Using an SMS Server to Provide a Robust Alerting Service for Nagios

I'm a big fan of the Nagios network monitoring system and rely on it to tell me if something goes wrong with the systems for which I am responsible. I have made a large investment in time configuring Nagios to monitor exactly what I am interested in, and this effort would be wasted if Nagios detected a problem, but failed to communicate that problem to me. To make Nagios more robust, I wanted to make sure that its alerting mechanism did not depend on connections to the Internet—this would include the physical connection itself and internal and external services, such as e-mail, routing and DNS.

I have relied on e-mail-based systems in the past to deliver alerts; however, my dilemma was that if I was not getting e-mail, I did not know if this meant everything was okay or if there was some problem preventing me from getting the e-mail alerts, such as a down Internet connection or another kind of e-mail failure. I found that I became uneasy after long periods of silence and felt compelled to “poll” the system to make sure everything was okay.

On the other hand, I felt that if my alerting system was robust and I could trust it, my thinking would become “no news is good news”, and the absence of alerts would mean everything was fine.

I've found that the Short Message Service (SMS) text service available on GSM cellular networks meets my requirements for a trusted alerting server. It is generally available and is unlikely to go down. A major disaster certainly could take down or overwhelm the cellular service, but I figure I would be aware of such an event and probably would have bigger and more pressing concerns than network management at that point.

There are several different ways to implement a Nagios-to-SMS service, and I certainly have not explored them all. This article describes the system I am using, which is the MultiTech Systems MultiModem iSMS Intelligent SMS server (Figure 1) in combination with a public domain Perl script running on a Linux-based Nagios server.

Figure 1. MultiTech Systems MultiModem iSMS Intelligent SMS Server

I selected this hardware and software combination for the following reasons:

As you can see in Figure 1, the iSMS is packaged in a sturdy metal enclosure. I used large plastic wire ties to mount the iSMS to a horizontal rack post, but it also can be mounted with screws. The antenna is visible on the top, and there is a little hatch on the bottom where the Subscriber Identity Module (SIM) card is plugged in.