"-" and "_" in domain name

[VERBEEK, Francois]

Please find below RFC2181 (http://www.ietf.org/rfc/rfc2181.txt?number=2181) section 11.
BTW Windows 2000 and active directory makes 'underscore' domains very important (in internal networks).


Note that for Internet host names this is restricted by RFCs 952 and 1123.


François Verbeek
ING South West Europe
BIS/ITI/HWE/NTW
Network Design Services
tel: +32 (0)2 738 22 83



11. Name syntax

   Occasionally it is assumed that the Domain Name System serves only
   the purpose of mapping Internet host names to data, and mapping
   Internet addresses to host names.  This is not correct, the DNS is a
   general (if somewhat limited) hierarchical database, and can store
   almost any kind of data, for almost any purpose.

   The DNS itself places only one restriction on the particular labels
   that can be used to identify resource records.  That one restriction
   relates to the length of the label and the full name.  The length of
   any one label is limited to between 1 and 63 octets.  A full domain
   name is limited to 255 octets (including the separators).  The zero
   length full name is defined as representing the root of the DNS tree,
   and is typically written and displayed as ".".  Those restrictions
   aside, any binary string whatever can be used as the label of any
   resource record.  Similarly, any binary string can serve as the value
   of any record that includes a domain name as some or all of its value
   (SOA, NS, MX, PTR, CNAME, and any others that may be added).
   Implementations of the DNS protocols must not place any restrictions
   on the labels that can be used.  In particular, DNS servers must not
   refuse to serve a zone because it contains labels that might not be
   acceptable to some DNS client programs.  A DNS server may be
   configurable to issue warnings when loading, or even to refuse to
   load, a primary zone containing labels that might be considered
   questionable, however this should not happen by default.

   Note however, that the various applications that make use of DNS data
   can have restrictions imposed on what particular values are
   acceptable in their environment.  For example, that any binary label
   can have an MX record does not imply that any binary name can be used
   as the host part of an e-mail address.  Clients of the DNS can impose
   whatever restrictions are appropriate to their circumstances on the
   values they use as keys for DNS lookup requests, and on the values
   returned by the DNS.  If the client has such restrictions, it is
   solely responsible for validating the data from the DNS to ensure
   that it conforms before it makes any use of that data.




	-----Original Message-----
	From:	Frank Reppin [SMTP:shauwn at relay.boerde.de]
	Sent:	vendredi 14 février 2003 06:12
	To:	spin022001
	Cc:	comp-protocols-dns-bind at isc.org
	Subject:	Re: "-" and "_" in domain name

	On 13 Feb 2003, spin022001 wrote:

	Hi,

	[...]
	> Are "-" and "_" valid in domain name? In which RFC I can find this inform=
	> ation?

	'_' not valid, at least not mentioned to be valid :)
	'-' valid

	as of:

	http://www.faqs.org/rfcs/rfc952.html

	> Thanks!
	> Frank

	Best regards,

	Frank Reppin

	-- 

	Heidestr. 15
	39112 Magdeburg
	Germany






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