Monitoring Windows

You install the Windows agent by installing the MSI package.

Before installation, you need to get the package and bring it to the host on which the agent will run (for example with scp or WinSCP).

The commercial editions have a software module, the Agent Bakery, for automatically packaging customized agents. A detailed description of this can be found in the general article on the agents. Installation of the baked MSI package is done in the same way as described above for the included package.

From Checkmk Cloud onwards you can additionally use the Agent Bakery to provide agent packages with a configuration for auto-registration, which facilitates the automatic creation of hosts. In this case, the agent registration is done automatically once the agent package has been installed, and manual registration, as described in the following chapter, will no longer be necessary.

If you use the Agent Bakery, you can also set up automatic updates of the agent. These updates are described in their own article.

During an installation the MSI package stores the program-specific files in C:\Program Files (x86)\checkmk\service\ and the host-specific files in C:\ProgramData\checkmk\agent\. You do not need to customize the program-specific files. The host-specific files are used to store plug-ins, log and configuration files and to configure the behavior of the agent.

Note: By default, the entire C:\ProgramData directory is hidden in Windows.

The agent reads three configuration files in succession:

If an option has been set in multiple files, then the last file read in determines the content of this option. For manual work with the agent, therefore, only the last configuration file check_mk.user.yaml is relevant, because it is read in last and thus has the last word. If the Agent Bakery is not used, this is in fact the only file in which customizations to the agent’s configuration may be made.

As you may have already recognized from the file extension of the configuration files, YAML is used as the file format.

After installing the agent including the Agent Controller, the next step is the registration, which sets up TLS encryption so that the encrypted agent output can be decrypted by the Checkmk server and displayed in monitoring.

There is a special feature when the agent has been installed with the Agent Controller for the first time. In such a case the agent will switch to the unencrypted legacy pull mode, so that the Checkmk server is not cut off from the monitoring data and can continue to display it. This applies to a new installation as well as to an update of an agent of version 2.0.0 or older.

It will look like this in the monitoring:

The Checkmk site recognizes from the agent output that the Agent Controller is present and thus that TLS encryption is possible — but not yet enabled. The Check_MK Agent service changes to the WARN state and remains so until you register it. After registration, only the encrypted pull mode will be used for communication. The legacy pull mode is switched off and will remain so. However, it can be switched on again by command if necessary.

The situation will be different when using a legacy agent on a very old Windows system. Without the Agent Controller a registration is not possible. Thus, for legacy agents the only relevant sections in the Registration chapter are adding the host to the Setup and then to the monitoring. In the chapter Testing and troubleshooting you must omit the test for calling the Agent Controller, because this is not available for a legacy agent. Since there is also no TLS encryption without the Agent Controller, you will need to use other methods of encryption if required. In this case we recommend using the built-in (symmetric) encryption using the Symmetric encryption (Linux, Windows) rule.

Note: In the Checkmk Agent installation auditing rule set you will find various settings to check the state of the agent and make it visible in monitoring. Among other things, here you can specify which state the Check_MK Agent service should have if TLS configuration has not yet been performed.

Immediately following the agent installation (also as an update of an agent of version 2.0.0 and older), only unencrypted communication is possible in the legacy pull mode. An exclusively encrypted data transmission can only be activated once a trust relationship has been established.

An exception to this are the packages preconfigured for the auto-registration and downloaded via the Agent Bakery. These packages perform the registration automatically after installation.

In all other cases, you perform the manual registration promptly after installing the agent. This chapter shows how to perform the registration.

The registration and thus the establishment of the mutual trust relationship is done under a Checkmk user with access to the REST-API. For this, a good choice is the automation user agent_registration which only has the permission to register agents and is automatically created with every Checkmk installation. You can randomize the corresponding automation password (automation secret) with the icon.

Note: Since there is no Agent Controller, and thus no registry and TLS encryption, on very old Windows systems you will need to use alternative encryption methods if required. In this case we recommend using the built-in (symmetric) encryption using the Symmetric encryption (Linux, Windows) rule.

First create the new host via Setup > Hosts > Add host. A host must exist in the configuration environment before it can be registered.

From Checkmk Cloud onwards you will find the Checkmk agent connection mode option in the properties of the host in the section on monitoring agents. Here you can activate the push mode for the Checkmk agent as an alternative to the pull mode, which is available in all editions.

