Planning for Emergencies -- A DNS Solution ?

[Barry Margolin]

In article <38D23F3D.24894.191C4CB at localhost>,  <wwebb at adni.net> wrote:
>> Actually, I susupect he wants to know how to use BIND to implement failover
>> for other servers, i.e. if the primary server fails, the DNS entry should
>> be updated automatically to point to the backup server's IP address.
>> Sorry, BIND doesn't have this type of functionality built-in.  Look into
>> products like Cisco Distributed Director.
>
>Does that take one of those  $50,000 Cisco 7500 series routers 
>that looks like a small refrigerator ? :-)

It's actually a 4000-series, I believe.

There are also other vendors who sell software-only solutions that are
similar.  That's why I said "products *like*" -- I just don't happen to
know the names of the other products.

>While perhaps not as elegant, what would be the drawbacks of this 
>DNS alternative:
>
>Place the primary DNS server on the same server as 
>the primary web server and the secondary DNS server  on the 
>same server as the secondary web server.  The primary DNS 
>server has an IP pointing to the primary web server. The secondary 
>dns server has an IP pointing it to the secondary  web server.  Both 
>dns records have very short TTL's.  
>
>If the primary goes down, then the primary dns server 
>fails as well and people would automatically be routed to the 
>secondary dns server, which would push them to the working 
>website on the secondary network..  The secondary server would 
>also que mail for the primary, until it came online. 
>
>Also if the both networks are up, and the primary's pipe becomes 
>congested, then the site would be served up by the secondary...
>
>I suspect this solution will not be found in any RFC ;-)

You seem to be assuming that secondary servers are only used when the
primary is unavailable.  The only difference between primary and secondary
servers is how they load the data (the primary has zone files that someone
edits manually, the secondary creates them by transfering the data from the
primary), not the order they're queried.  That's why we now refer to them
as "master" and "slave" -- the old terms implied a precedence that doesn't
actually exist.

Clients normally query whichever of the nameservers has given them the best
response time in the past.  If they've been about the same, it will choose
arbitrarily.

-- 
Barry Margolin, barmar at bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
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