Why is Ubuntu Mate using Google DNS servers?

Solution 1:

After much investigation (with help from @Terrance) I discovered that Ubuntu Mate is using avahi-dnsconfd and some other device on my network (possibly a Raspberry Pi) was broadcasting Google's DNS servers over mDNS / Bonjor / Avahi.

avahi-dnsconfd was picking this up and when avahi-dnsconfd.action ran, it was calling resolvconf to add the DNS servers is discovered over mDNS to /etc/resolf.conf

I disabled avahi-dnsconfd using:

sudo systemctl stop avahi-dnsconfd.service
sudo systemctl disable avahi-dnsconfd.service

for good measure I also edited /etc/default/avahi-daemon and set AVAHI_DAEMON_DETECT_LOCAL=0 (because I know my network doesn't have unicast DS servers that serve the .local TLD)