Date: Wed, 07 May 2008 14:04:46 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Bill Moran <wmoran@potentialtech.com> Cc: questions@freebsd.org, Christopher Key <cjk32@cam.ac.uk> Subject: Re: Underscores in host names Message-ID: <4821A8EE.8030704@infracaninophile.co.uk> In-Reply-To: <20080507083549.2a4b1d3b.wmoran@potentialtech.com> References: <4821896B.60005@cam.ac.uk> <20080507083549.2a4b1d3b.wmoran@potentialtech.com>
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Bill Moran wrote:
> In response to Christopher Key <cjk32@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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