The registration is done using the Agent Controller cmk-agent-ctl, which provides a command interface for configuring the connections. You can display command help with cmk-agent-ctl help, also for specific available subcommands, for example with cmk-agent-ctl help register.

Whether the host is configured for the pull mode or the push mode makes no difference for the command examples. The Agent Receiver tells the Agent Controller in which mode it should operate during registration.

Now go to the host that is to be registered. Here you have to make a request to the Checkmk site with administrator rights:

The host name following the --hostname option must be exactly the same as it was when it was created in the Setup. The --server and --site options specify the name of the Checkmk server and the site. The server name may also be the IP address, the site name (here mysite) corresponds to the one you see in the URL path for the web interface. The options are completed by the name and password used by the automation user. If you omit the --password option, the password will be requested interactively.

Caution, trap for the unwary: If you primarily administer Unix machines, you are used to enclosing paths or parameters with spaces or special characters in single quotes (apostrophes, 0x27). Windows interprets this character as part of the call — in this case the password — and the registration will fail. Use double quotes (quotation marks, 0x22) instead.

If the specified values were correct, you will be asked to confirm the identity of the Checkmk site to which you want to connect. For clarity here, we have abbreviated the server certificate to be confirmed:

Confirm with Y to complete the process.

If no error message is displayed, the encrypted connection will have been established. All data will now be transmitted in compressed form via this connection.

If you want to disable the interactive check of the certificate—​for example to fully automate the registration—​you might use the additional parameter --trust-cert. The transferred certificate will be automatically trusted in this case. Keep in mind that you should take other measures to verify the integrity of the certificate. This can be performed (manually or scripted) by inspecting the file /var/lib/cmk-agent/registered_connections.json.

From Checkmk Cloud onwards, Checkmk offers the facility to create hosts automatically at registration. For such auto-registration, in addition to a user with permission to register hosts, you need at least one folder configured to hold the hosts that are to be created automatically.

If these conditions have been met, you can also carry out the registration, including automatic host creation, via the command line.

Usually you will use the Agent Bakery settings procedure, which includes the /var/lib/cmk-agent/pre_configured_connections.json configuration file in the agent package and which performs the registration automatically during installation. The command line call presented here is therefore primarily used for testing and debugging, for example trying out your own agent labels with the --agent-labels <KEY=VALUE> option.

The biggest difference here is the modified register-new sub-command, which is used to request the registration and creation of a new host in the Checkmk site. The name of the host is the one stored in the %COMPUTERNAME% environment variable. The subsequent confirmation of the certificate is the same as shown in the last section.

Whether the host is created in pull mode, push mode or not at all is defined by your settings in the Agent registration rule set. Following a successful registration, it may take several minutes before the host appears in the monitoring.

The cmk-agent-ctl status command now shows exactly one trust relationship with the Checkmk server:

If the information is needed in a machine-readable format, append the additional parameter --json to retrieve the output formatted as a JSON object.

Note: There can only ever be one trust relationship between host and site. For example, if you register an already registered host mynewhost under a different name (mynewhost2) but with the same IP address, then the new connection will replace the existing one. The connection from mynewhost to the site will be disconnected and no more agent data will be supplied to the host for monitoring.

For easier registration of multiple hosts, any host on which the agent is installed can perform a registration on behalf of other hosts. The registration process exports a JSON file, which can then be transferred to the target host and imported there. Again, as before, the host registered in the job must already be set up on the site.

First, on any host in the Setup, the registration is performed by proxy. Here, of course, the Checkmk server comes in handy, as it is usually the first host to be set up. As with the example above, you can pass the password by option or be asked for it interactively if you omit the --password option. We redirect the JSON output to a file in the example:

Next we transfer the /tmp/mynewhost3.json file to the host we registered for and import that file:

Once the registration is complete, perform a connection test and a service discovery in the Checkmk server Setup. Then, as the last step include the discovered services in the monitoring by activating the changes.

If the connection test fails, refer to the following chapter for testing and troubleshooting information.

You can also deregister a host.

On a host connected to the Checkmk server, you can revoke the trust. Here, in the following command, the Universally Unique Identifier (UUID) to specify is the one output by the cmk-agent-ctl status command:

To delete all connections from the host and additionally restore the legacy pull mode, enter the following command:

After that, the agent will behave as it did following the initial installation and before the first registration, and sends its data unencrypted.

