Open source monitoring tools for Linux are:
NagiosNagios monitors almost all types of components, including network protocols, operating systems, web servers, and websites. By consuming less server resources, it provides high performance. Third party applications can be integrated with a plugin.
Zabbix With Zabbix, you can monitor the performance and availability of servers, web applications, and databases. It is enterprise level software.
It uses a server-agent architecture, where it installs an agent on a server (client) to be monitored by the Zabbix server. FTP, SSH, HTTP, and DNS do not require agents. SNMP is a network protocol used to manage and monitor devices connected to the network. Provides better reports via SNMP.
CactiIt is an open source network monitoring tool that can be installed on Linux or Windows. Connected to RRDTool, it generates graphs of relevant network data. In the form of graphs, it presents the statistics of the network. A PHP compatible database such as MySQL, Apache or IIS is required. It makes it easy for administrators to create different levels of users and give them different levels of permissions based on their authority.
IcingaIcinga-2 is a free and open source monitoring tool that provides monitoring and alerting capabilities for servers, network devices, and applications. It supports plugins to monitor various aspects of the server and can be configured to send email alerts.
NetdataNetdata provides real-time performance monitoring and troubleshooting of system and applications. Using thousands of metrics, interactive visualizations, and detailed status alarms, Netdata is fast and efficient. It runs on all systems without causing interruptions.
PrometheusIt is a free and open source monitoring and alerting software built on SoundCloud.
Its architecture consists of several modules called exporters that use metrics and key-value pairs to identify data.
The system consists of single independent server nodes. Prometheus consists of several components, including the server, a push gateway, client libraries, and an alert manager.
MonitorixMonitorix is an open source system monitoring tool designed for Linux or UNIX systems. It is a lightweight tool to track and monitor multiple devices. It is also suitable for embedded systems due to its size and simplicity. Monitorix wrote in Perl.Monitorix regularly collects system and network data and displays graphs in its web interface. It also monitors the temperature and health of the disk drive.
There are several other open source tools available, but the ones mentioned above are some of the best open source tools available to monitor your server.