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

How to implement a Nagios-to-SMS service.

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:

  • Another company had done all the required work to integrate the iSMS device with Nagios, clearly documented the process and made this freely available on the Web, including the Perl script described in this article.

  • A major feature of the Perl script is the ability to “ACK” or acknowledge a Nagios alert. This means you don't have to have any kind of IP connection to your Nagios server to perform acknowledgements. The ability to acknowledge alerts is helpful when you are off the IP network, as it stops any future alerts and can prevent the alerts from going to others if you have configured Nagios to do this. The script also can force a service or host back to an “OK” state if desired.

  • The iSMS device is a standalone “appliance” and does not depend on any infrastructure other than a (local) Ethernet connection, GSM cellular service and electrical power. Most other products in this area are similar to traditional analog modems in that they have serial connections hard-wired to a specific host. As the iSMS is connected via Ethernet, it can be accessed and shared by multiple hosts. The particular model I used has a single GSM modem, but four- or eight-modem versions are also available.

  • Other Nagios users are using conventional mobile phone handsets in this role, but I feel that consumer-level power supplies and some kind of jury-rigged mounting of a phone in a machine room would undermine the reliability I want. The iSMS has a robust metal case and can be attached securely to a rack. The power plug is threaded to the chassis to prevent accidental unplugging.

  • The iSMS has a Web-based administration interface and supports multiple methods of communication, including a “Telnet” interface to connect directly to the GSM modem for use of “AT” commands and multiple APIs. These include both TCP and HTTP APIs for sending and receiving SMS messages or querying the status of queued messages. Certainly, you could use Web-based or e-mail-based tools to create a similar alerting functionality, but SMS is somewhat unique in that it does not require an IP connection and is generally available wherever a modern cellular infrastructure exists.

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.