address-to-names

[Kevin Darcy]

Hongbo Shi wrote:

> Dear all,
>
>  I have thought about the PTR RR for long time.
>  I still not quite sure why PTR RR is based on the "one IP one domain"
>  consideration in [RFC1034].
>  I hope somebody can answer this question for me.

I'm not sure what you mean by "one IP one domain". If you mean the
misconception that a PTR record should have only 1 RR, then this is
specifically debunked in RFC 2181, section 10.2 ("PTR records").

> [RFC1034]
> cname & aliases:
>  "Most of these systems have a notion that one of the equivalent set of
>   names is the canonical or primary name and all others are aliases."
>
> PTR:
>  "Domain names in RRs which point at another name should always point a=
t
>  the primary name and not the alias.  This avoids extra indirections in
>  accessing information."
>
>  Furthermore a lot of persons regard the domain name used in the A reco=
rd
> as a cname. Is it correct? Is it misunderstanding? Is there some new RF=
Cs
> already permitted the consideration?

There is some confusion about CNAMEs and "canonical names". This too is
addressed in RFC 2181. See section 10.1.1 ("CNAME terminology").

>  And then based on the cosideration above, if the following A records e=
xist,
>
>  A.ISI.EDU    IN   A    10.0.0.52
>  B.ISI.EDU    IN   A    10.0.0.52
>  C.ISI.EDU    IN   A    10.0.0.52
>
>  of course "address-to-names" should exist.
>
>  52.0.0.10.IN-ADDR.ARPA  IN      PTR     A.ISI.EDU
>  52.0.0.10.IN-ADDR.ARPA  IN      PTR     B.ISI.EDU
>  52.0.0.10.IN-ADDR.ARPA  IN      PTR     C.ISI.EDU

There is no formal requirement that all 3 of these PTR RR's exist.
Administrators may choose to create any or none of them, or different PTR=
's
entirely.

In practice, no client software (as far as I know) looks beyond the first
PTR in a PTR RRset. So it's a waste to create the others. If you want to =
see
how wasteful this can become, trying doing a reverse lookup of 209.164.15=
.191
on the Internet sometime (credit goes to Peter H=E5kanson for discovering=
 this
abomination).


- Kevin