Install gnupg and add OpenNMS repository GPG key sudo apt -y install gnupg ca-certificates sudo apt-key adv --fetch-keys https://debian.opennms.org/OPENNMS-GPG-KEY Add apt repository sudo apt -y install software-properties-common sudo add-apt-repository -s 'deb https://debian.opennms.org stable main' sudo apt update You can safely ignore the message about conflicting distributions (expected stable but got opennms-xx). Installing the of Horizon Sentinel sudo apt -y install opennms-sentinel Disable the OpenNMS Horizon repository after installation to prevent unwanted upgrades when upgrading other packages on the server. After upgrade, Horizon requires manual steps to upgrade configuration files or migrate database schemas to a new version. For this reason, it is recommended to exclude the Horizon packages from update except when you are planning on performing an upgrade. sudo apt-mark hold opennms-sentinel Verify directory structure with the tree command sudo apt -y install tree tree /usr/share/sentinel -L 1 Directory structure after successful installation /usr/share/sentinel ├── bin ├── data -> /var/lib/sentinel/data ├── deploy -> /var/lib/sentinel/deploy ├── etc -> /etc/sentinel ├── lib └── system Enable Horizon Sentinel on system boot and start immediately sudo systemctl enable --now sentinel