Last explicit TTL is used instead of $TTL

3.1.0 includes this change:

omitted TTL value is correctly set to the last explicitly stated value (RFC 1035)

I think the value of the $TTL directive, if set, should be used instead. That would be consistent with Knot DNS 3.0.8, PowerDNS Authoritative Server, and BIND's documentation. (I didn't check BIND's implementation.)

I think $TTL is nonstandard, so there's no official reference for what to do.

Example zone:

$ORIGIN example.com.
$TTL 3600
@               SOA     example.com. dnsadmin.example.net. 1 1800 900 2678400 3600
@       7200    NS      example.com.
@               AAAA    2001:db8::

In Knot DNS 3.0.8 (I believe) and PowerDNS Authoritative Server, the TTL of example.com./AAAA would be 3600. In Knot DNS 3.1.0, it's 7200.

BIND's documentation says:

The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.