How to Add dnsmasq and keep systemd-resolved (18.04 to 20.04)

I wanted to get fast dns resolution with dnsmasq and keep the default systemd-resolved/NetworkManager setup untouched for future use. Yes the huge dns caching of dnsmasq can improve browsing speed. Yes the goal was to keep the default featured dns setup of 18.04

1. Install dnmasq
2. Configure it (listen address and dns servers)
3. Configure NetWorkManager for manual dns server address
4. Check verify

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1 - With sudo

apt-get -y install dnsmasq

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2 - With sudo

tee -a /etc/dnsmasq.conf << ENDdm
interface=lo
bind-interfaces
listen-address=127.0.0.1
# DNS server from OpenDns. Use yours...
server=208.67.222.222
server=208.67.220.220
ENDdm

systemctl restart dnsmasq
systemctl enable dnsmasq

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3 - With USER, configure NetworkManager

# Get NM first active profile name
NetManProfile=$(nmcli -t  connection show --active | cut -f 01 -d ':')
# remove, if exists, current dns servers
nmcli con mod "$NetManProfile" ipv4.dns ""
# set 'manual' dns server
nmcli con mod "$NetManProfile" ipv4.ignore-auto-dns yes
# set dnsmasq as manually set dns server
nmcli con mod "$NetManProfile" ipv4.dns 127.0.0.1
# i also disabled ip6, do what u want
nmcli con mod "$NetManProfile" ipv6.method ignore
# reconnect to take effect
nmcli connection down "$NetManProfile"
nmcli connection up "$NetManProfile"

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4 - Check verify

• systemd-resolved listen on 127.0.0.53 as should by default
• dnsmasq listen on 127.0.0.1 as set in /etc/dnsmasq
• systemd-resolved took 127.0.0.1 from NetworkManager

netstat -antup
Proto Recv-Q Send-Q Adresse locale          Adresse distante        Etat       PID/Program name    
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1036/dnsmasq        
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      766/systemd-resolve
cat /run/systemd/resolve/resolv.conf 
nameserver 127.0.0.1

I tried to find a reasonable solution and looks that there are different approaches.

I wanted to stay at most within the distribution layout while keeping all business requirements fulfilled. This is what I collected around and tested to work on clean Ubuntu 18.04 and KDE Neon flavour:

# Install required package and reconfigure service plans (i.e. disablesystemd-resolved, enable dnsmasq
sudo apt-get install dnsmasq
sudo systemctl disable systemd-resolved
sudo systemctl stop systemd-resolved
sudo systemctl enable dnsmasq

# These two lines should work on most environments, but .. :-) - so I kept them commented out for less experienced users
# Just add or change 'dns=dnsmasq' to your NetworkManager.conf to the section [main]
# and yes, the sed expression can be better :-)

#sudo cp /etc/NetworkManager/NetworkManager.conf /etc/NetworkManager/NetworkManager.conf.backup
#sudo bash -c 'cat /etc/NetworkManager/NetworkManager.conf.backup |sed -e "s/^dns=.*//"| sed -e "s/\[main\]/\[main\]\ndns=dnsmasq/" >/etc/NetworkManager/NetworkManager.conf'

# Restart NetworkManager to make the change above applied
sudo systemctl restart NetworkManager

# This removes the systemd resolv.conf link only if it has NetworkManager replacement :-)
ls /var/run/NetworkManager/resolv.conf && sudo rm /etc/resolv.conf

# And add NetworkManager's resolv.conf available for the system resolver
sudo ln -s /var/run/NetworkManager/resolv.conf /etc/resolv.conf

(please note that the only general difference with the above answers is that the NetworkManager handle the dnsmasq DNS server assignments automatically