Re: Valid charecter set in DNS

["Eric A. Hall" <ehall@ehsco.com>]

dhiraj Dhiraj wrote:

>                I have a question regarding the valid character set in
> DNS. I have seen RFC 1034, 1123, 2181. It seems RFC 2181 removes the
> restrictions of RFC 1034,1123 which says that only  letters, digits, and
> hyphen are allowed. I wanted to know whether this interpretation is
> correct or not

The restriction has always been on the names that applications use, rather
than on the data that DNS can provide. RFC 2181 doesn't change the rules
so much as it clarifies the distinction.

> what are the applications that require other characters? I am aware of
> that underscores are required bcoz of the SRV RR.

Email addresses are often stored in DNS labels (as data to the SOA and RP
resource records), and the allowable syntax for the local-part element of
an email address is significantly greater than the allowable syntax for
hostnames.

As another example, TXT resource records do not reference hosts, and
therefore are not subject to the hostname rules. Any characters can be
provided as the owner domain name of a TXT RR and it will be fully legal.

Another situation which can result in funky names getting stored in DNS is
with NetBIOS names (specifically, those NetBIOS names which are not
encoded according to STD 19).

> RFC 1034 section 3.5: The labels must follow the rules for ARPANET host
> names.

That is a reference to the hostname rules (RFC 952, updated by 1123).
However, it is not the only rule for data in the DNS. See RFC 1035,
section 3.1. Name space definitions:

 | Although labels can contain any 8 bit values in octets that make up a
 | label, it is strongly recommended that labels follow the preferred
 | syntax described elsewhere in this memo, which is compatible with
 | existing host naming conventions.

RFC 2181 provides the necessary additional clarification.

-- 
Eric A. Hall                                        http://www.ehsco.com/
Internet Core Protocols          http://www.oreilly.com/catalog/coreprot/