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Re: History of fallback to A

2008-04-02 12:27:17

--On Wednesday, April 02, 2008 11:21 AM +0200 Alessandro Vesely <vesely(_at_)tana(_dot_)it> wrote:

John Levine wrote:
RFC 821 came out in 1982.  It makes only passing references
to DNS, because at the time the transition from HOSTS.TXT to
the DNS had not yet started.  RFC 883, the first description
of the DNS came out over a year later in late 1983.  It
described the now abandoned MD and MF records.  According to
RFCs 897 and 921, the transition to DNS started in 1983, but
HOSTS.TXT didn't go away until the end of 1985.

In January 1986, RFC 973 and 974 deprecated MD and MF records,
replaced them with MX, and defined the MX lookup with
fallback to A. While rereading 974, I note that it also
recommends that clients do a WKS lookup on each MX host to
see if it actually supports SMTP and discard the MX entry if
it doesn't, but as far as I can tell, nobody ever did that.
Too bad.

So this means that SMTP had been in use for at least a year
using HOSTS.TXT, and then another couple of years using A,
MD, and MF, before MX came along, and I get the impression
that MD and MF were clunky enough that a most people just
used the A record.  Under the circumstances, MX without
fallback to A wouldn't have worked because of the substantial
installed base of mail servers using A records.

I think the above provides quite some insight into
regarding the MX record, so I took the liberty of copying it in

be a little careful because, for several reasons of which only a few were related to their being a bad idea, MD and MF were never really used on large scale. See Dave's note about the actual deployment realities -- my recollection is that MX was used as an inter-network gateway mechanism somewhat earlier than his not would indicate, but, for on-Internet relay purposes, MXs were essentially useless (and hence little used) until the DNS was widely deployed and widely used.


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