Michał Kępień authored Oct 09, 2018 and Ondřej Surý committed Oct 24, 2018

Since mirror zone data is treated as cache data for access control
purposes, configuring a mirror zone and disabling recursion at the same
time would effectively prevent mirror zone data from being used since
disabling recursion also disables cache access to all clients by
default.  Even though this behavior can be inhibited by configuration,
mirror zones are a recursive resolver feature and thus recursion is now
required to use them.

Ignore the fact that certain configurations might still trick named into
assuming recursion is enabled when it effectively is not since this
change is not meant to put a hard policy in place but rather just to
prevent accidental mirror zone misuse.