Re: [exim-dev] [Bug 1026] New: dnslists doesn't allow unders…

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Author: Graeme Fowler
To: exim-dev
Subject: Re: [exim-dev] [Bug 1026] New: dnslists doesn't allow underscores
On Thu, 2010-09-30 at 08:49 +0200, Thomas Hochstein wrote:
> Wasn't it disallowed to use underscores in DNS entries?

Host and domain names are defined in RFC1034 as follows:

"The labels must follow the rules for ARPANET host names. They must
start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen. There are also some
restrictions on the length. Labels must be 63 characters or less."

However many subsequent RFCs exist which update this to extend to other
RR types, such as the TXT RR (defined in RFC1035) which is defined as:

"TXT-DATA        One or more <character-string>s."

And a character-string is further defined in RFC1035 in a very long
paragraph which I'll not quote here.

In short, a dnslist can be used to contain items which are neither host
nor domain names; ergo characters not permissible in those items should
be permissible to enable the lookup of records such as they type Mike
originally mentioned.

Apologies if that's a bit long-winded but it might help others who come
along later and look in the archives!