Underscores in host names

[Bill Moran]

In response to Christopher Key <cjk32 at cam.ac.uk>:

> Hello,
> 
> I've a host on the network called "GC100_000C1E00AC3F_GlobalCache", and 
> I'm getting interesting behaviour when I try to do DNS lookups on it.
> 
> Under FreeBSD, ping fails with 'Unknown server error'(distinct from the 
> standard 'Unknown host'), and nslookup succeeds.  OSX and Windows 
> machines will do a DNS lookup on it quite happily
> 
> The best explanation I can manage is that ping etc. are using different 
> code from nslookup, and only nslookup is allowing the underscores within 
> the hostname.
> 
> Is this behaviour by design?  My understanding is that underscores are 
> not strictly permitted, but that most implementations choose to allow 
> them unless there's a specific reason not to.

I had this discussion with some colleagues a short time back.  Our
conclusion (based on some research and experimentation):
1) Underscores are not valid in domain names.
2) _most_ DNS systems will work with them anyway.
3) Just enough DNS systems don't work with _, that it's a really bad
   idea to use them in domain names.

-- 
Bill Moran
http://www.potentialtech.com