Literal IP address as domain name (was Re: Feedback on blacklist rule I plan to write)

["David F. Skoll" <df...@roaringpenguin.com>]

On Wed, 30 Oct 2013 20:07:16 +0000
RW <rw...@googlemail.com> wrote:

> But does anything actually say that 1.2.3.4 can't be treated as a
> hostname.

Sec. 4.1.2 of RFC 5321 allows this according to the BNF grammar.  But
I don't think the semantics are well-defined.

Both of these adhere to the grammar:

     example@1.2.3.4
     example@1.2.3.444

How are they to be interpreted?  1.2.3.4 is a legal IP address.  It's
also a legal "domain" according to the BNF, but is it a legal domain
name in the DNS?  RFC 1035 says a valid "label" in a domain
name has to start with a letter... but we all know that "3m.com" is a
valid domain name (or at least, that it resolves!)  Then RFC 2181 muddies
the water and says: 

   "Those [length] restrictions
   aside, any binary string whatever can be used as the label of any
   resource record. [...]
   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."

1.2.3.444 means... what?  It's not an IP address, and if we take
RFC 2181 at it's word, should we do MX and A lookups on "1.2.3.444"?

All in all, I would avoid email addresses that match the perl regex
[^@]+@\d+\.\d+\.\d+\.\d+

Regards,

David.