Path: utzoo!censor!geac!torsqnt!lethe!yunexus!davecb
From: davecb@yunexus.YorkU.CA (David Collier-Brown)
Newsgroups: comp.protocols.tcp-ip.domains
Subject: Re: Domain names in resource records (was: PTR records ...)
Message-ID: <20520@yunexus.YorkU.CA>
Date: 16 Jan 91 15:00:50 GMT
References: <1120@nikhefh.nikhef.nl>
Organization: York U. Computing Services
Lines: 57

e07@nikhefh.nikhef.nl (Eric Wassenaar) writes:
| RFC1034, 3.6.2 Aliases and canonical names, page 15, states:
| "Domain names in RRs which point to another name should always
| point at the primary name and not the alias."

| This means that in records like
| $ORIGIN your.domain.
| foo	IN	NS	bar.your.domain.
| foo	IN	MX	10 bar.your.domain.
| 'bar.your.domain' must NOT be a CNAME, but must have an A record.

  Interesting...  

  I've requested the exact opposite of our network manager in at least one
case:  mailhub.yorku.ca (a CNAME) is the target of an MX record
(cox.yorku.ca) because the machine that ``is'' mailhub at any given time is
the machine from which cox will get its mail.  
  Cox is **not** MX'd to nexus, despite the fact that cox mounts nexus' disk.
Cox mounts the disk of the mailhub of its department, whichever machine that
might be.

  This is in compliance with the ``write once'' rule, which says that
one has a single, authoritative source of any piece of information, and
others refer to it, not to a private copy. (Note that ``single'' can be
weakened in the case of a replicated, distributed database like the DNS)
  The write-once rule is not part of the broader internet mindset: it's
origin is Multics...


  I can see the advantages of not allowing an arbitrary number of levels of
indirection in the DNS, for at least the avoidance of loops.  However,
breaking this rule is neither detected nor diagnosed, which makes it
statistically probably that someone else has broken it, hopefully for an
equally justifiable reason.

  I wonder if the community can comment:
	1) do MX->CNAME->A record chains break any application
		using MX records?
	2) does any application or service-provider detect RR->RR loops?
	3) does any application break trying to follow RR->RR loops?

  I can comment on (3): berzerkley sendmail breaks on MX->MX, without even a loop.
It evaluates the first MX and stops there.


  I'd also like to know about other reasons than loops for this restriction.
As you might guess, I'd argue for weakening the requirement to ``no RR
record can point to an RR record of the same type'', claiming that the other
existing constraints prevent a loop of multiple types of RR records (eg,
CNAME->MX->CNAME is impossible).

--dave
-- 
David Collier-Brown,  | davecb@Nexus.YorkU.CA | lethe!dave
72 Abitibi Ave.,      | 
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