local_unbound: How to prevent caching of SERVFAIL?

[wolfgang at lyxys.ka.sub.org]

Hi,

using local_unbound on FreeBSD 11.3 I have the problem that an
occasional SERVFAIL is cached for several minutes (not sure about the
exact length of time but something like 5 to 15 minutes). Is it possible
to reduce this to something like 10 seconds?
I did set cache-max-negative-ttl but that apparently only sets the ttl
for caching NXDOMAIN.

Wolfgang