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History of fallback to A
I wasn't there, so I hope people who were will correct me if I'm
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.
Now fast forward 20 years. How many domains are there on the Internet
that accept mail from the outside world with only AAAA records?
Considering that most of the net still can't connect to IPv6 hosts,
it's rather unlikely that there are any at all other than perhaps a
few test setups. The issues of installed base that mattered in 1986
just don't exist now.
The transition to v6 is going to involve, among other things,
renumbering nets from v4 into v6 address space, and then adding AAAA
records for all the hosts that have A records. Some of those hosts
are mail servers. Is it really an unreasonable burden to ask people
who are redoing their DNS anyway to add MX records for the small
number of mail hosts that don't already have them? Maybe somewhere
there is DNS management software so wierdly broken that it can install
AAAA records but not MX, but even if there is, that doesn't seem like
much of an argument compared to the advantages laid out in my last
message of having hosts default to not being mail servers.