In this recipe, we'll create a new service to check on an existing host. Specifically, we'll check our sparta.naginet server to see if it's responding to HTTP requests on the usual HTTP TCP port 80. To do this, we'll be using a predefined command called check_http
, which in turn uses one of the standard set of Nagios Core plugins, also called check_http. If you don't yet have a web server defined as a host in Nagios Core, then you may like to try the Creating a new network host recipe in this chapter.
After we've done this, not only will our host be checked for a PING response by check_command, but Nagios Core will also run a periodic check to ensure that an HTTP service on that machine is responding to requests on the same host.
You'll need a working Nagios Core 3.0 or greater installation with a web interface, all the Nagios Plugins installed, and at least one host defined. If you need to set up a host definition for your web server first, then you might like to read the Creating a new network host recipe in this chapter, for which the requirements are the same.
It would be a good idea to test that the Nagios Core server is actually able to contact the web server first, to ensure that the check we're about to set up should succeed. The standard telnet tool is a fine way to test that a response comes back from TCP port 80 as we would expect from a web server:
tom@olympus:~$ telnet sparta.naginet 80 Trying 10.128.0.21... Connected to sparta.naginet. Escape character is '^]'.
We can create the service definition for sparta.naginet as follows:
Change to the directory containing the file in which the
sparta.naginethost is defined, and edit it as follows:# cd /usr/local/nagios/etc/objects # vi sparta.naginet.cfg
Add the following code snippet to the end of the file, substituting in the value of the host's
host_namedirective:define service { host_name sparta.naginet service_description HTTP check_command check_http max_check_attempts 3 check_interval 5 retry_interval 1 check_period 24x7 notification_interval 60 notification_period 24x7 contacts nagiosadmin }Restart the Nagios Core server:
# /etc/init.d/nagios restart
If the server restarted successfully, the web interface should show a new service under the Services section, in PENDING state as the service awaits its first check:
Within a few minutes, the service's state should change to OK once the check has run and succeeded with an HTTP/1.1 200 OK response, or similar:
If the check had problems, perhaps because the HTTP daemon isn't running on the target server, then the check may show CRITICAL instead. This probably doesn't mean the configuration is broken; it more likely means the network or web server isn't working:
The configuration we've added adds a simple service check definition for an existing host, to check up to three times whether the HTTP daemon on that host is responding to a simple HTTP/1.1 request. If Nagios Core can't get a response to its check, then it will flag the state of the service as CRITICAL, and will try again up to two more times before sending a notification. The service will be visible in the Nagios Core web interface and we can check its state at any time. Nagios Core will continue testing the server on a regular basis and flagging whether the checks were successful or not.
It's important to note that the service is like a property of a particular host; we define a service to check for a specific host, in this case, the sparta.naginet web server. That's why it's important to get the definition for host_name right.
The directives we defined in the preceding configuration are as follows:
host_name: This references the host definition for which this service should apply. This will be the same as thehost_namedirective for the appropriate host.service_description: This is a name for the service itself, something human-recognizable that will appear in alerts and in the web interface for the service. In this case, we've usedHTTP.check_command: This references the command that should be used to check the service's state. Here, we're referring to a command defined in Nagios Core's default configuration calledcheck_http, which refers to a plugin of the same name in the Nagios Core Plugins set.max_check_attempts: This defines the number of times Nagios Core should attempt to re-check the service after finding it in a state other thanOK.check_interval: This defines how long Nagios Core should wait between checks when the service isOK, or after the number of checks given inmax_check_attemptshas been exceeded.retry_interval: This defines how long Nagios Core should wait between retrying checks after first finding them in a state other thanOK.check_period: This references the time period during which Nagios Core should run checks of the service. Here we've used the sensible24x7time period, as defined in Nagios Core's default configuration. Note that this can be different fromnotification_period; we can check the service's status without necessarily notifying a contact.notification_interval: This defines how long Nagios Core should wait between re-sending notifications when a service is in a state other thanOK.notification_period: This references the time period during which Nagios Core should send notifications if it finds a host in a problem state. Here we've again used24x7, but for some less critical services it might be appropriate to use a time period such asworkhours.
Note that we added the service definition in the same file as defining the host, and directly after it. We can actually place the definition anywhere we like, but this happens to be a good way of keeping things organized.
The service we've set up to monitor on sparta.naginet is an HTTP service, but that's just one of many possible services we could monitor on our network. Nagios Core defines many different commands for its core plugin set, such as check_smtp, check_dns, and so on. These commands, in turn, all point to programs that actually perform a check and return the results to the Nagios Core server to be dealt with. The important thing to take away from this is that a service can monitor pretty much anything, and there are hundreds of plugins available for common network monitoring checks available on the Nagios Exchange website: http://exchange.nagios.org/.
There are a great deal more possible directives for services, and in practice it's more likely for even simple setups that we'll want to extend a service template for our service. This allows us to define values that we might want for a number of services, such as how long they should be in a CRITICAL state before a notification event takes place and someone gets contacted to deal with the problem.
One such template that Nagios Core's default configuration defines is called generic-service, and we can use it as a basis for our new service by referring to it with the use keyword:
define service {
use generic-service
host_name sparta.naginet
service_description HTTP
check_command check_http
}This may work well for you, as there are a lot of very sensible default values set by the generic-service template, which makes things a lot easier. We can inspect these values by looking at the template's definition in /usr/local/nagios/etc/objects/templates.cfg. This is the same file that includes the generic-host definition that we may have used earlier.