Bind response to query's very small edns udp payload size

[John Wobus]

What does bind try to do if the client specifies a udp size of less than 512?
I’ve been trying queries and here is what I’ve seen:

I have a query that ordinarily receives a response with an answer section
and an authority section, the response length being ~ 500.

If I specify a udp size of 200, then I receive the same answer section,
but minus the authority section.  But the received length is greater than 200,
and the tc flag is not set.

(In contrast to this, if I try a different query that gets a truly long answer,
specifying a udp size of 512, then I do get a response with the
tc flag set and with no answer-section lines.)

I’ve been looking at a customer's reported problem,
testing scenarios and behavior that might explain it, so this is a bit
of an academic question just to know what to expect from bind.
The actual problem is likely (in my mind) to be a firewall or
client configuration.

FYI:
$ ./named -v
BIND 9.9.8-P4 (Extended Support Version) <id:deea0d7>

John Wobus
Cornell University IT