Underscores in host names

Mark Andrews Mark_Andrews at isc.org
Wed May 18 01:08:03 UTC 2005


In article <1116377042.592906.137650 at 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



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