Date: Sun, 12 Mar 2000 15:01:27 -0800 From: Doug Barton <Doug@gorean.org> To: Ryan Thompson <ryan@sasknow.com> Cc: Matthew Joseff <mjoseff@hellenco.com>, FreeBSD Questions List <freebsd-questions@FreeBSD.ORG> Subject: Re: CNAME vs A records (clarification) Message-ID: <38CC21C7.4216AC42@gorean.org> References: <Pine.BSF.4.21.0003121607290.7591-100000@ren.sasknow.com>
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I knew this post was coming as soon as I saw your post to the list.
What you're discussing are style issues. I wouldn't ever bring up the
topics you're discussing with someone new to DNS, they just get too far
into details that shouldn't be addressed with someone who doesn't even
know what A and CNAME records are.
Ryan Thompson wrote:
>
> Doug Barton wrote to Matthew Joseff:
>
> > Matthew Joseff wrote:
> > >
> > > Can someone correct and/or confirm my understanding of CNAME vs A records:
> > >
> > > CNAME should be used for a host that exists on (potentially) another
> > > server but uses that domain.
> > >
> > > A records should be used as an alias for a host but co-exists with other
> > > hosts.
> >
> > There is no such concept as "alias" in DNS. Erase it from your
> > mind.
>
> Sorry, Doug, but I'm afraid you are wrong. CNAME records are just
> that--aliases that point to a host's canonical name.
Show me the word "alias" in the definition of any RR in any RFC. The
fact that "DNS and BIND" chooses to use this extremely bad and
potentially confusing definition aside.
> To understand all of the implications, I suggest EVERYONE who uses BIND
> should pick up O'Reilly's DNS and BIND, 3rd edition.
I agree completely, and neglected to mention that in my post, thanks.
> Consider this, Doug (snippets from a zone transfer for freebsd.org):
Sorry, "someone else is doing it that way" is never a sound argument
for any aspect of system administration. I can point you to thousands of
examples of bad configurations. That doesn't make any of them right.
However, a quick perusal of your examples indicates to me that
freebsd.org is using CNAME's for one of their valid uses, namely
pointing a hostname that may have moved at it's new home. Beyond that,
I'm not going to comment other than to say look again at the first
sentence in this paragraph.
> > I don't really understand your example, or what you're trying to get
> > at. But there is no reason at all you can't have multiple A records in
> > multiple zones pointing at the same IP address. If I've missed the
> > essence of your question, feel free to try and restate it...
>
> What about reverse lookups?
Every IP should reverse to at least one hostname, usually the "main"
hostname for that machine. You can also add more than one PTR record for
the IP if needed. Other than the fact that IP's should reverse so that
they're easier to track down, mail and ftp are really the only
applications anymore that really NEED solid forward <=> reverse
mappings.
Beyond this we're talking DNS style issues, which are outside the scope
of this list. I will restate my original point, namely that people who
don't understand what CNAME's are shouldn't use them. There is no DNS
application in the world that requires CNAME's, and they can cause a lot
more harm than good. That doesn't mean that for an experienced DNS
Administrator they can't be useful, I use them all the time. But this
isn't a DNS list, so I try to keep it simple. You're free to offer any
advice you want, just be sure you're not making an already bad situation
worse.
Doug
--
"Welcome to the desert of the real."
- Laurence Fishburne as Morpheus, "The Matrix"
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