[Pdns-users] varios questions on how a DNS resolver would react.

[Marc Haber]

On Wed, Jun 19, 2013 at 09:50:42PM -0600, Alex Flex wrote:
> a.)  Suppose I have  a domain name example.org it is hosted under
> the nameservers: ns1.dns.com and ns2.dns.com. Suddenly ns1.dns.com
> starts lagging severly (not down) / or is completely down. How well
> do resolvers coop with this? Do they query both at the same time ,
> or randomly choose and wait ? If so how much would they wait
> usually?

They choose and wait. A couple of seconds. Noticeable fo clients, but
it's only the first query as the resolver will cache the answer.

> b.) Same scenario as above but this time ns2.dns.com is
> misconfigured or the zone for example.org is erased giving a non
> existing domain or another possible error along that line. Would
> resolvers try to query the second nameserver and try?

An authoritative NXDOMAIN is treated as such and the second server is
not queried. That negative answer is also cached by the resolver
("negative caching").

If you just remove the data and leave the server configured for that
domain, it's going to return SERVFAIL, which causes a resolver to
query the other servers.  This is the really bad case because if all
your servers are broken the same way (which frequently happens if you
use non-DNS replication) you'll have an uncached n-way timeout.

Greetings
Marc

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