Nagios is an open source monitoring system to monitor small to enterprise computer networks. With Nagios you can monitor servers, switches, applications and services. Nagios alerts you whenever there is a problem, and sends a notification when the problem has been resolved. You can receive notifications via email, SMS messages and even phone calls. Nagios is licensed under GNU GPLv2.
Monitor network services like SMTP, HTTP, SNMP, FTP, SSH, POP, etc.
Monitor Servers resources like disk drives, memory, processor, server load usage, system logs, etc. you can extend Nagios capabilities with hundreds of plugins found in its official website repository.
With Nagios you can define network hosts as parents (Hyper-V, or VMware Hosts for example) so you don’t get flooded with alerts that all systems (VMs hosted in the hosts) are down. You will only receive notifications about the parent hosts.
You can set it to send you Emails, SMS, and even voice calls whenever there is a problem in the network.
You can setup Nagios in a distributed system if you need to have a monitoring system in multiple locations.
When you install Nagios it automatically installs a web portal where you can view current network status, problem history, log files, notifications that have been triggered by the system, etc.
Agent for Windows hosts
NSClient++ is the preferred Agent to monitor Windows server hosts. NSClient++ listens on theTCP port 12489, and the command you use to collect information from the host is check_nt. With NSClient++ you can monitor CPU load, RAM usage, disk drive usage, server running processes, etc. there are hundreds of custom plugins you can download from the Nagios official site repository to make Nagios monitor many other custom devices, services.
Agent for Linux Hosts
NRPE (Nagios Remote Plug-Ins Executor) is the preferred agent to monitor Linux hosts. The NRPE daemon binds to the 5666 port, and you collect information from the remote hosts using the check_nrpe command. With NRPE you can remotely execute Nagios plugins on other Linux/Unix machines. This allows you to monitor remote machine metrics (disk usage, CPU load, etc.).
Agent for other devices
There is no Nagios agent for embedded devices like Routers, Switches, UPSs, and many other devices were no custom software can be installed. In those devices Nagios use the NNTP protocol to gather information.