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Re: History of fallback to A

2008-04-02 10:36:20

Alessandro Vesely 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.
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,

Looking at things in very broad strokes:

1.  hosts.txt remained in use for much of the 80s

2. The pre-MX specifications for A-record surrogates were experiments that culminated in the MX record. Hence, they were not relevant to real-world operation.

3.  MX did not gain substantial deployment and use until the latter 80s.

So that is at least 5 years of hosts.txt or A record, for doing email "routing".



  Dave Crocker
  Brandenburg InternetWorking

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