Underscores in host names

Wed May 18 09:15:44 UTC 2005


> Mark,
> 
> Grump.
> 
> I used to be in the 952/1123 sect, but I have since reformed and  
> continue to do penance for my sins.
> 
> The "hostname is not a domain name" dodge is simply wrong.  If you  
> like, I can get a signed affadavit from the author of the DNS  
> specifications (assuming he's in the office tomorrow) to the effect  
> that it was always his intent that domain names be composed of any 8- 
> bit value.  That's the whole reason for length encoding the labels.   
> RFC 2181, for all its other warts, explicitly clarified this  
> particular issue.

	No one is saying that a domain name can't be any 8 bit value.

	A hostname is not a domainname.  It's all through RFC's 1033,
	RFC 1034 and RFC 1035.  There are references that make it clear
	that a domain name is not the same as a hostname.

	I quoted one of them.  I can find other references.

	Proctor&Gamble.com anyone?  That is the logical concusion of
	saying hostnames are arbitary 8 bit strings.
 
> The whole reason for check-names was because of very seriously broken  
> software that would allow shell meta-characters in in-addr.arpa  
> labels to do bad things.  I have come to the opinion that if such  
> software still exists, then the people who run that software deserve  
> what they get. Check-names was a bad idea that might have been  
> justified at the time, but pretending it remains justified by  
> 952/1123 has got to stop sometime.

	We tried hard to kill check-names.  The only reason it still
	exists is that people wouldn't move from BIND 8 without it.

	I havn't run with "check-names answer" enabled in years.
 
> However, that rant was mostly irrelevant.  Can you point to _ANY_  
> application, operating system, or anything else that has any issues  
> whatsoever with an "_" of all characters?

	The original query was about a OS / application that had
	problems with underscores.

	The point of RFC's is to promote interoperability.  People
	who attempt to name there machines with underscores either
	don't know better or don't care about interoperability or
	both.
	
	The simplest way to fix this is for application that
	configure hostnames, real or virtual, to reject by default
	illegal hostnames.  Apache should not allow virtual sites
	with illegal hostnames without explicit overrides.  Similarly
	for your favourite MTA, DNS server etc.  If people want to
	use them they need to know they are stepping out of the
	area where interoperability should occur.

	Note: SRV and Active Directory *both* depend on underscore
	not being legal in hostnames to keep their names spaces
	seperate from the hostname namespace.

	Half the anti-spam/DNS schemes depend upon underscore not
	being legal in a hostname.

	Mark

> Rgds,
> -drc
> 
> On May 17, 2005, at 6:08 PM, Mark Andrews wrote:
> >     RFC 952 and RFC 1123 describe what is currently legal
> >     in hostnames.
> >
> >     Underscore is NOT a legal character in a hostname.
> 
--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at isc.org