Complete the deregistration on the Checkmk server: In the Setup, on the Properties of host page, select the Host > Remove TLS registration menu item and confirm the prompt.

If you prefer using the command line: On the Checkmk server, for each connection of a host that is in monitoring, there is a soft link with the UUID that points to the folder with the agent output:

From Checkmk Cloud onwards you can switch hosts from push to pull mode and vice versa. This may be necessary in individual cases if changes to the network topology are pending, or a downgrade to Checkmk Enterprise — in which only the pull mode is possible — is to be carried out.

First specify the access mode in the Setup, in the properties of the host, with the Checkmk agent connection mode option. Within the next minute, all services will assume the UNKNOWN status since no monitoring data is being received. Then perform a new registration. During this re-registration, the Checkmk server’s Agent Receiver tells the Agent Controller whether it expects data in pull or push mode. A subsequent check using cmk-agent-ctl status will then show a new UUID and a mode consistent with the change made in the Setup.

To check that the configuration has been read in as expected, call the agent program with the showconfig option. With this option you will not only get the configuration as it is currently used by the agent, but additionally, the environment variables as well as the configuration files in use will always be displayed.

If only a certain part of the configuration is of interest, you can limit the output to that specific part. Here, for example, it is checked whether the options in the ps section have been set correctly:

In this way you get a quick overview of how the three different configuration files have been merged and used by the agent program. Errors will be immediately visible.

If registering a host fails even before a certificate is presented, knowledge about the methods of communication can help identifying the problem — and of course solving it.

Once the cmk-agent-ctl register command has been entered, the Agent Controller first asks the Checkmk server for the Agent Receiver port using the REST API. As the second step a connection to the Agent Receiver is established to request the certificate. You can simulate the first request on the host with a program such as curl:

The parameter --insecure instructs curl to skip the certificate check. This behavior reflects the behavior of the Agent Controller in this step. The response is only a few bytes, containing the port number of the Agent Receiver. For the first site this is usually just 8000, for the second 8001 and so on. Common problems regarding this request are:

When the curl command fails you might change routing or firewall settings to enable access.

In case the host you are trying to register uses an HTTP proxy, curl will use it, but cmk-agent-ctl won’t do so with default settings. Use the additional --detect-proxy option to instruct cmk-agent-ctl to use a proxy configured via system settings.

However it may often be easier to identify the port of the Agent Receiver and note it down. To do so, on the Checkmk server, logged in as site user, run:

Now you can specify the port when entering the command for registration. This skips the first request to the REST API. Communication then takes place directly with the Agent Receiver without any detours:

Port 8000 also must be reachable from the host. In case it is not, you will get this error message:

Equivalent to port 443 (respectively 80) mentioned above, you can now adjust routing or firewall settings so that the host to be registered can reach the Checkmk server on the Agent Receiver’s port (8000 or 8001…​).

In the case of a registration in push mode the following applies: If the registration has worked, the minute-by-minute transfer of the agent output will also be successful.

Should security policies prohibit access to the Agent Receiver, there is still the possibility to use registration by proxy on the Checkmk server.

As the agent program must be called under the LocalSystem account in order to deliver exactly the data that arrives in the monitoring, you should never start it in a shell. If you want to examine the agent output locally, use the Agent Controller in dump mode (subcommand dump). This starts the agent program with the correct environment and under the correct user ID and then outputs the result.

Since the output can be a bit longer, the 'more' pager is very handy here. You can exit it with the Q key:

This allows you to verify that the data from the agent program has arrived at the Agent Controller. This output does not yet prove that the agent is also accessible over the network.

If in pull mode it has been verified that the agent program and its installed plug-ins are executing correctly, you can next check via netcat (or nc) whether port 6556 is reachable via the external IP address of the host:

The output 16 indicates whether the connection was successfully established and that the TLS handshake can now take place. Since everything else here is TLS encrypted, no more detailed check is possible.

If the communication between agent and Checkmk server is still unencrypted (as in legacy pull mode) or is and remains unencrypted (as in legacy agent), you will get the complete unencrypted agent output with this command instead of the 16.

For more diagnostics to run on the Checkmk server, see the general article on the monitoring agents. In particular, you can also perform a connection test using the Checkmk interface. You will get the result in the Agent: box.

If you get no information or only a timeout error message during the connection test, as in the example above, you should check the Inbound Rules of the Windows firewall on the host.

