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This is a good discussion with lots of useful information.
Re: Honest Base Config
From: Peter Nilsson
Date: Friday, March 10, 2000
Time: 7:59:00 am
I'm just surprised that nobody yet has mentioned the reverse lookups.
Depending on the type of service in use, the server may do reverse lookups
_for every unique user_ that visits the site. This would cause a lot more
traffic "in the other direction" than external DNS servers looking for that
simple, cachable A-record, or that zone transfer that occurs maybe once a
day.
/petern
> ----------
> From: Global Homes Webmaster
> Reply To: QuickDNS Talk
> Sent: fredag 10 mars 2000 01.06
> To: QuickDNS Talk
> Subject: Re: Honest Base Config
>
> On 03/09/00 at 17:24, Clarence Kwei wrote:
>
> > Maybe I'm not fully understanding DNS traffic... please let me know if
> I'm
> > off-base with this...
> >
> > I thought that every time a user visits your site, your DNS server gets
> > queried for information. That happens only once per unique visitor since
> > after the first successful query, the information is pretty much set for
> the
> > entire session - that is future hits from the user during the session do
> not
> > need repeated DNS queries.
>
> Close, but not quite. Users (i.e. web browsers) should never be querying
> your
> name server directly. When someone visits your web site, their browser
> asks
> the name server that _their_ system is configured to use (usually their
> ISP's
> name server or a name server on their LAN) to resolve the domain name of
> the
> web site. If their local name server already has the relevant information
> cached (from a previous look-up), it will respond to the user with the
> cached
> info without having to send a query to your name server. If the user's
> name
> server does _not_ already have the info cached, then it will send a query
> to
> your name server. This is where the TTL comes into play. The TTL is the
> maximum amount of time that a name server can keep a given record in its
> cache.
>
> For example, say you have a web site at www.domain.com, and the SOA record
> for
> the domain.com. zone has a TTL of 1 day. Consider what happens when User A
> and
> User B, who both use the same ISP, visit your site. Suppose User A makes a
> visit to your site at 8:00 a.m. His browser will query his ISP's name
> server
> to resolve 'www.domain.com'. The ISP's name server will send a query to
> your
> name server, save the response in its cache, and send the answer to User
> A's
> browser (User A's computer should also cache the response so that it won't
> have to keep asking the ISP name server for it). Now suppose that User B
> visits your site at 10 p.m. that evening. User B's browser will query the
> ISP's name server to resolve 'www.domain.com'. Since the ISP name server
> looked up that name earlier in the day, and the TTL (1 day) has not
> expired,
> it will still have the answer in its cache and will not need to query your
> name server. So the ISP name server will be able to send a response to
> User B
> without any traffic to your name server.
>
> The above is somewhat simplified, but what it boils down to is that the
> number
> of requests your name server receives should be significantly smaller than
> the
> number of hits your web server receives. In my example, your name server
> would
> have received only one request for two visits to your web site. In the
> real
> world, medium to large ISP's have name servers caching information for
> hundreds or thousands of users, so the ratio of DNS queries to web hits
> for a
> popular site could be very small indeed.
>
> > That's one part of DNS traffic, the other part is SOA information, which
> I
> > really don't have a clue how it works. Do the main DNS servers hit your
> DNS
> > server according to the refresh value in your SOA record? Is it the
> > Expiration or the TTL value?
>
> I'm not sure what you mean by 'the main DNS servers.' Name servers should
> cache information for no longer than the TTL value. Longer TTLs will
> result in
> less traffic to your name server, since other servers can serve your data
> from
> their caches for longer periods of time. What you set your TTLs to depends
> on
> how often your DNS data changes. For a domain that's relatively stable,
> you'll
> probably want your TTLs to be a day or two. For a domain that sees
> frequent
> changes, you may want shorter TTLs. For a domain that's about to undergo a
> one-time major change, you may want to temporarily shorten the TTL in
> order to
> minimize your 'down-time' when the change occurs. But that's another
> discussion... 8^]
>
> The 'refresh' and 'expire' values in the SOA are used by the secondary
> name
> servers for a given zone. The 'refresh' is how often the secondary should
> check with the primary to see if the zone data it has is up-to-date. The
> 'expire' value is the maximum amount of time a secondary server should
> hold
> onto data when the primary is unreachable.
>
> Christopher Bort
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