Re: DNS Question/Help

From: Lester B.
Sent on: Sunday, December 12, 2010 10:40 PM
I'm going to make some guesses here, for what they are worth.

> 1. When a DNS cache server needs to pull new records, does it pull individual
>    record types like all the MX records but not the A records, or does it
>    pull all the DNS records? ��(My theory is that it would pull all)

Not really following here.  A caching server has no special relation
with a nameserver, so it normally can't do a zone transfer, as between a
master and slave.  It can't know in advance what all the records in the
zone are.  I would think it must cache record by record as they are
returned from queries.

> 2. If you change the Nameservers for a domain name is it fair to say that you
>    would either get the old A records or the new ones and not have missing
>    records? Since the new DNS info would replace the old, I don't see how the
>    system would come back with a "missing" record, unless the new information
>    sent to the Authoritative server was messed up.

Changing nameservers operates outside the DNS protocol.  The only thing
specified is the protocol transfers, and the only wholesale transfer of
zone data is from master to slavei (AXFR).  How the master records are
loaded is up to implementation software, whether they are ASCII lines in
a flat file or SQL records or whatever.  I think zone transfers from
master to slave are atomic, but the atomicity and/or timing of other
operations loading the master or switching masters seems to be opaque.

I think the only way to know some result is to examine the nameserver
directly via a query tool like dig or nslookup, specifically querying
that @server.  Anything else, like the UI of a web page interface of a
nameserver management tool, is suspect.  If you are not directly
querying that specific server, then all sorts of caching and negative
caching effects may kick in.  I know I have updated a record via a web
page and not seen the actual nameservers settle down for some time,
usually at the next scheduled nameserver data refresh.  That was for a
bulk nameserver, like, not for a private nameserver as part
of a web host.

Hope this helps.



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