[ILUG] Resolvers and DNS

[Thomas Bridge]

On Tue, 27 Apr 1999, Kevin Dunleavy wrote:

> Colm Buckley wrote:

> > Given "salmon.maths", the same principle would apply.  It contains one
> > dot, which is less than two, so it applies the search paths again:
 
> > salmon.maths.tuatha.cs.tcd.ie.
> > salmon.maths.tuatha.org.
> > salmon.maths.cs.tcd.ie.
> > salmon.maths.tcd.ie.     -- this succeeds
> > salmon.maths.
 
> Does this mean that when you use domain names like slashdot.org or
> ibm.com (without the www) that you are doing unnecessary lookups in the
> tuatha/cd/tcd etc domains?

Depends on how you define "unneccesary".    There is of course always the
risk that the administrators of tcd.ie for some reason would have a
subdomain of slashdot.org.tcd.ie and you would then be taken there rather
than to the site you were actually trying to reach. [1]

> Does this have any performance overhead?

A little - but the chances are that the lookups you are doing in
"unneccessary" zones won't use up essential bandwidth as you'd probably be
making all those requests over ethernet (or if you're dialled up to the
net over your phone line and your providers ethernet).     As its not
outgoing bandwidth until the DNS starts looking outside the network,
there's no problem with the unneccessary lookups, as it'll only make one
request[2].

In any case the size of a DNS response packet would be fairly small.

[1] If the administrators of tcd.ie were to do such a thing however, they
should be taken out and told to read DNS & BIND where it specifically says
not to use TLD names as subdomains.

[2] Not exactly true, but if you really want to know more, look at DNS &
BIND.     It's an O'Reilly Book, and I did hear that the third edition was
due out soon.

Thomas Bridge			NOC Engineer, Medianet Ireland.
thomas at medianet.ie

"Spurs fans?  Try to think of it less as a cup final misses, more a Chas and
Dave comeback averted." - The Sun after Newcastle beat Spurs in the FA Cup
semi final.