BIND caching of nxdomain responses

[Dan Hanks]

Greetings,

As I understand RFC 2308, when receiving an NXDOMAIN response, and when
deciding how long to cache that NXDOMAIN response, a resolver should use
whichever value is lower of the SOA TTL, and the SOA.minimum value as the
length of time to cache the NXDOMAIN.

I have a situation where I am seeing different behavior from that in BIND.
Given the following SOA record:

azure.mongodb.net.      900     IN      SOA     ns-1430.awsdns-50.org.
awsdns-hostmaster.amazon.com. 1 7200 900 1209600 60

I am finding that BIND (9.11.x) is caching the NXDOMAIN response for 900s
(SOA TTL), instead of the expected 60s (SOA.minimum).

I have noticed that many auth servers out there will drop the SOA TTL to
match the SOA.minimum value when attaching the SOA record to an NXDOMAIN
response. Is BIND expecting this to happen, and just opting to use the SOA
TTL value (and not the SOA.minimum value if they disagree)?

Thanks for any insight,

Dan
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