All about CNAME record

agentnnn at my-deja.com agentnnn at my-deja.com
Wed Jun 7 01:21:47 UTC 2000


Sorry for replying to old posts...so what reasons would someone want to
use a CNAME?  I can only think of a few mail issues but I may not even
be right about that.  Is it for organization puposes?

-Nathan


In article <393550CC.3E1C7C3 at daimlerchrysler.com>,
  Kevin Darcy <kcd at daimlerchrysler.com> wrote:
> mgourite at my-deja.com wrote:
>
> >   Hello,
> >
> >   I would like to have informations about the CNAME record :
> >
> >   1) Why we can't associate an alias at NS, SOA, CNAME, MX records ?
> > It's because, it's recommended by the RFC or it's a piece of advice
to
> > simplify DNS administrator's life.
>
> Today, it is generally considered *mandatory* for labels in the RDATA
of
> an RR to be canonical names rather than aliases, even though RFC 1034
> uses the term "should" -- back when 1034 was written, the IETF wasn't
> quite so picky about requirements-levels terms, i.e. should/must.
> RFC 2119 nailed all of this down eventually.
>
> The main rationale for the _de_facto_ mandate? Efficiency. CNAMEs
> increase packet size and require more nameserver -- and in some cases
> application -- processing than canonical names. While they may have
valid
> uses at an application level, i.e. it may be desirable for users to
know
> the same server by multiple names, there really is no legitimate
reason
> to use them in zonefile RDATA's, which are mostly transparent to the
> users and often maintained programmatically. So, in the absence of any
> visible benefit, do things the efficient way.
>
> >   2) What are the problems generated by this use of CNAME ? I heard
> > about problems with MTA (sendmail), old BIND versions, ...
>
> Yes, when following the RFC 974 logic for MX record processing,
mailers
> need to identify "self" references in a given MX list in order to
prevent
> loops. This is greatly complicated if CNAMEs are allowed as MX
targets.
> Note that sendmail is not the only mailer that is impacted by this:
> *any* mailer that uses MX records is going to have a tougher time
> discerning "self" references if CNAMEs are permitted as MX targets.
The
> problem is not implementation-specific.
>
> You are certainly welcome to lobby for a relaxation of this _de_facto_
> mandate, but I think the "namedroppers" list is probably a better
forum
> for that than "bind-users", since it's not at all BIND-specific.
>
> - Kevin
>
>


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