Nagios is an industry-standard open-source IT infrastructure and network monitoring software which can monitor a variety of targetdevices and services, including server hardware (e.g., CPU, memory, disk usage), network equipment (e.g., switches or routers), application protocols and services (e.g., HTTP, SMTP, POP3). Nagios can be deployed in either agent-based or agentless mode. In the former, agents are installed on target devices, and report back to the management server. In the latter, existing protocols are leveraged by Nagios to emulate an agent. There exists a strong Nagios ecosystem consisting of numerous plugins, front-ends and add-ons, which extend the capabilities of Nagios in terms of monitoring protocols, visualization, alerting and notifications (e.g., e-mail, SMS, mobile apps), supported hardware, security (e.g., SSH/SSL encryption), database/webserver integration, etc.