Failover

[jesk]

Hello,

im thinking about a failover setup of webservices at different locations via
DNS.
I got some questions about the possibilities of this:

1. how is "IN NS" cached and used by other bind nameservers if one of the NS
is down? f.e. the TLD server has two "IN NS" records for my zone, now a
nameserver is looking up this zone and will get this 2 records. first i
think its trying to resolv via the first nameserver of the replyorder, but
what would be if this one is down and not reachable, will the resolving
nameserver try to query via the second one a second time? what would be if
the first nameserver can succesfully answer, then will be cached by the
resolving nameserver, but then in the future of the life of the cached "IN
NS" record the nameserver will be down, is the second nameserver still in
the cache and the failover will work if this will happen?

2. is the only solution to get a global dns failover without the use of
routing protocols like BGP to use two or more nameservers at different
locations(AS or something else) which will then answer queries f.e. of
webservers with its own specific A-records? f.e. if nameserver A is down in
cause of a routing problem, then a resolver will query nameserver B(located
at a different provider) which then will answer a query for www.domain.tld
with a specific A-record which will be reachable, because its in the same
physically network.

3. if the "IN NS" failover is possible, whats about caching nameservers
which are caching A-records? are them also failover possible, if yes would
it be possible to return the A-records for the webserver of both locations
so that a client will try webserver A first and when not reachable webserver
B (i think its a implementation thing and too much risk)? or is the only
solution to create a zone with a TTL of zero?

thanks for any hints and explanations to get this fully understanding :)


best regards,
christian