You'll need a working Nagios Core 4.0 or greater installation with a web interface and all the Nagios Core plugins installed. If you have not yet installed Nagios Core, you should start with the quick start guide at http://nagios.sourceforge.net/docs/nagioscore/4/en/quickstart.html that is appropriate to your operating system.

We'll assume that the configuration file Nagios Core reads on startup is at /usr/local/nagios/etc/nagios.cfg, as is the case with the default installation. It shouldn't matter where you include this new host definition in the configuration, as long as Nagios Core is going to read the file at some point. However, it might be a good idea to give each host its own file in a separate objects directory, which we'll do here. You should have access to a shell on the server and be able to write text files using an editor of your choice; I'll use vi. You will need root privileges on the server via su or sudo.

You should know how to reload Nagios Core on the server so that the configuration you're going to add gets applied. It shouldn't be necessary to restart the whole machine to do this! A common location for the startup/shutdown script on Unix-like hosts is /etc/init.d/nagios, which I'll use here. On modern GNU/Linux systems, it may be a better practice to use system nagios reload.

You should also get the hostname or IP address of the server you'd like to monitor ready. We'll use IP addresses rather than DNS hostnames here, which means that our checks will keep working even if DNS is unavailable. You may prefer to use hostnames if your addresses change regularly. You shouldn't need the subnet mask or anything like that; Nagios Core will only need whatever information the ping(8) tool would need for its own check_ping command.

Finally, you should test things first; confirm that you're able to reach the host from the Nagios Core server using ping(8) by checking directly from the shell, to make sure your network stack, routes, firewalls, and netmasks are all correct:

user@olympus:~$ ping 192.0.2.21
PING sparta.example.net (192.0.2.21) 56(84) bytes of data.
64 bytes from sparta.example.net (192.0.2.21): icmp_req=1 ttl=64 time=0.149 ms

We can create the new host definition for sparta.example.net as follows:

If the server restarted successfully, the web interface should now show a brand new host in the hosts list and a PENDING state as it waits to verify that the host is alive:

In the next few minutes, the host's background should change to green to show that the verification was complete and the host status should change to UP, assuming that the checks succeeded:

If the test failed and Nagios Core was not able to get a PING response from the target machine after three tries, for whatever reason, it would probably look something like this:

The configuration we included in the preceding adds a host to Nagios Core's list of hosts to check, Nagios Core will periodically send a PING request to 192.0.2.21, checking whether it receives a reply, and will update the status as shown in the Nagios Core web interface appropriately. We have neither defined any other services to check for this host yet, nor have we specified what action it should take if the host is down. However, the host itself will be automatically checked at regular intervals by Nagios Core and we can view its state in the web interface at any time.

The directives we defined in the preceding configuration are as follows:

Note that we added the definition in its own file called sparta.example.net.cfg and then referred to it in the main configuration file nagios.cfg. This is simply a conventional way of laying out hosts and it happens to be a tidy way to manage things to keep definitions in their own files.

There are a lot of other useful parameters for hosts, but the ones we've used include everything that's required.

While this is a perfectly valid way of specifying a host, it's more typical to define a host based on a template, with definitions of how often the host should be checked, who should be contacted when its state changes and on what basis, and similar properties. Nagios Core defines a simple template host called generic-host, which could be used by extending the host definition, as with the use directive:

define host {
    use                 generic-host
    name                sparta
    host_name           sparta.example.net
    address             192.0.2.21
    max_check_attempts  3
    contacts            nagiosadmin
    check_period        24x7
    check_command       check-host-alive
}

This uses all the parameters defined for generic-host and then adds on the details of the specific host that needs to be checked. If you're curious to see what's defined in generic-host, you'll find its definition by navigating to /usr/local/nagios/etc/objects/templates.cfg.

You'll need a working Nagios Core 4.0 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, 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, so we know that the test we're about to set up should succeed. The telnet(1) tool is a fine way to test that a response comes back from the TCP port 80 as we would expect from a web server:

user@olympus:~$ telnet 192.0.2.21 80
Trying 192.0.2.21...
Connected to 192.0.2.21.
Escape character is '^]'.

