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My Experience with Nagios
I am using Nagios from past two years & monitoring 700 services, over 100 Servers / Network devices & over 200 remote hosts.
One of my colleague told me about Nagios & its capability. At that time I was using MRTG to monitor link & server utilization, & was looking for Linux based application which can monitor all my critical servers, network & storage devices with alerts.
At that time we were dependent on technical staff to check server health & to monitor MRTG graphs.
This was tedious job & often people make mistakes when it comes to monitoring. Thus Monitoring & Altering application was mandatory for us.

During this time I was totally dependent on Online forums & installation guides to implement Nagios, it took me few hours to setup Nagios & to monitor few critical services. At that time I was looking for guide to setup Nagios more efficiently.
I did started Nagios & used it for a while, during failure of branch office connectivity where I have few servers running, I realised Nagios should be configured with service / host dependency.
Over a time I found few addons, such as Nagvis to replace Nagios Status Map & pnp4nagios to replace MRTG.

I used to monitor Network & Server resources using MRTG , which consumed 40% CPU & increased Disk I/O. Replacing MRTG with Nagios & pnp4nagios reduced CPU to 25% & reduced Disk I/O.

This book is nice guide to setup Nagios in efficient manner,
this book covers followings
Nagios Installation
Configuring Hosts
Configuring Services
Configuring Commands
Configuring Time Periods
Configuring Contacts

There are examples to monitor services like HTTP, FTP, POP3, SMTP, Tcp & Udp services, DHCP & SQL

Explains how to maintain Nagios configurations.
Notifications & Escalations
External Commands

Passive check
Check using SSH
Check using NRPE

Check using SNMP
Check Windows terminals

Service & Host templates
Service & Host dependency

Click here to get book extracts.

Conclusion
This book covers most crucial parts of Nagios, which helps admin to configure Nagios in less time with more efficiency.

Uncovered Topics
The only missing part is Nagvis & pnp4nagios, for any admin pnp4nagios is must. It helps you to replace applications like MRTG, Cacti, PRTG by integrating graph in Nagios.