Are underscores no longer supported in Bind 9.4?

Mark Andrews Mark_Andrews at
Sun Mar 4 13:32:27 UTC 2007

> Jake Morrisey wrote:
> > On 2/28/07, Mark Andrews <Mark_Andrews at> wrote:
> >>        No.  It's a 20 year old rule.  See RFC 952 and RFC 1123.
> >>        The later extends the syntax to allow leading digits.
> >>
> >>        Labels in hostnames are restricted to letters, digits and
> >>        hypens (LDH).
> > 
> > Thank you for the previous references.
> > As a followup are underscores allowed in CNAME records?  Are CNAME
> > records not bound by these RFCs?  I would think they would have the
> > same rules but the named-checkzone and the named-compilezone have no
> > problem with a CNAME having an underscore.
> > 
> CNAME's point to hostnames and therefore must follow the same rules. Are
> you asking about the label of the CNAME record? It should be the same as
> it acts as a hostname for all purposes. If it doesn't complain then it's
> probably a bug. I didn't see anything in any of the RFC's that makes
> that explicit but it would make sense.
> Danny

	CNAME's provide aliasing of domain names.  Hostnames are
	a subset of domain names.  If a CNAME is being used as
	a hostname then it needs to follow the rules in RFC 952 +
	RFC 1123.  If it is not being used as a hostname then it

	If people actually read and followed the RFC's then there
	wouldn't be all this discussion.  RFC 1035 says to use RFC
	952 syntax for hostnames.  RFC 1123 which came later, relaxed
	it slightly to allow leading digits.


	RFC 1035.

The DNS specifications attempt to be as general as possible in the rules
for constructing domain names.  The idea is that the name of any
existing object can be expressed as a domain name with minimal changes.

However, when assigning a domain name for an object, the prudent user
will select a name which satisfies both the rules of the domain system
and any existing rules for the object, whether these rules are published
or implied by existing programs.

For example, when naming a mail domain, the user should satisfy both the
rules of this memo and those in RFC-822.  When creating a new host name,
the old rules for HOSTS.TXT should be followed.  This avoids problems
when old software is converted to use domain names.

> Danny
> > Thanks,
> > -Jake
> > 
> > 
> > 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at

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