On 2/16/23 12:56 PM, Barry Leiba wrote:
Okay. What's the value for X - T that prevents this problem, but doesn't cause DKIM
signatures of "normal" mail to fail?
There's not one "right" value; we're talking about distributions
of timings for normal mail vs. replay, and yes, there's some
overlap there. In practice I've seen many signers choose
expirations in the range of 1hr to a few days. 1hr can be very
good at limiting the opportunity for high volume replay, but I
estimate "normal" signature breakage at that level is on the
order of 0.1%. 24hr is probably effectively zero breakage, but
with greater opportunity for replay.
I think you're way off on these numbers, especially for the 1-hour
case. While normal circumstances get mail delivery in less than an
hour, I have seen *many* cases of legitimate mail delayed by hours --
sometimes quite a few hours. I would consider anything less than two
days to be unacceptable, and with that sort of gap you don't do much
to prevent a spam blast.
I dunno, I would think it has to do with how much of a boost reputation
is actually giving deliverability. For typical marketing email that's
not too spammy maybe it doesn't make much difference? Adding signatures
and a SPF record is pretty easy to do these days, so it's sort of a
no-brainer to just do it. But if some small percentage with slow
delivery fall through the cracks, how adverse is that to delivery,
assuming they don't have a p=reject policy? That seems like it should be
a pretty measurable thing for an ESP.
Sounds like something that Evan might know. Actually it might well be
worth adding that kind of estimate to the problem statement to judge how
much of a problem it is. What we have now is very hand waving and makes
it impossible to judge about how heroic we need to be.
Mike
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