delegation - pri and sec dns

John Tan d_name at hotmail.com
Wed Jul 26 00:45:29 UTC 2000


Hi Nishal,

Thx for your answer and your time.

best rgds,


>From: Nishal Goburdhan <nishal at is.co.za>
>To: John Tan <d_name at hotmail.com>
>CC: bind-users at isc.org, phil46 at pacific.net.sg
>Subject: Re: delegation - pri and sec dns
>Date: Tue, 25 Jul 2000 14:02:48 +0200
>
>
>
>taken from www.acmebw.com/askmrdns/ - you will find answers to most dns 
>questions there, in one form or another, if you try the search utility.
>
>
> > Hi all,
> >
> > just would like to ask a simple question :
> >
> > do the primary and secondary authoratative DNSes 'share' the load in 
>that
> > sometimes the pri dns gets hit or the sec dns gets 'hit' depending on 
>server
> > response time. OR is it the primary always gets the request and the
> > secondary gets the request only if the named daemon on the primary is 
>down.
> >
> > appreciate any helpful answers. Appreciate if u reply all to
> > phil46 at pacific.net.sg as well.
> >
> > thx.
>
>
>The mirror site you refer to is a "secondary" (the older term) or a "slave"
>(the newer term).  The authoritative name servers for a zone--the primary
>master and the slaves--are advertised to other name servers via NS (name
>server) records.   (Actually, there isn't always an NS record for the
>primary master, but that's another question and another answer.)
>
>A name server looking up a name first needs the list of NS records for the
>zone the name belongs to.  The name server might already have the NS
>records in its cache or it will retrieve them in the course of answering
>this query.  These multiple NS records are the heart of your question: How
>does the querying name server decide which NS record to use, which
>authoritative name server to ask?  Mr. DNS is glad you asked.
>
>The querying name server calculates and stores a round trip time (RTT) for
>each name server for a zone (i.e., for each NS record).  The RTT is the
>time--in milliseconds--that it takes that name server to respond to
>queries.  When faced with multiple sources of information about a zone in
>the form of multiple NS records, the querying name server asks the name
>server with the lowest RTT.  In the case of a tie, it picks among them at
>random.  After a name server is queried, the querying name server updates
>its RTT.
>
>When a name server first caches a list of NS records, the RTT for all of
>them is zero.  Because it chooses among name servers with equal RTTs at
>random, each name server will be queried once.	From then on, it favors the
>name server with the lowest RTT.  But what if something happens to that
>name server with the lowest RTT?  Fortunately, whenever a querying name
>server consults the name server with the lowest RTT, it decrements all the
>other name server's RTTs a little bit.	Over time, the other RTTs creep
>down and will eventually be queried.  Of course, if they are slow to
>respond, their RTT goes back up and they're not consulted again for a
>while.
>
>So to answer your question, it isn't just the primary master who is
>consulted--all the authoritative name servers for a zone are consulted by
>other name servers.  A querying name server will eventually favor the
>authoritative name server which responds the fastest.
>
>Mr. DNS
>
>
>

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