Re: Underscores in host names

[Mark Andrews <Mark_Andrews () isc org>]

In article <1116377042.592906.137650 () g44g2000cwa googlegroups com> you write:
> Hello all.
> We have a client containing an underscore in the email address domain
> name.  Our email server rejects it because of it's violation of the RFC
> standard.  This individuals claim is that he doesn't have problems
> anywhere else and if this is going to be a problem he's "going to take
> his business elsewhere"!
>
> I understand it's a violation of the standard, but does it pose a
> security hole to the email server to allow this sort of mail?
>
> Thanks

        RFC 952 and RFC 1123 describe what is currently legal
        in hostnames.

        Underscore is NOT a legal character in a hostname.

        Before anyone says that domain names allow underscore which
        they do.

        RFC 1034 Section 3.3

For hosts, the mapping depends on the existing syntax for host names
which is a subset of the usual text representation for domain names,  
together with RR formats for describing host addresses, etc.  Because we
need a reliable inverse mapping from address to host name, a special
mapping for addresses into the IN-ADDR.ARPA domain is also defined.

        Mail domains follow the same rules as for hostnames.  RFC
        821 and its replacement RFC 2821 havn't extended the syntax
        to include underscores.

        Mark