Add DNSSEC and force Quad9 DNS on Ubuntu:
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This will force Quad9 DNS with added DNSSEC support on ubuntu 19.x and above that is not enabled by default.

edit this file by:
sudo nano /etc/systemd/resolved.conf

Replace contents with this:
------------------------------
[Resolve]
DNS=9.9.9.9
#FallbackDNS=149.112.112.112
Domains=dns.quad9.net
#LLMNR=no
#MulticastDNS=no
DNSSEC=yes
DNSOverTLS=yes
#Cache=no-negative
#DNSStubListener=yes
#ReadEtcHosts=yes

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Save changes then input this command to restart:
sudo systemctl restart systemd-resolved

To verify DNSSEC Support, type:
systemd-resolve --status | grep DNSSEC

as well as:
grep DNSSEC /etc/systemd/resolved.conf

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Sources: 
https://stanislas.blog/2018/06/enable-dnssec-support-in-systemd-resolve/
https://serverok.in/systemd-resolved

Date: February 8th, 2021