AdguardHome_address already in use error when trying to install on Ubuntu

This happens because the port 53 on localhost, which is used for DNS, is already taken by another program. Ubuntu comes with a local DNS called systemd-resolved, which uses the address 127.0.0.53:53 and thus prevents AdGuard Home from binding to 127.0.0.1:53. You can see that by running:

sudo lsof -i :53
The output should be similar to:

COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
systemd-r 14542 systemd-resolve 13u IPv4 86178 0t0 UDP 127.0.0.53:domain
systemd-r 14542 systemd-resolve 14u IPv4 86179 0t0 TCP 127.0.0.53:domain
To fix this, you need to either disable the systemd-resolved daemon or choose a different network interface and bind to an accessible IP address on it, for instance, the IP address of your router inside your network. But if you do need to listen on localhost, there are several solutions.

Firstly, AdGuard Home can detect such configurations and disable systemd-resolved for you if you press the “Fix” button, which is shown near the address already in use message on the installation screen.

Secondly, if that doesn't work, follow the guide below. Note that if you're using AdGuard Home with docker or snap, you'll have to do it yourself.

Create the /etc/systemd/resolved.conf.d directory, if necessary:

sudo mkdir -p /etc/systemd/resolved.conf.d
Deactivate DNSStubListener and update DNS server address. To do that, create a new file, /etc/systemd/resolved.conf.d/adguardhome.conf, with the following content:

[Resolve]
DNS=127.0.0.1
DNSStubListener=no
Specifying 127.0.0.1 as DNS server address is necessary because otherwise the nameserver will be 127.0.0.53 which doesn't work without DNSStubListener.

Activate another resolv.conf file:

sudo mv /etc/resolv.conf /etc/resolv.conf.backup
sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
Restart DNSStubListener:

sudo systemctl reload-or-restart systemd-resolved
After that, systemd-resolved shouldn't be shown in the output of lsof, and AdGuard Home should be able to bind to 127.0.0.1:53.