LibreNMS

LibreNMS finds what to monitor on its own. Give it SNMP access to a network and it walks the devices, identifies the hardware and operating system, and starts collecting without per-device setup. That autodiscovery spans a broad vendor list, including Cisco, Linux, FreeBSD, Juniper, Brocade, Foundry, and HP.

Under the hood it is a PHP and MySQL application that stores metrics in RRD and renders them as graphs, with alerting on top. To make trials easy, the project ships a ready-to-run VM image based on Ubuntu, available as a VirtualBox download.

LibreNMS is a community-built, GPL-licensed project maintained by individual contributors. It is self-hosted on your own infrastructure, with documentation, forums, and Discord for support.