We can create the service definition for sparta.example.net as follows:

If the server restarted successfully, the web interface should now show you a new service under the Services section and a 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 a similar response:

If the check had problems, perhaps because the HTTP daemon isn't running on the target server, the check may show CRITICAL instead. This probably doesn't mean that the configuration is broken; it more likely means that 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 test, it will flag the state of the service as CRITICAL and will try again two more times before sending a notification. The service will be visible in the Nagios Core web interface, we can check its state any time, and 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.example.net 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:

Note that we added the service definition in the same file as defining the host, directly after it. We can actually place the definition anywhere we like, but this happens to be a good way to keep things organized.

The service we've set up to monitor on sparta.example.net is an HTTP service, but that's just one of the 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 others; all these commands, in turn, point to the programs that actually perform the 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. In practice, we often want to have a service template object with common values, and then extend it for each service we need to check. 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.example.net
    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 at /usr/local/nagios/etc/objects/templates.cfg. This is the same file that includes the generic-host definition that we may have used earlier.

You should have a working Nagios Core 4.0 or better server running with a web interface and at least one host to check. If you need to do this first, refer to the Creating a new network host recipe in this chapter.

For this particular kind of contact, you'll also need to have a working SMTP daemon running on the monitoring server, such as Exim or Postfix. You should verify that you're able to send messages to the target address and that they're successfully delivered to the host you expect them to be delivered to.

We can add a simple new contact to the Nagios Core configuration as follows:

When we are done with the preceding steps, the next time our host changes its state we should receive messages like the one shown in the following screenshot:

When the host becomes available again, we should receive a recovery message as follows:

If possible, it's worth testing this setup with a test host that we can safely bring down and then up again to verify that we receive appropriate notifications.

This configuration adds a new contact to the Nagios Core configuration and references it in one of the hosts as the appropriate contact to be used when the host has problems.

We've defined the required directives for the contact and a couple of others:

Note that we placed the definition for the contact in contacts.cfg, which is a reasonably sensible place. However, we can place the contact definition in any file that Nagios Core will read as part of its configuration; we can organize our hosts, services, and contacts any way we like, but it helps to choose some sort of system, so we can easily identify where definitions are likely to be when we need to add, change, or remove them.

If we define a lot of contacts with similar options, it may be appropriate to have individual contacts extend contact templates, so they can inherit those common settings. The default Nagios Core configuration includes such a template, called generic-contact. We could instead define our new contact as an extension of this template as follows:

define contact {
    use           generic-contact
    contact_name  spartaadmin
    alias         Administrator of sparta.example.net
    email         nagios@example.net
}

To see the directives defined for generic-contact, you can inspect its definition in the /usr/local/nagios/etc/objects/templates.cfg file.

You should have a working Nagios Core 4.0 or better server running.

We can verify the Nagios Core configuration as follows:

The configuration is parsed as though Nagios Core were about to start up, to check that the configuration all makes sense. It will run basic checks, such as looking for syntax errors, and will also check things such as having at least one host and service to monitor. Some of the things it reports are warnings, meaning that they're not necessarily problems, such as hosts not having any services monitored or not reporting to any contacts.

This is the quickest way to get an idea of whether the Nagios Core configuration is sane and will work correctly and whenever, there's trouble restarting the Nagios Core server, it's a good idea to check the output of this command.

The program at /usr/local/nagios/bin/nagios is actually the same program that runs the Nagios Core server, but the -v part of the command is a switch for the program that, instead of starting the server, verifies the configuration and shows any problems with it. The second path is to the configuration file with which Nagios Core starts, which in turn imports configuration files for objects such as the contact, host, and service definitions.

You should have a working Nagios Core 4.0 or better server running with a web interface.

You should also have at least two hosts that form a meaningful group; perhaps they're similar kinds of servers, such as web servers, or are monitored by the same team, or both at a physical location.

In this example, we have two web servers, sparta.example.net and athens.example.net, and we're going to add them to a group called webservers.

We can add our new hostgroup webservers to the Nagios Core configuration as follows:

We should now be able to visit the Host Groups section of the web interface and see a new hostgroup with two members:

The preceding configuration that we have added includes a new file with a new hostgroup into the Nagios Core configuration and inserts appropriate hosts into the group. The hostgroup creates a separate section in the web interface for us to get a quick overview of only the hosts in that particular group.

