Silly DNS question

[Ted Roche]

On 01/22/2010 11:50 AM, Thomas Charron wrote:
>    Is an _ allowed in a DNS name?
>
>    I didn't think so, and my home DNS proxy doesn't think so, but other
> networks seem fine with it.
>
> http://www.thingiverse.com/image:8662
>
>    Above is an example, where the image is stored by amazon at
> http://thingiverse_beta.s3.amazonaws.com/renders/fe/2a/15/49/75/0.5mm_single_wall_calibration_piece_display_medium.jpg
>
>    I sent Makerbot an email about the use of the _, but I just wanted
> to make sure I wasn't wrong that _'s in a domain name aren't allowed,
> as their reserved for special use.
>
>    
Underscore appears to be allowed in DNS, however, it does not appear to 
be legal in a hostname. From Wikipedia's interpretation of the RFCs:

"The Internet standards (Request for Comments 
<http://en.wikipedia.org/wiki/Request_for_Comments>) for protocols 
mandate that component hostname labels may contain only the ASCII 
<http://en.wikipedia.org/wiki/ASCII> letters 'a' through 'z' (in a 
case-insensitive manner), the digits '0' through '9', and the hyphen. 
The original specification of hostnames in RFC 952 
<http://tools.ietf.org/html/rfc952>, mandated that labels could not 
start with a digit or with a hyphen, and must not end with a hyphen. 
However, a subsequent specification (RFC 1123 
<http://tools.ietf.org/html/rfc1123>) permitted hostname labels to start 
with digits. No other symbols, punctuation characters, or blank spaces 
are permitted.

"While a hostname may not contain other characters, such as the 
underscore character (/_/), names entered into the DNS, may contain the 
underscore. Systems such as DomainKeys 
<http://en.wikipedia.org/wiki/DomainKeys> and service records 
<http://en.wikipedia.org/wiki/SRV_record> use the underscore as a means 
to assure that their special character is not confused with hostnames. 
For example, |_http._sctp.www.example.com| specifies a service pointer 
for an SCTP capable webserver host (www) in the domain example.com.

"A notable example of non-compliance with this specification, Windows 
systems often use underscores in hostnames."

Entertaining reading. From http://en.wikipedia.org/wiki/Hostname

-- 
Ted Roche
Ted Roche&  Associates, LLC
http://www.tedroche.com

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