Re: [exim] mx selection (possibly off topic)

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著者: Philip Hazel
日付:  
To: Exim Mailing List
題目: Re: [exim] mx selection (possibly off topic)
On Thu, 18 Aug 2005, Steven Wayne wrote:

> >     I didn't know mx records couldn't be ip addresses....


RFC 1034:

  "The MX RRs have an RDATA section which consists of a 16 bit number     
  followed by a domain name."


RFC 974:

"For the purposes of message routing, the system stores RRs known as
MX RRs. Each MX matches a domain name with two pieces of data, a
preference value (an unsigned 16-bit integer), and the name of a
host."

Note that, as a result of pressure long ago from a certain multinational
company whose name starts with the digit 3, components of domain names
are permitted start with digits. Thus, an IPv4 address is a
syntactically valid domain name. A nameserver cannot therefore object to
being told to load a record such as

xxx MX 1 1.2.3.4

because, syntactically, "1.2.3.4" *could* be a domain name. Sigh.

-- 
Philip Hazel            University of Cambridge Computing Service,
ph10@???      Cambridge, England. Phone: +44 1223 334714.
Get the Exim 4 book:    http://www.uit.co.uk/exim-book