Underscores in host names

[Matthew Seaman]

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Bill Moran wrote:
> 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.
> 

DNS is perfectly happy with underscores in RRs generally -- it's just forbidden
for them to appear in hostnames in the DNS specifically.  This is to distinguish
between hostnames and other data such as SRV records.

	Cheers,

	Matthew

- -- 
Dr Matthew J Seaman MA, D.Phil.                       Flat 3
                                                      7 Priory Courtyard
PGP: http://www.infracaninophile.co.uk/pgpkey         Ramsgate
                                                      Kent, CT11 9PW, UK
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