single-character host names

Mark Andrews Mark_Andrews at
Thu Feb 26 22:55:03 UTC 2009

In message <E740AAF63C0B4481BD2A542EA7B981C7 at>, "Mike Bernhardt" writes:
> Another wrinkle: RFC 1035 states "The labels must follow the rules for
> ARPANET host names.  They must start with a letter, end with a letter or
> digit, and have as interior characters only letters, digits, and hyphen.
> There are also some restrictions on the length.  Labels must be 63
> characters or less."
> RFC 1123 states " The syntax of a legal Internet host name was specified in
> RFC-952 [DNS:4].  One aspect of host name syntax is hereby changed: the
> restriction on the first character is relaxed to allow either a letter or a
> digit.  Host software MUST support this more liberal syntax.
> Host software MUST handle host names of up to 63 characters and SHOULD
> handle host names of up to 255 characters."
> It does not mention RFC 1035 label requirements also being updated, though
> obviously using digits as first characters is now quite common. It also
> implies that RFC952 is indeed the reference document to work from. Mighty
> confusing!

	RFC 1035 was giving general advice in that section.
	###$!@@!@#!$@! is a legal domain name.
	It doesn't fit the *preferred* syntax but there is NOTHING
	wrong with it.  It however isn't a legal hostname.

	The DNS is a distributed, replicated, loosely coherent
	database.  Entries in that database have a "domain name",
	"type" and "class".  You can store many things in that
	database.  One of those things is a hostname.  When you are
	storing hostnames you use the subset of domain names that
	match the syntax of hostnames.

	When you read RFC's you need to take care to see if what
	the author really meant when they say "domain name".  Often
	they are confusing it with "heirachical hostname".

> Mike
> -----Original Message-----
> From: Matthew Pounsett [mailto:matt at] 
> Sent: Wednesday, February 25, 2009 2:34 PM
> To: Evan Hunt
> Cc: Mike Bernhardt; bind-users at
> Subject: Re: single-character host names
> >The OP should be looking to RFC1035 section 2.3.1 for the  
> >specification for modern host names.
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> bind-users at
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at

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