[isp-dns] Re: Should I have to restart BIND for updates?

[Paul Robinson <prispdns@...]

On Wed, Nov 20, 2002 at 11:19:03AM -0600, Chris Clements wrote:
> 
> 
> 	We were not any sort of authority for that domain.  I should not have to
> restart DNS every time a completely unrelated domain changes hosts.  How
> would I even have known, had one customer not been trying to access the new
> host?  Perhaps my cache TTL should be shortened?


Such is the problem with caches, at least in DNS.  The only mechanism
available for determining whether a record has changed is to connect to the
authoritative server.  To do so defeats the purpose of caching the record in
the first place.  Instead, a zone owner is permitted to specify a per-record
TTL (or a per-zone default) to let recursive caches know how long to keep
information cached.

If that TTL is very low, there will be little benefit to caching the record.
If it is too high, then frequent visitors may not see DNS changes you make for 
a very long time, potentially making sites/services appear down.

Depending on implementation, you may be able to force-expire cached records 
sooner than their specified TTL, but I wouldn't recommend it.  Typically,
high traffic sites will either reduce the TTL of changing records in
advance, or continue to respond to traffic directed to the old, cached
address for at least $TTL seconds after the change.  If they take these
steps, you and your customer never notice the change.

Cheers,
Paul