For those of you that doesn’t use ipv6 yet there’s a chance it’s still enabled in your system. This isn’t really a bad thing but the log files might get some extra unnecessary cruft in them. I found this to be true with bind, I was seeing a lot of these errors in my logs:
error (network unreachable) resolving '': 2600:9000:5307:100::1#53
and found it was caused by IPv6 being enabled in bind, a quick google search will lead you to modifying start up file in /etc/defaults/bind9 with OPTIONS="-4 -u bind" but for some reason this doesn’t work. It’s because there’s a bug in debian bind systemd file which ignores these options. Actually using /etc/defaults/bind9 is deprecated for systemd, where they want us to put runtime configuration now is above me.

As a quick fix so you could replace the ExecStart line in /lib/systemd/system/bind9.service with this ExecStart=/usr/sbin/named -f -4 -u bind, but this might get overwritten by a new package release.

Here’s another workaround which was suggested in the linked thread above:
mkdir /etc/systemd/system/bind9.service.d
echo "[Service]\nExecStart=\nExecStart=/usr/sbin/named -4 -f -u bind" > /etc/systemd/system/bind9.service.d/local.conf
systemctl reenable bind9.service
This is a more permanent solution and will not get overwritten.

There’s one more file you need to modify to prevent bind from querying ipv6, edit /etc/bind/named.conf.options and comment listen-on-v6 { any; };

Now you can reload systemd daemon and restart with:
systemctl daemon-reload
systemctl restart bind9.service