The way we've added hosts to the preceding groups is actually not the only way to do it. If we prefer, we can instead name the hosts for the group inside the group definition, using the members directive, so we could have something like the following:

define hostgroup {
    hostgroup_name  webservers
    alias           Webservers with Greek names
    members         athens.example.net,sparta.example.net
}

We can also make a hostgroup that always includes every single host, if we find that useful:

define hostgroup {
    hostgroup_name  all
    alias           All hosts
    members         *
}

If we're going to be using hostgroups extensively in our Nagios Core configuration to add hosts to groups, we should use whichever of the two methods we think is going to be easiest for us to maintain.

It's worth noting that a host can be in more than one group and there is no limit on the number of groups we can declare, so we can afford to be quite liberal with how we group our hosts into any sort of useful categories. examples of hostgroups could be organizing servers by a function, manufacturer, or colocation customer or organizing routers by BGP or OSPF usage; it all depends on what kind of network we're monitoring.

You should have a working Nagios Core 4.0 or better server running with a web interface.

You should also have at least two defined services that form a meaningful group; perhaps they're similar kinds of services, such as mail services, are monitored by the same team, or they are both on the same set of servers at a physical location.

In this example, we have three servers performing mail functions: smtp.example.net, pop3.example.net, and imap.example.net, running an SMTP, POP3, and IMAP daemon, respectively. All three of the hosts are set up in Nagios Core, and so are their services. We're going to add them into a new servicegroup called mailservices.

Here are the definitions of the hosts and services used in this example, so you can see how everything fits together:

define host {
    use                 linux-server
    host_name           smtp.example.net
    alias               smtp
    address             192.0.2.31
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            smtp.example.net
        service_description  SMTP
        check_command        check_smtp
    }
define host {
    use                 linux-server
    host_name           pop3.example.net
    alias               pop3
    address             192.0.2.32
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            pop3.example.net
        service_description  POP3
        check_command        check_pop
    }
define host {
    use                 linux-server
    host_name           imap.example.net
    alias               imap
    address             192.0.2.33
    hostgroups          webservers
}
    define service {
        use                  generic-service
        host_name            imap.example.net
        service_description  IMAP
        check_command        check_imap
    }

We can add our new servicegroup with the following steps:

We should now be able to visit the Service Groups section of the web interface and see a new servicegroup with three members:

The configuration we've added in the preceding section includes a new file with a new servicegroup into the Nagios Core configuration and inserts appropriate services into the group. The servicegroup creates a separate section in the web interface for us to get a quick overview of only the services in that particular group.

The way we've added services to the preceding groups is not the only way to do it. If we prefer, we can instead name the services (and their applicable hosts) for the group inside the group definition using the members directive, so we could have something like the following:

define servicegroup {
    servicegroup_name  mailservices
    alias              Mail services
    members            smtp.example.net,SMTP,pop3.example.net,POP3,imap.example.net,IMAP
}

Note that we need to specify both the host that the service is on and the services to be monitored on it. This string needs to be comma-separated. The hostname always comes first, but it can be followed by any number of its services that are to be monitored.

This allows us to make a servicegroup that always includes every single service on every host, if we find that useful:

define servicegroup {
    servicegroup_name  all
    alias              All services
    members            *
}

If we're going to use servicegroup definitions extensively in our Nagios Core configuration to add services to groups, we should use whichever of the two methods we think is going to be easiest for us to maintain.

It's worth noting that a service can be in more than one group and there is no limit on the number of groups we can declare, so we can afford to be quite liberal with how we group our services into any sort of useful categories. A few examples of this could be organizing services by the appropriate contact for their notifications, internal functions, or customer-facing functions.

You should have a working Nagios Core 4.0 or better server running.

