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Wikipedia states that underscores are used by both Android and Microsoft Windows Systems in hostnames. My question is, what are these host names used for? Do you know list of these strange hostnames? Have you ever even seen one of these?

Rook
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3 Answers3

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Per RFC952, hostnames are:

A "name" (Net, Host, Gateway, or Domain name) is a text string up to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-), and period (.). Note that periods are only allowed when they serve to delimit components of "domain style names".

Microsoft in the usual "Embrace, Extend and Extinguish™" method they use, have these for certain functions within their DNS system they use for domains and extended it to include allowing hostnames to resolve, probably because NETBIOS names allowed underscores and it was easier just to slap it lower case onto the front of the domain to come up with a machine hostname. It sometimes makes zone transfers a real pain as the entries are not RFC compliant.

I would assume that Google, being the 900 lb Gorilla, has done the same thing for similar reasons. Which means that access points that follow the RFC will decline connections among other misery that it causes..

Community
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Fiasco Labs
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  • DNS records with underscores shouldn't cause any problems with domain transfers as they have always been legal DNS entries. The original DNS RFCs recommended *but did not require* that DNS labels follow the rules for names ala RFC952. In other words, a telnet client might legitimately refuse to connect to a host whose name had an underscore, but if it doesn't do so, the DNS should be able to handle the name perfectly well. – Harry Johnston Jun 09 '12 at 23:01
  • You might want to qualify answers like this. _Insofar as your Non-Windows DNS supports DNS A and CNAME entries with underscores..._ This is one of those "Been There Done That" answers where I've personally run into the problem. – Fiasco Labs Jun 10 '12 at 03:07
  • Sorry; I overspoke. There's nothing improper about DNS server software refusing to accept A or CNAME records with underscores, so you might indeed run into trouble with zone transfers in this situation. But there's nothing improper about DNS server software accepting these records either. My point is that this isn't a proprietary or non-standard extension to DNS as your second paragraph claims, it's just a differing implementation. – Harry Johnston Jun 10 '12 at 22:27
  • I'm not sure under what circumstances you'd run into trouble with access points, could you expand on this? – Harry Johnston Jun 10 '12 at 22:27
  • Mentioned on code.google.com as an issue among certain Android using vendors whose devices have problems accessing Wireless hotspots because they chose to put an underscore in the device name. Other mentions were problems getting the underscored name to register with dynamic DNS on college networks. – Fiasco Labs Jun 10 '12 at 22:35
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Erik Fair has answered this extensively over at Quora:

The keyboard of the Teletype (TTY) ASR-33 (a very common terminal interface of the period from whence this standard hails) doesn't have underscore on it: http://www.pdp8.net/asr33/pics/k... which gives one a nasty problem: how do you type (input) a hostname if the hostname in question contains a character that your terminal interface can't generate?

In the article a few more links are mentioned.

adamo
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Microsoft Windows does not prohibit the underscore in the name of a computer. Some system administrators, either not knowing or not caring about RFC952, use underscores in the name of a computer that is connected to the internet, and either put this name in the DNS explicitly or allow it to be published via dynamic DNS.

There's nothing particularly strange about these names. Examples might be JOHNS_COMPUTER or ACCOUNTING_DEPT_PC1.

Harry Johnston
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