario: I'm also using fictitious IP 
> addresses to make it easier to explain.
> 
> I have two internet routers 1 each from different providers. The first has 
> network of  1.2.3.0 the second has network 8.9.10.0
> The two servers run bind v9. The primary dns has an IP address of 1.2.3.1 
> with a secondary IP of 8.9.10.1
> The second dns which is slave to the primary has an IP of 8.9.10.2 with a 
> secondary IP of 1.2.3.2.
> 
> Ok here is my real question. If network provider network 1.2.3.0 fails I 
> want the dns servers to continue serving dns. If network provider 8.9.10.0 
> fails I want the dns's to continue serving dns. If I have the servers set up 
> this way will it work? Or does anyone out there have a better way of doing 
> this. Any help appreciated.

So...  If network provider 1.2.3.0 fails does 8.9.10.0 continue serving
DNS?  Yes.

If 8.9.10.0 fails and 1.2.3.0 is still up, does 1.2.3.0 continue
serving DNS?  Yes.

If 1.2.3.0 fails and 8.9.10.0 fails, does either continue serving DNS?
Not as far as the external world is concerned.  (Your network fails
the "I route, therefore you are" test.  In other words, if both
networks went down, how are the queries going to get to your DNS
servers?)

To your internal network, both servers may be up, so I guess you could
say that your servers are answering queries, but...

If either network provider fails, but the other remains up, the
external world will continue to query your servers providing the
network does what it is supposed to do and provides routes TO your
servers.  They will try one then the other server.  Whichever one
provides the 'fastest' (relatively) response time is the one that is
actually going to get queried.

Hope this helps.

Regards,
GRegory Hicks
---------------------------------------------------------------------
Gregory Hicks                           | Principal Systems Engineer
Cadence Design Systems                  | Direct:   408.576.3609
555 River Oaks Pkwy M/S 9B1
San Jose, CA 95134

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