Numbers are legal characters for a DNS name and best I can tell from my understanding of the RFCs, there is nothing against names with all numbers.  In fact, I have customers that use number strings for their workstation names and I haven’t heard any screams.  I realize that saying that “nothing bad has happened” doesn’t necessarily mean that it is “good” but we go by the info we have.  What I do know is that all-numeric names in Active Directory sites is problematic – at least for the netlogon service.

Dashes for Numbers

1.  Let’s start by verifying that everything is okay. 

1a.  Here’s the original output from netlogon.dns found at %systemroot%\system32\config.

1b.  Here’s a normal DNS.

2.  All’s well.  So now let’s go into AD Sites and Services and change the Default-First-Site-Name to 123456.

3.  We’ll stop and start the netlogon service

4.  Now let’s take a look at how that impacts DNS.

4a.  Here’s the netlogon.dns file.

4b.  And here’s a view of DNS

In both the log file and in DNS, you can see that the site name has been replaced with a series of ‘dashes’ (-).  The number of dashes is directly equivalent to the count of numeric characters in the site name.  This will go anywhere from the minimum of one dash for one character:

to sixty-three dashes for the maximum number of characters in a site name:

This does negatively impact site-aware services (including authentication) as you may imagine.  As soon as you add an alphabetic character, this issue goes away. 

Numbers can be anywhere in the site name and there can be as many of them as you’d like – just remember to add a letter to the name somewhere.

NOTE:  I have not yet tested this for Windows 2008 and 2008 R2 behavior.