Re: Last Call: On-Demand Mail Relay (ODMR) to Proposed Standard

[Graham Klyne <GK@xxxxxxxxxxxxxx>]

While RFC 2181 clarifies the intent that the DNS resolution service can
handle names containing arbitrary characters, it also recognizes that
particular applications can constrain the form of name allowed.

Specifically, RFC 1123 (sections 6.1.3.5 and 2.1) and RFC 952 jointly
indicate the restricted form indicated below as the valid form for Internet
host names.  I attach relevant excerts at the end of this message.

In particular, the examples you give are not valid host/domain names, even
though they are valid as DNS resolution service keys.

#g

At 18:03 07/04/99 -0700, Randy Bush wrote:
>    [ABNF] for ATRN:
>    atrn          = "ATRN" [domain *("," domain)]
>    domain        = sub-domain 1*("." sub-domain)
>    sub-domain    = (ALPHA / DIGIT) *(ldh-str)
>    ldh-str       = *(ALPHA / DIGIT / "-") (ALPHA / DIGIT)
>
>this precludes use of this protocol to pick up mail for perfectly valid
>domains, e.g. feen_bar.foo, let alone much more interesting cases with
>which i could make you ill.  or, to be more politically correct, for
>domain names not representable in 7-bit ascii, such as göran.com.
>
>see rfc 2181 sec 11.
>
>randy
>



Citations:
----------

>From RFC 2181, section 11:

   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.



>From RFC 1123, section 2.1:

   2.1  Host Names and Numbers

      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.


>From RFC 1123, section 6.1.3.5:

                 The DNS defines domain name syntax very generally -- a

             string of labels each containing up to 63 8-bit octets,

          separated by dots, and with a maximum total of 255

  octets.  Particular applications of the DNS are

permitted to further constrain the syntax of the domain

names they use, although the DNS deployment has led to

some applications allowing more general names.  In

particular, Section 2.1 of this document liberalizes

slightly the syntax of a legal Internet host name that
                 was
defined in RFC-952 [DNS:4].


>From RFC 952:

   1. A "name" (Net, Host, Gateway, or Domain name) is a text string up

to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
   sign
(-), and period (.).  Note that periods are only allowed when
   they serve
to delimit components of "domain style names". (See
   RFC-921, "Domain
Name System Implementation Schedule", for
   background).  No blank or
space characters are permitted as part of a
   name. No distinction is made
between upper and lower case.  The first
   character must be an alpha
character.  The last character must not be
   a minus sign or period.  A
host which serves as a GATEWAY should have
   "-GATEWAY" or "-GW" as part
of its name.  Hosts which do not serve as
   Internet gateways should not
use "-GATEWAY" and "-GW" as part of
   their names. A host which is a TAC
should have "-TAC" as the last
   part of its host name, if it is a DoD
host.  Single character names
   or nicknames are not allowed.

------------
Graham Klyne
(GK@xxxxxxx)