You should also have at least two contacts that form a meaningful group. In this case, we have two staff members, John Smith and Jane Doe, who are both part of our network operations team. We want them both to be notified for all the appropriate hosts and services, so we'll add them to a group called ops. Here are the definitions with which we're working:

define contact {
    use           generic-contact
    contact_name  john
    alias         John Smith
    email         john@example.net
}
define contact {
    use           generic-contact
    contact_name  jane
    alias         Jane Doe
    email         jane@example.net
}

We can create our new ops contactgroup as follows:

With this group set up, we are now able to use it in the contactgroups directive for hosts and services to define which contacts notifications should be sent to. Notifications are sent to all the addresses in the group. This can replace the contacts directive, where we name contacts individually.

As an example, consider the following service definition:

define service {
    use                  generic-service
    host_name            sparta.example.net
    service_description  HTTP
    check_command        check_http
    contacts             john,jane
}

Instead of having a service definition as the preceding one, we could use this:

define service {
    use                  generic-service
    host_name            sparta.example.net
    service_description  HTTP
    check_command        check_http
    contact_groups       ops
}

If John Smith were to leave the operations team, we could simply remove his contact definition and nothing else would require changing; from then on, only Jane would receive the services notification. This method provides a layer of abstraction between contacts and the hosts and services for which they receive notifications.

You should have a working Nagios Core 4.0 or better server running. You should also have a clear idea about when the boundaries for the time period you want to define are.

We can set up our new time period, which we'll call weekdays, as follows:

In our host and service definitions, there are two directives, check_period and notification_period, which are used to define the times during which a host or service should be checked and the times when notifications about it should be sent. The two examples normally defined in the stock Nagios Core configuration are the 24x7 period and the workhours period, which are defined in the timeperiods.cfg file that we just edited and used in several of the examples and templates.

We've just added two more of these time periods that we can now use in our definitions for hosts and services. The first is called weekdays, which corresponds to any time during a weekday; the second is called weekends, which corresponds to any time that's not a weekday. Note that, in both cases, we specified the dates and times by naming each individual day and the times to which it corresponded.

The definitions for dates are flexible, and can be defined a variety of ways. The following are all valid syntaxes to define days and time periods:

In a typical practice, it's likely that the standard 24x7 and workhours definitions will be fine for day-to-day monitoring, maybe with a weekdays and weekends definition added. However, there may come a time when we need a specific host or service monitored on an unusual schedule, particularly if we're debugging a specific problem that only manifests around a certain time, or have a lot of contacts to manage, or a complex on-call roster.

Note that Nagios Core can behave in unusual ways, particularly with uptime reporting, if the time periods for our monitoring of hosts and services don't add up to 24 hours. Ideally, we should check and notify all our hosts and services in some way around the clock, but by dealing with the notifications in different ways depending on the schedule, for example paging the systems administrators about a noncritical system during work hours but just e-mailing them when they're asleep!

You should have a working Nagios Core 4.0 or better server running with a web interface. You should be familiar with adding services to individual hosts.

You should also have at least one hostgroup defined, with at least one host in it; we'll use a group called webservers with the sparta.example.net and athens.example.net hosts defined in it.

For reference, here is the hostgroup definition and the definitions for the two hosts in it:

define hostgroup {
    hostgroup_name  webservers
    alias           Webservers
}
define host {
    use         linux-server
    host_name   athens.example.net
    alias       athens
    address     192.0.2.22
    hostgroups  webservers
}
define host {
    use         linux-server
    host_name   sparta.example.net
    alias       sparta
    address     192.0.2.21
    hostgroups  webservers
}

We can create the service definition for the webservers group as follows:

It's important to note that if we were already monitoring these hosts with a per-host service of the same name, we will need to remove the definitions as well; Nagios Core will not start if a service of the same description is already defined on the same host.

Adding a service to a hostgroup works exactly in the same way as adding it to an individual host, except it only requires one definition, which is then individually applied to all the hosts in the group. This means that it's a very good way to keep a Nagios Core configuration tidier. If we have a group with 50 different web servers in it and we need to monitor their HTTP services on the same basis for every one of them, we don't need to create 50 service definitions as well; we can just create one for its hostgroup, which amounts to a smaller and more easily updated configuration.

Like the host_name directive for services, hostgroup_name can actually have several hostgroups defined, separated by commas. This means that we can apply the same service to not just one group but several of them. For services that we would want to run on several different groups, for example basic PING monitoring, this can amount to a much more flexible configuration.