[NTLUG:Discuss] "Back up dns server"?

[MadHat Unspecific]

Chris Cox wrote:
> m m wrote:
> ....
>> My questions is:
>>  
>> How can I have a "back up" (web) server (at location A city) take over the 
>> job when the primary web server (at location B city) can not reachable
>> (because of firewall, power down ...) by using same domain name.
> 
> This is VERY hard to do.  Especially if it's a "job", that is an
> inflight session with state information.  Easier if you just want
> a different non-related web server to answer back and the site
> is 100% static content, or even dynamic content where you have
> some kind of mirroring happening to keep them insync (and don't
> mind if they are a tad out of sync if one goes down).
> 
> The problem with trying to handle this with DNS is caching.
> If everyone is caching www as 1.2.3.4 as your web server address, nobody
> will be able to reach you if www changes to 4.3.2.1 since everyone
> thinks your at 1.2.3.4.  You could set a really low value of time to
> live on your DNS for other to cache, but it's generally not nice
> to have extremely low TTLs.... defeats the scalability of DNS.
> 
> If you just need load balancing or failover across sites, you
> will have some kind of front end that redirects appropriately
> (btw, the frontend can NEVER go down.... that's just life).
> With a frontend, even clustering and session management might
> be possible.  Somebody already mentioned one kind of frontend...
> a "proxy".  But likewise, that "proxy" can't go down, or you
> still have the original problem.

Akamai makes shitloads doing the load balancing and availability.  That 
is who Yahoo and other major plays use to offload traffic.  Akamai 
answers for www.yahoo.com and redirects to one of their server farms, 
the one logically closest to you and the one with the best response time 
and  what ever other details you choose.  It's a great (and expensive) 
service.  Yahoo syncs data/states between colos on a regular basis and 
if one disappears they are not guaranteed to be synced, but they will be 
close.  There is no perfect answer.