CoreDNS on clients

Notes

This guide was designed for a fresh install of Ubuntu desktop 20.04

Edit the file /etc/sysctl.conf and add (or edit) the following line:

net.core.rmem_max=8388608

Reload sysctl

sudo sysctl -p

Install CoreDNS

core_version=1.7.0
sudo useradd -M -s /bin/false coredns
cd ~
wget https://github.com/coredns/coredns/releases/download/v$core_version/coredns_"$core_version"_linux_amd64.tgz
tar -xvf coredns_"$core_version"_linux_amd64.tgz
sudo mv coredns /usr/local/bin
sudo chown coredns:coredns /usr/local/bin/coredns
sudo setcap CAP_NET_BIND_SERVICE=+eip /usr/local/bin/coredns

Create the file /etc/systemd/system/coredns.service and add the following:

[Unit]
Description=CoreDNS Server
Wants=network-online.target
After=network.target network-online.target

[Service]
Type=simple
Restart=on-failure
User=coredns
Group=coredns
StandardOutput=syslog
StandardError=syslog
ExecStart=/usr/local/bin/coredns -conf /etc/coredns/Corefile
KillSignal=SIGTERM

[Install]
WantedBy=multi-user.target

Reload systemctl

sudo systemctl daemon-reload

Create the config directory

sudo mkdir -p /etc/coredns

Create the file /etc/coredns/Corefile and add the following, adjusting for your subnet:

.:53 {
        forward . /var/run/NetworkManager/resolv.conf
        dnssec

        cache
        cancel
        bufsize 1232

        log
        errors
}

chown/chmod the config directory and files

sudo chown -R coredns:coredns /etc/coredns
sudo chmod -R 0664 /etc/coredns
sudo chmod 0775 /etc/coredns

Create the log file for CoreDNS

sudo touch /var/log/coredns.log
sudo chown syslog:adm /var/log/coredns.log

Create /etc/rsyslog.d/coredns.conf and add the following:

if $programname == 'coredns' then /var/log/coredns.log
& stop

Resatrt syslog

sudo systemctl restart syslog

Disable resolved

sudo systemctl disable systemd-resolved
sudo systemctl stop systemd-resolved

Delete resolved files

Yes, this is important. No, you cannot skip it. Yes, it's fine.

sudo rm /etc/resolv.conf
sudo touch /etc/resolv.conf

Reconfigure NetworkManager

Create the file /etc/NetworkManager/conf.d/disableresolv.conf and add the following:

[main]
dns=none

Create the file /etc/NetworkManager/dispatcher.d/coredns and add the following:

#!/bin/bash

case "$2" in
  up)
    sleep 3
    service coredns restart
  ;;
  down)
    sleep 2
    service coredns restart
  ;;
esac

exit $?

Make sure the new file has proper permissions:

sudo chown root:root /etc/NetworkManager/dispatcher.d/coredns
sudo chown 0755 /etc/NetworkManager/dispatcher.d/coredns

Restart NetworkManager:

sudo systemctl restart NetworkManager

Enable CoreDNS

sudo systemctl enable coredns
sudo systemctl restart coredns

And you're done!