what's a valid domain name?

Kevin Darcy kcd at chrysler.com
Mon Jan 31 23:00:05 UTC 2011

           "Dotted hostnames" is a term coined by certain limited 
DNS-management tools that strive to split up fully-qualified DNS names 
into a "hostname" part and a "zone" (sometimes referred to as "domain") 
part. If one chooses to extend a given name for 2 or more labels below 
the closest-enclosing zone cut, then in the standard representation used 
by those tools, the "hostname" portion of the name will have a dot in 
it, hence "dotted hostnames", which are treated by such limited tools as 
somehow weird and special. At least one tool I'm familiar with, requires 
a special non-default option setting to even allow "dotted hostnames" to 
exist at all in its database.

DNS itself has never had a problem with multiple labels beneath a zone 
cut. Better tools recognize the inherent flexibility of DNS naming and 
don't try to deprecate or otherwise discourage so-called "dotted hostnames".

                         - Kevin

On 1/31/2011 10:50 AM, Vyto Grigaliunas wrote:
> Although we're moving away from it, we found it useful for naming router
> interfaces, i.e. te1-2.routername.company.com, without having to create a
> separate sub-domain for each router.
> I thought at some point periods were allowed in hostnames, and they do work
> without escaping them.
> Thanks...
> Vyto
>> -----Original Message-----
>> From: bind-users-bounces+vyto=fnal.gov at lists.isc.org [mailto:bind-
>> users-bounces+vyto=fnal.gov at lists.isc.org] On Behalf Of Barry Margolin
>> Sent: Sunday, January 30, 2011 4:14 PM
>> To: comp-protocols-dns-bind at isc.org
>> Subject: Re: what's a valid domain name?
>> In article<mailman.1586.1296424051.555.bind-users at lists.isc.org>,
>>   Vytautas Grigaliunas<vyto at fnal.gov>  wrote:
>>> What is the status of "dotted" hostnames - i.e. a period in the
>> hostname
>>> portion of a domain name ?
>>> At one point they were allowed, I believe ? What is the latest
>> official RFC ?
>> I don't think they've every been allowed.  Why would you need to do
>> this, instead of just using another level of subdomain?  To get a
>> literal period into a label, you need to escape it, e.g.
>> foo\.bar.company.com instead of foo.bar.company.com.  The latter is
>> allowed, always has been, and is quite common.
>> --
>> Barry Margolin, barmar at alum.mit.edu
>> Arlington, MA
>> *** PLEASE don't copy me on replies, I'll read them in the group ***
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