Unbound strange stub_zone behavior?

Andrew Forgue andrew at forgue.io
Sat Jul 11 15:49:06 UTC 2020

Hi there,

I have an unbound server that acts as a recursive resolver for clients and also acts as a target for fully delegated DNS (i.e. unbound is the NS record).

For the fully-delegated domain it is a simple stub zone with an upstream of localhost on a different port.  Let's call it "blah.example.com".

Occasionally, unbound (has happened on versions 1.10.1 and 1.7.3) will start responding to non-recursive queries with the list of root zones instead of a response from the stub-zone.  It seems that clients that use the `rd` flag are fine and continue to be able to resolve records in the stub-zone.  Only recursive desired clients will receive correct records from unbound (using the stub server).  All records in seemingly all stub zones have this behavior simultaneously.

I don't know what triggers it, but a full restart of unbound is the only thing that fixes it.  I've tried flushing cache, flushing infra, and everything, nothing seems to matter.

I've seen only 2 things that may point to the issue.

- With verbosity turned up to 10, there's an entry produced in strace (but not in the actual log - maybe a misconfig):

	"unbound[2213085:5] debug: answer from the cache failed"

- stracing the "broken" unbound process is a very tight recvmsg() (of the request) and sendmsg() (with the root servers) with no syscalls in between.

Again, Using dig with +recurse works all the time, even when unbound gets in this state.  So seems like an unbound bug / cache corruption or something?

Any ideas?

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