What would be happen if one of two dns was down?

[Chris Cox]

On Sat, 2008-08-09 at 07:43 +0000, MontyRee wrote:
> Hello, all.
> 
> 
> I have operated two dns(primary and secondary) for one domain like below.
> 
> 
> example.com    IN       NS        ns1.example.com
> example.com    IN       NS        ns2.example.com
> 
> 
> and there was a event that ns1.example.com dns was down.
> As I know, if ns1 dns is down, all requests go to the ns2.example.com.

Depending on what "down" means, it could take some time before
the request is sent to ns2.  So there will likely be a delay, even
if not much (it will feel like forever to some users).

> 
> But  when ns1.example.com dns was down, actually some people can't lookup the domain.

Sounds like a configuration issue.  However realize there is a zone
cache and if ns2 is slaving zones of ns2 (typical bind master slave
scenario) and the zone cache expires, then ns2 will refuse to
trust the slaved zone it had... and thus nothing works.

> 
> What's the problem and how can I solve this problem?
> 
> If I adding some dns server like ns3,ns4.ns5, it would be helpful to solve this problem?

No if it's the cache issue... but that's if we're talking about
authoritative zones that you're serving.

If you're just needing some nameservers that answer recursively
for generic requests and many are of dubious quality, then adding
extra "might" help.... but I'd suspect something else is wrong
before I'd do that.