The agent already creates a rule in the Windows Firewall during its installation, so that the Agent Controller can be reached from the outside via port 6556. When using the push mode, it is usually not necessary to change its settings. If you use a very strict firewall configuration, the Outbound Rules for connections to the monitoring server must be configured so that at least port 8000 (for easier registration, additionally ports 80 or 443) is reachable.

In current versions of Windows you can find the Windows Defender Firewall with Advanced Security via Windows Settings (Settings > Windows Security) or start it by calling wf.msc from the command line:

If you do not find such an entry in the Windows Firewall settings, you can add it at this exact location. To do this, click on New Rule in the Action menu.

This opens a wizard for creating a new firewall rule. Set the five choices as follows:

Alternatively, you can automate this step and set the rule directly on the command line. Modify the following command to your customized installation path if necessary:

Note: The command has been split into four lines for readability.

In your Checkmk site’s ~/var/agent-receiver/received-outputs/ folder, for each registered host you will find a soft link that uses the host’s UUID as its name. For hosts in push mode this soft link points to the folder with the agent output, for pull hosts it points to a non-existent file with the name of the host as used in the monitoring.

Based on the age of the cached agent output, you can determine whether the regular transmission was successful or is being interrupted by sporadic network problems, for example.

Furthermore, you can take a look at the log file C:\ProgramData\checkmk\agent\log\check_mk on the host (paths may be configured differently). Lines such as the following indicate connection problems:

If a host has been configured for auto-registration with the Agent controller auto-registration rule set and the Keep existing connections option is set to no, whenever the cmk-agent-ctl-daemon service is restarted (for example, when a host is restarted), all other connections will be removed — except the connection configured for auto-registration. This affects, for example, hosts where connections to multiple sites were set up before the baked agent package was installed, or connections were manually added after the agent package was installed.

You can temporarily override this behavior by setting the keep_existing_connections variable to true in the C:\ProgramData\checkmk\agent\pre_configured_connections.json file on the host. You can achieve a permanent change across an agent package update by setting Keep existing connections to yes in the above rule set.

When auto-registering a host, typically about two minutes pass before the host appears in the monitoring.

Security is an important criterion for any software, and monitoring is no exception. Since the monitoring agent is installed on every monitored server, a security problem here would have particularly serious consequences.

This is why security was emphasized in the design of Checkmk and has been an absolute principle since the earliest days of Checkmk: The agent does not read data from the network. Period. This means that it is impossible for an attacker to inject any commands or script components via the monitoring port 6556.

For an attacker, however, even a process list can be a first approach for drawing conclusions about worthwhile targets. Therefore, transport encryption between agent and Checkmk server with Transport Layer Security (TLS) is mandatory from Checkmk version 2.1.0. Here, the Checkmk server 'pings' the monitored host, which then establishes the TLS connection to the Checkmk server and transmits the agent output over it. Since only Checkmk servers with which a trust relationship exists can initiate this data transfer, there is no risk of data falling into the wrong hands.

To secure the TLS connection, Checkmk uses a self-signed certificate that is automatically replaced shortly before its validity expires. The Agent Controller takes care of renewing the certificate in time before it expires. Only agents that have been inactive for a longer period of time, i.e., without a running Agent Controller, can lose their registration upon expiration and must then be registered again. The lifetime of the certificate can be specified via the Agent Certificates > Lifetime of certificates global setting.

Note: Since there is no Agent Controller, and thus no registry and TLS encryption, on very old Windows systems, you will have to choose other ways of encryption if needed. In this case we recommend using the built-in (symmetric) encryption using the Symmetric encryption (Linux, Windows) rule.

Restricting access to specific IP addresses can also be configured via the firewall. However, the agent itself also offers the possibility to simply ignore requests from foreign IP addresses. Just add the following restriction to the configuration file in the global options. Note that there may be other parameters set in the configuration file before or after this and this is just a snippet:

As you can see in the example, you can allow any number of subnets. For example, with /32 you specify a subnet of size 1, so that only this one address is allowed, while with 192.168.42.0/24 you allow all addresses between 192.168.42.0 and 192.168.42.255.

In Agent Bakery, you can configure the allowed IP addresses using the following rule set: Setup > Agents > Windows, Linux, Solaris, AIX > Agent rules > Allowed agent access via IP address (Linux, Windows).

