[dns-operations] about the underline in hostname

Mark Andrews marka at isc.org
Thu May 29 22:01:21 UTC 2014

In message <88F4B55A-26A7-491E-AE4E-B8F40EF495CC at rfc1035.com>, Jim Reid writes:
> On 29 May 2014, at 18:56, wbrown at e1b.org wrote:
> > Out of curiosity, where did the prohibition of underscores in host names 
> > come from.  I'm sure there's a historical reason for it, but I've never 
> > heard it.  Or is it really as simple as "RFC 952 only listed thirty seven 
> > characters"
> I believe this was a limitation of the (pre?) NCP-era ARPANET and may well ha
> ve been a side-effect of the capabilities of operating systems from that era.
>  See RFCs 226 and 229.

One needs to decide what forms a name and what doesn't.  Is $ part
of a name.  Is @ part of a name.  Etc.

Letters, digits and hyphen let you express labels with joined words.
In English if you are joining words you use a hyphen.  It was logical
to join words using a hyphen to form labels from that.  Additionally
when you underscore a name you loose the distinction of whether a
underscore is part of a name or not.  Many current users have never
used equipment where this distinction is lost.  With modern displays
you can have a underscore with a underline.

For some reason someone decided to use underscore rather than hyphen
to join words when making a labels despite hyphen being there to
perform that role.  This took off in the DOS/Windows world.  What
didn't help was software that didn't sanity check names.


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Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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