A-label definition

[Frank Ellermann]

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 ?

 Frank