[OT] Reverse DNS

[Michael H. Warfield]

On Fri, Jun 25, 2004 at 04:05:56PM -0400, Mark Haney wrote:
> I'm having a discussion with my boss over reverse DNS and I want to know  
> if my logic is correct.  Who exactly resolves the reverse DNS queries?  I  
> say it's the name server that serves up the regular DNS, but my boss  
> insists it is our ISP which isn't our primary DNS provider.  Is there any  
> DNS FAQ's that explain this.  I thought I knew how this worked, but now  
> I'm confused.

	Your boss is probably closer to right.  Unless your ISP has
delegated the reverse zone to you, then they probably control the
reverse look-ups.  It COULD be the same name server if you had both
your forward domain and your reverse pointer zone in the in-addr.arpa.
zone served by the same name server, but nothing requires it.  But
it doesn't HAVE to be your ISP either.  They can delegate it to you,
generally if you have a big enough block (/24 or better).  Blocks
of /25 or smaller get stickier to delegate and the ISP probably would
prefer to manage it themselves.

	To reverse lookup an IPv4 address, you reverse the octets and
append the .in-addr.arpa. domain to it and do a type PTR lookup (which
"host" does implicitly when you give it an IPv4 address).

[mhw at alcove mhw]$ host alcove.wittsend.com
alcove.wittsend.com has address 130.205.12.10

[mhw at alcove mhw]$ host 130.205.12.10
10.12.205.130.in-addr.arpa domain name pointer alcove.wittsend.com.

	Sooo...  Who has the name servers for those zones (drop the last
component and look up the NS records):

[mhw at alcove mhw]$ host -t NS wittsend.com.
wittsend.com name server banshee.wittsend.com.
wittsend.com name server ehecatl.iss.net.
wittsend.com name server ns.commandcorp.com.

[mhw at alcove mhw]$ host -t NS 12.205.130.in-addr.arpa
12.205.130.in-addr.arpa name server banshee.wittsend.com.
12.205.130.in-addr.arpa name server ns.commandcorp.com.

	Not quite the same.  Close, in my case, since I do control both
the domain and the netblock so I control both the forward and reverse
lookup zones.

	IPv6 reverse lookups get even groddier...  Reverse the nibbles...
All 32 of them...

> -- 
> Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

> Mark Haney
> Network, Database and Systems Administration
> DoctorDirectory.com Inc.

	Mike
-- 
 Michael H. Warfield    |  (770) 985-6132   |  mhw at WittsEnd.com
  /\/\|=mhw=|\/\/       |  (678) 463-0932   |  http://www.wittsend.com/mhw/
  NIC whois:  MHW9      |  An optimist believes we live in the best of all
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