## <a id="monitoring-remote-clients"></a> Monitoring Remote Clients

### Agent-less Checks

If the remote service is available using a network protocol and port,
and a [check plugin](#setting-up-check-plugins) is available, you don't
necessarily need a local client installed. Rather choose a plugin and
configure all parameters and thresholds. The [Icinga 2 Template Library](#itl)
already ships various examples.

### Agent-based Checks

If the remote services are not directly accessible through the network, a
local agent installation exposing the results to check queries can
become handy.

#### SNMP

The SNMP daemon runs on the remote system and answers SNMP queries by plugin
binaries. The [Monitoring Plugins package](#setting-up-check-plugins) ships
the `check_snmp` plugin binary, but there are plenty of [existing plugins](#integrate-additional-plugins)
for specific use cases already around, for example monitoring Cisco routers.

The following example uses the [SNMP ITL](#itl-snmp) `CheckCommand` and only defines
additional macros (note the `+=` operator) as command parameters for the `oid`
(`community` is already set):

    object Host "remote-snmp-host" inherits "generic-host" {
      ...
      services["uptime"] = {
          templates = [ "generic-service" ],
          check_command = "snmp",
          macros += {
            "oid" = "1.3.6.1.2.1.1.3.0"
          }
      },
      macros = {
        "address" = "192.168.1.101"
      }
    }

#### SSH

Calling a plugin using the SSH protocol to execute a plugin on the remote server fetching
its return code and output. `check_by_ssh` is available in the [Monitoring Plugins package](#setting-up-check-plugins).

    object CheckCommand "check_by_ssh_swap" inherits "plugin-check-command" {
      command = [ "$plugindir$/check_by_ssh",
                  "-l", "remoteuser",
                  "-H", "$address$",
                  "-C", "\"/usr/local/icinga/libexec/check_swap -w $warn$ -c $crit$\""
                ]
    }

    object Host "remote-ssh-host" inherits "generic-host" {
      ...
      services["swap"] = {
          templates = [ "generic-service" ],
          check_command = "check_by_ssh_swap",
          macros = {
            "warn" = "50%",
            "crit" = "75%"
          }
      },
      macros = {
        "address" = "192.168.1.102"
      }
    }


#### NRPE

[NRPE](http://docs.icinga.org/latest/en/nrpe.html) runs as daemon on the remote client including
the required plugins and command definitions.
Icinga 2 calls the `check_nrpe` plugin binary in order to query the configured command on the
remote client.

> **Note**
>
> The NRPE daemon uses its own proprietary configuration format in nrpe.cfg while `check_nrpe`
> can be embedded into the Icinga 2 `CheckCommand` configuration syntax.

Example:

    object CheckCommand "check_nrpe" inherits "plugin-check-command" {
      command = [ "$plugindir$/check_nrpe",
                  "-H", "$address$",
                  "-c", "$remote_nrpe_command$",
                ],
    }

    object Host "remote-nrpe-host" inherits "generic-host" {
      ...
      services["users"] = {
          templates = [ "generic-service" ],
          check_command = "check_nrpe",
          macros = {
            "remote_nrpe_command" = "check_users"
          }
      },
      macros = {
        "address" = "192.168.1.103"
      }
    }

nrpe.cfg:

    command[check_users]=/usr/local/icinga/libexec/check_users -w 5 -c 10


#### NSClient++

[NSClient++](http://nsclient.org) works on both Windows and Linux platforms and is well
known for its magnificent Windows support. There are alternatives like the WMI interface,
but using `NSClient++` will allow you to run local scripts similar to check plugins fetching
the required output and performance counters.

> **Note**
>
> The NSClient++ agent uses its own proprietary configuration format while `check_nt`
> can be embedded into the Icinga 2 `CheckCommand` configuration syntax.

Example:

    object CheckCommand "check_nscp" inherits "plugin-check-command" {
      command = [ "$plugindir$/check_nt",
                  "-H", "$address$",
                  "-p", "$port$",
                  "-v", "$remote_nscp_command$",
                  "-l", "$partition$",
                  "-w", "$warn$",
                  "-c", "$crit$",
                  "-s", "$pass$"
                ],
      macros = {
        "port" = "12489",
        "pass" = "supersecret"
      }
    }

    object Host "remote-windows-host" inherits "generic-host" {
      ...
      services["users"] = {
          templates = [ "generic-service" ],
          check_command = "check_nscp",
          macros += {
            "remote_nscp_command" = "USEDDISKSPACE",
            "partition" = "c",
            "warn" = "70",
            "crit" = "80"
          }
      },
      macros = {
        "address" = "192.168.1.104"
      }
    }

For details on the `NSClient++` configuration please refer to the [official documentation](http://www.nsclient.org/nscp/wiki/doc/configuration/0.4.x).

> ** Note **
> 
> The format of the `NSClient++` configuration file has changed from 0.3.x to 0.4!


#### Icinga 2 Agent

A dedicated Icinga 2 agent supporting all platforms and using the native
Icinga 2 communication protocol supported with SSL certificates, IPv4/IPv6
support, etc is on the [development roadmap](https://dev.icinga.org/projects/i2?jump=issues).
Meanwhile remote checkers in a [Cluster](#cluster) setup could act as
immediate replacement, but without any local configuration - or pushing
their standalone configuration back to the master node including their check
result messages.


### Passive Check Results and SNMP Traps

> **Note**
>
> The host and service object configuration must be available on the Icinga 2
> server in order to process passive check results.

#### NSCA-NG

[NSCA-ng](http://www.nsca-ng.org) provides a client-server pair that allows the
remote sender to push check results into the Icinga 2 `ExternalCommandListener`
feature.

The [Icinga 2 Vagrant Demo VM](#vagrant) ships a demo integration and further samples.


#### SNMP Traps

SNMP Traps can be received and filtered by using [SNMPTT](http://snmptt.sourceforge.net/) and specific trap handlers
passing the check results to Icinga 2.