Philip Hazel writes ("Re: several messages"):
> On Wed, 10 Jul 1996, m.hampson wrote:
...
> > > user@[155.198....]
> >
> > Oh bugger, and it was looking so good. We can probably work around the
> > bracket thing but we will always have to support @[] from the Internet.
>
> Sigh. I suppose I might have to do something about this one as well. We
> have never supported this form of addressing, not for the last 5 years,
> and nobody has yet complained! I suppose Exim ought to have the ability
> to do it, crazy though it is in today's Internet.
Well, it's not true that nobody has yet complained - I have. Perhaps
you don't remember it :-).
Furthermore, I have the RFCs to back me up; see below.
I have had cause to use this style of addressing when trying to get
through to systems when the DNS in their neighbourhood was very
broken, and in certain other abnormal cases.
I agree that users should be strongly discouraged from using this
form, but supporting it is an absolute must, and the authors of
RFC1123 `Requirements for Internet Hosts -- Application and Support'
agree with me on this point.
No mailer is ready for use in a production context unless it complies
with the MUSTs in 1123, and this is one of them.
Ian.
RFC 1123:
2.1 Host Names and Numbers
...
Whenever a user inputs the identity of an Internet host, it SHOULD
be possible to enter either (1) a host domain name or (2) an IP
address in dotted-decimal ("#.#.#.#") form. The host SHOULD check
the string syntactically for a dotted-decimal number before
looking it up in the Domain Name System.
DISCUSSION:
This last requirement is not intended to specify the complete
syntactic form for entering a dotted-decimal host number;
that is considered to be a user-interface issue. For
example, a dotted-decimal number must be enclosed within
"[ ]" brackets for SMTP mail (see Section 5.2.17). This
notation could be made universal within a host system,
simplifying the syntactic checking for a dotted-decimal
number.
If a dotted-decimal number can be entered without such
identifying delimiters, then a full syntactic check must be
made, because a segment of a host domain name is now allowed
to begin with a digit and could legally be entirely numeric
(see Section 6.1.2.4). However, a valid host name can never
have the dotted-decimal form #.#.#.#, since at least the
highest-level component label will be alphabetic.
...
5.2.17 Domain Literals: RFC-822 Section 6.2.3
A mailer MUST be able to accept and parse an Internet domain
literal whose content ("dtext"; see RFC-822) is a dotted-
decimal host address. This satisfies the requirement of
Section 2.1 for the case of mail.
An SMTP MUST accept and recognize a domain literal for any of
its own IP addresses.
RFC822:
3.4.6. BRACKETING CHARACTERS
...
o Square brackets ("[" and "]") are used to indicate the
presence of a domain-literal, which the appropriate
name-domain is to use directly, bypassing normal
name-resolution mechanisms.
...
6.2.3. DOMAIN TERMS
A domain-ref must be THE official name of a registry, network,
or host. It is a symbolic reference, within a name sub-
domain. At times, it is necessary to bypass standard mechan-
isms for resolving such references, using more primitive
information, such as a network host address rather than its
associated host name.
To permit such references, this standard provides the domain-
literal construct. Its contents must conform with the needs
of the sub-domain in which it is interpreted.
Domain-literals which refer to domains within the ARPA Inter-
net specify 32-bit Internet addresses, in four 8-bit fields
noted in decimal, as described in Request for Comments #820,
"Assigned Numbers." For example:
[10.0.3.19]
Note: THE USE OF DOMAIN-LITERALS IS STRONGLY DISCOURAGED. It
is permitted only as a means of bypassing temporary
system limitations, such as name tables which are not
complete.
The names of "top-level" domains, and the names of domains
under in the ARPA Internet, are registered with the Network
Information Center, SRI International, Menlo Park, California.
--
Ian Jackson ijackson@??? These opinions are my own.
Cambridge University Computer Laboratory, New Museums Site. + 44 1223 3 34676
Home: ijackson@??? Churchill College, CB3 0DS. + 44 1223 3 31579
PGP2 public key available. http://www.cl.cam.ac.uk/users/iwj10/
