In order to enable logging in the Unbound DNS resolver, you have to add the following lines to your /etc/unbound/unbound.conf configuration file:

server:
    chroot: ""
    logfile: /var/log/unbound.log
    verbosity: 1
    log-queries: yes
    ...

Then, create the file and make sure it’s owned by the unbound process:

touch /var/log/unbound.log
chown unbound:unbound /var/log/unbound.log

Finally, restart Unbound:

/etc/init.d/unbound restart

Now you should be able to see the log:

tail -f /var/log/unbound.log
[1553775590] unbound[32655:0] info: 127.0.0.1 googlemail.l.google.com. A IN
[1553775609] unbound[32655:0] info: 127.0.0.1 acp-ss-ew1.adobe.io. A IN
[1553775695] unbound[32655:0] info: 127.0.0.1 clients4.google.com. A IN
...

The reason you have to add chroot: "" is because by default unbound runs in a chroot and can’t write to /var/log.

This post was tested on OpenWRT.