their DNS data in one query. This doesn't seem quite right to me.
DMP tried to make things easier for domains publishing data and only to dig
deeper when there's uncertainty. As I had it described once: "Front-load the
pain now, and it'll ease up as it's adopted."
I didn't see a way around it when the argument around wildcard records came
about. Sure an unusual implementation of DNS could synthesise records in a
new way to avoid the second lookup, but I didn't want to count on that.
It is my understanding that DMP requires you to fetch a TXT record
from _smtp-client.$FQDN and also an A record at
$REV-ADDRESS.$ADDRESS-TYPE._smtp-client.$FQDN. Similarly, FSV appears
to need to fetch either an A record at _fsv.$FQDN and either a TXT
record from the same location, or another A record
$REV-ADDRESS._fsv.$FQDN.
Doesn't this mean that DMP and FSV require a minimum of 2 DNS queries?
More like a maximum of two, minimum of one[1]. DMP got a little worse, with
a maximum of four queries, when a receiver queries for hostnames as well as
domain names. If a receiver were prepared to sacrifice Store and Forward,
that got back down to a maximum of two.
I want to know if there's a better way to check if a domain publishes
LMAP-type data when the first query returns NXDOMAIN, so we are back down to
a maximum of one query per e-mail.
FSV sticks with domains instead of hosts, from what I read, so it looks like
it has a maximum of two regardless.
[1] Subject to DNS caching.
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