[wplug] OT - request for comment on an idea

[Chester R. Hosey]

On Fri, 2005-06-03 at 14:52 -0400, Poyner, Brandon wrote:
> The concept of primary and secondary in name servers only applies to
> which server has the master copy of the zone record.  When a 3rd party
> name server does a recursive lookup for your domain it is going to get a
> list of NS records for your domain and choose a name server at "random"
> (it's not entirely random but that's another matter).  You can't set a
> priority or make one name server more authoritative than another.  
> 
> As for your question about caching records, the best you can do there is
> set the time to live (TTL) as low as you can tolerate.  Set it too low
> and could have many DNS requests to deal with, and possibly more failed
> or slower DNS lookups.  Another factor is that some name servers will
> completely ignore your specified TTL and cache answers for much longer
> (sometimes days), there is nothing you can do about that.  They're
> typically far and few between however.
> 
> Brandon Poyner
> Network Engineer III
> CCAC - College Office
> 412-237-3086

Thank you, Brandon, for clarifying my explanation of the primary-
secondary distinction.

I would add to your statements the fact that if you drop TTL too much
and ISPs don't ignore your limits, they will have to query your servers
instead of their local cache more often, which will introduce some level
of user delay. There are drawbacks to a low TTL other than your own
increased traffic, and I'd rather set a high TTL and know that I can't
depend on fast propagation than set a low one.

A few years ago I did some migration of some heavily frequented services
from a deprecated machine to a new one, and was quite surprised by how
long it took requests to the old machine to drop to zero. Had I made
assumptions I wouldn't have kept checking log files to make sure that
service wasn't interrupted. It is with this experience in mind that I
caution against relying on TTL to indicate anything other than the
absolute minimum time to expect results to be cached.

Chet