/sys/src/cmd/ndb/dns.h:

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To: 9front@9front.org
Date: Sun, 07 Feb 2021 14:56:39 +0100
From: kvik@a-b.xyz
Subject: Re: [9front] transient dns errors cause smtp failure
Reply-To: 9front@9front.org

I think I found a reason for DNS failing on known good domains.

/sys/src/cmd/ndb/dns.h:156,157
	/* tune; was 60*1000; keep it short */
	Maxreqtm=	8*1000,	/* max. ms to process a request */

So, 8 seconds is how much the resolver will bother with a request it
has been handed, before dropping it on the floor with little
explanation.

It seems quite possible that this is too short a timeout on a machine
during a spam queue run, which predictably stresses the compute and
network resources.

In turn, negative response caching might explain why a particular
unlucky domain would basically stop receiveing any mail for a while.

I'm dying to know if bumping this limit would clear up the queue of
such DNS errors.

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[narrator: it did.]

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