Create a config file with FQDN of nameservers as masters in auth-zone section. Current version of default unbound.config suits this condition. Start unbound with `unbound -d` command and terminate it in short period of time. You should see a segfault. It's fixed in upstream: https://github.com/NLnetLabs/unbound/commit/8a87fc6ae7e17490b9bbb79c31f78977e2744ac3
There is error in previous comment. Current version of default unbound.config should be modified to reproduce. The following line: master: 199.9.14.201 # b.root-servers.net Should be replaced with: master: b.root-servers.net