Mirror zone data may not be used during resolution despite being available

Consider a server configured with two mirror zones:

• bar., which has its data loaded,
• foo.bar., which is not loaded (has expired, has not yet been transferred etc.)

(To make the example more dramatic/realistic, use . instead of bar. and org. instead of foo.bar..)

If a query then arrives for a name at or below foo.bar, named will not use bar zone data for resolution purposes, even though it is available.

This happens due to the way dns_zt_find() was modified to support mirror zones (see 8d996fd7).

I am opening this ticket mostly to publicly indicate that this is a known issue. While handling this scenario more elegantly is possible¹, IMHO it is not critical enough to warrant the added complexity. Feel free to prove me wrong.

For now, I plan to resolve this ticket by adding a code comment explaining the issue. If this ever becomes a real-world problem, we will tackle it then.

1. e.g. by employing dns_rbt_findnode() instead of dns_rbt_findname() when the options argument passed to dns_zt_find() has the DNS_ZTFIND_MIRROR bit set and then going up the node chain in case the deepest match in the zone table is not loaded ↩