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From Reindl Harald <>
Subject Re: Underscores in hostnames
Date Thu, 02 Feb 2017 13:26:25 GMT

Am 02.02.2017 um 14:22 schrieb Reindl Harald:
> Am 02.02.2017 um 13:53 schrieb Joe Orton:
>> Another 2.4.25 regression reported from a Fedora user is that
>> underscores in hostnames are rejected by default now.  I couldn't see a
>> specific discussion of this, was it deliberate?
> underscores are not allowed in host names by RFC and many things will
> break at all with them because in different layers of client software
> things just break by using them
> we had that more than once in devel environments where strange bugs
> turend out to be another case where sombody used a underline in
> /etc/hosts and his local webserver

here you go:

The Internet standards (Requests for Comments) for protocols mandate 
that component hostname labels may contain only the ASCII letters 'a' 
through 'z' (in a case-insensitive manner), the digits '0' through '9', 
and the hyphen ('-'). The original specification of hostnames in RFC 
952, mandated that labels could not start with a digit or with a hyphen, 
and must not end with a hyphen. However, a subsequent specification (RFC 
1123) permitted hostname labels to start with digits. No other symbols, 
punctuation characters, or white space are permitted.

While a hostname may not contain other characters, such as the 
underscore character (_), other DNS names may contain the underscore.[4] 
Systems such as DomainKeys and service records use the underscore as a 
means to assure that their special character is not confused with 
hostnames. For example, specifies a service 
pointer for an SCTP capable webserver host (www) in the domain Note that some applications (e.g. Microsoft Internet 
Explorer) won't work correctly if any part of the hostname contains an 
underscore character.[5]

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