Underscores in host names
wmoran at potentialtech.com
Wed May 7 12:50:32 UTC 2008
In response to Christopher Key <cjk32 at cam.ac.uk>:
> 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.
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