[NTLUG:Discuss] Use DNS for redundant geographic sites?

[Patrick R. Michaud]

On Sun, Nov 19, 2006 at 10:52:47PM -0600, Neil Aggarwal wrote:
> Hello:
> I am trying to figure out a way to create a web site
> that is multi-homed geographically.
> ...
> Machine A will run:
> 	dnsA.jammconsulting.com (DNS server)
> 	webA.jammconsulting.com (Web Server)
> 
> Machine B will run:
> 	dnsB.jammconsulting.com (DNS server)
> 	webB.jammconsulting.com (Web Server)
> ...
> dnsA will always resolve www.jammconsultng.com to 
> the IP address of webA and dnsB will always resolve
> www.jammconsulting.com to the IP address of webB.
> 
> This way, if either machine goes down, the machine
> will not respond to DNS queries.  The DNS will query
> the other machine and all traffic will go to it.
> ...
> Will DNS clients handle a situation where the 
> nameservers give different responses?

Depends on what "handle a situation" means.  Does this
mean that the client will treat the differing responses
as being the same server?  Depends on the client.

> Are there other downsides I have not considered?

What about DNS caching?  Most domains allow DNS entries
to be cached for very long periods of time (a week or so),
so if one of the machines goes down, then it's not
as though existing clients will immediately redirect
to the new host.  (New clients that haven't looked
up the www.jammconsulting.com domain will of course
get only the address for the one machine that is up.)

Pm