A-label definition

[Mark Andrews]

> Mark Andrews wrote:
> 
> > Hostnames are allowed to be up to 255 characters in length
> > and labels up to 63 characters (RFC 1123).
>  
> > The DNS is only capable of representing hostnames that are
> > up to 253 characters in length which is what you get back
> > to when you take the maximal wire format and convert it
> > back into a presentation format that is only LDH for the
> > labels.
> 
> Right, we talked temporarily about different things.  For the
> at the moment existing www.example.com I'd sometimes say that
> www is the host name, and www.example.com the FQDN.  And at
> other times I'd say that this is host www.example.com, which
> is clearly inconsistent.  You had www.example.com (the FQDN)
> in mind, I had www (the label) in mind.
> 
> Yes, I'm aware of the 253 limit, RFC 4408 mentions it.  And
> of course the 255 in (among others) 2821bis, which doesn't
> elaborate how this can ever work if DNS doesn't support it.
> 
> It used to be irrelevant, but with those long xn--... labels
> it might be something "draft-idnabis-952bis" should mention.
> 
> > Domain names may have a trailing period.
> 
> Even in RFC 4408, after my appeal about last minute changes
> in AUTH48 met Dave Null.  Nobody ever said that this caused
> a problem, fortunately it turned out that I was paranoid.
> 
> > For a arbitary domain name the presentation format is 
> > anything up to ~1k in length.
> 
> Now you've lost me, 253 or 255 are not ~1K.  Is what you are
> talking about the maximal length expected to work in an
> /etc/hosts file ?

	As I said, a arbitrary *domain name*.  I did not say an
	arbitary *hostname*.

	Hostname and domain names are NOT the same things.  There
	are lots of RFC's which confuse the two.  There are lots
	of people which confuse the two.

	A domain name is the owner of data in the DNS.
	A hostname is a name which is restricted to LDH labels and
	has associated addresses.

	_sip._udp.isc.org is a domain name, it is not a hostname.
	isc.org is a domain name, it is also a hostname.

	\000.example.net is a domain name.  It is not a hostname.
	The first label in wire format is "0x01 0x00", length 1
	with a octet of all zero bits.

	Mark

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