Monitoring

If you run servers, you need to know they're healthy before users tell you they aren't. Monitoring is one of the most common day-one responsibilities in sysadmin and hosting jobs — and Zabbix and Nagios are the two open-source systems you're most likely to inherit at work.

Guides

1. Zabbix — Server, Agents & SNMP — what the Zabbix server is, installing Agent 2, passive vs active checks, SNMP devices, and every other item type
2. Nagios — Core, Agents & SNMP — what Nagios Core is, the plugin/exit-code model, adding hosts with NCPA and NRPE, SNMP and agentless checks

Build on the foundations

Monitoring leans on skills from other tracks: systemd services to run the daemons, firewalls to open agent ports, and TCP/UDP & ports plus DNS to understand what you're watching.