Especially when updating the agent, it may be that the built-in (symmetric) encryption is active, which is performed by the agent program itself. If TLS encryption and built-in encryption are active at the same time, then the entropy of the transmitted data is so high that compression, which is active from version 2.1.0 onwards, will not save any transmitted data — and will burden the CPUs of both the host and the Checkmk server with additional encryption and decryption processes.

For this reason, you should deactivate the built-in encryption promptly after switching to TLS.

In the first step, deactivate the encryption in the existing rule under Setup > Agents > Access to agents > Checkmk agent > Symmetric encryption (Linux, Windows).

In the second step, on the agent’s host, in the C:\ProgramData\checkmk\agent\check_mk.user.yml configuration file, change the value of the encrypted parameter to no:

In the third and last step, use the Enforce agent data encryption rule to specify that the Checkmk server only accepts data encrypted via TLS. To do this, select the Accept TLS encrypted connections only value in the rule.

Turning off encryption with the Agent Bakery proceeds like this: With the first step, changing the Symmetric encryption (Linux, Windows) rule, you are almost done. You only need to bake and distribute new agents. The configuration file C:\ProgramData\checkmk\agent\check_mk.user.yml will be automatically changed for you and included in the agent packages. All that remains is the third step, i.e. modifying the Enforce agent data encryption rule.

Following the next automatic agent update, the encryption of the agent program is disabled, but encryption is guaranteed by the Agent Controller. Note that after the automatic agent update, only registered hosts will be able to provide monitoring data.

The agent program check_mk_agent.exe contains a whole set of sections which provide monitoring data for various check plug-ins which are then automatically found by the service discovery. This includes all important monitoring of the operating system.

In addition, there is the possibility to extend the agent with agent plug-ins. These are small scripts or programs that are called by the agent and extend it with additional sections with additional monitoring data. The Checkmk project provides a number of such plug-ins, which — if installed and configured correctly — automatically provide new services in the service discovery.

Why aren’t these plug-ins simply hard-coded into the agent? For each of the plug-ins there is one of the following reasons:

The included plug-ins for Windows can all be found on the monitored host in the agent installation directory under C:\Program Files (x86)\checkmk\service\plugins. They are stored there so that they are directly available. Alternatively, the plug-ins for Windows are also located on the Checkmk server under ~/share/check_mk/agents/windows/plugins.

They are also available from the agent download page in the Setup menu (as described in the Installation chapter) in the Plugins box:

For all agent plug-ins we provide, there are matching check plug-ins that can evaluate their data and generate services. These are already installed, so that newly found services are immediately recognized and can be configured.

Note: Before you install a plug-in on the host, take a look at the corresponding file. Often you will find important information there about the correct use of the plug-in.

The actual installation is then simple: Copy the file to C:\ProgramData\checkmk\agent\plugins.

Once the plug-in is in the correct directory, it will be called automatically by the agent and a new section will be created in the agent output. This usually has the same name as the plug-in. Complex plug-ins (e.g. mk_oracle.ps1) even create a whole set of new sections.

Some plug-ins need a configuration file in C:\ProgramData\checkmk\agent\config to work. For others, a configuration is optional (e.g. mssql.vbs) and allows special features or customizations. Still others just work that way. You have several sources to get information:

For special (scripting) languages, it may be necessary to enable them first in the agent configuration. For example, Python scripts will not be executed unless they are explicitly enabled. You can do this by extending the file extensions in the check_mk.user.yml configuration file in the global section, as shown in the following excerpt:

Important: The use of such plug-ins requires that the files can also be called in a regular command line without special paths. In the case of Python, it must be installed correctly and the path to the interpreter must be present in the environment variables. Instructions on how to set up Python correctly can be found directly on the pages of the Python Software Foundation.

Each plug-in can be executed in different modes. The following options are available for entry in the configuration file.

A configuration for the Veeam plug-in looks then for example like this (the excerpt is shortened and contains only the relevant part for the example):

According to the above exemplary configuration the plug-in in the data directory C:\ProgramData\checkmk\agent\plugins is executed asynchronously every five minutes (300 seconds) and may run for a maximum of two minutes (120 seconds). If the plug-in runs into this timeout, it will try a second time to get a result.

In the commercial editions, the included plug-ins can be configured via the Agent Bakery. This takes care of both the installation of the plug-in itself and the correct creation of the configuration file, should one be needed.

Each plug-in is configured via an agent rule. You can find the appropriate rule sets in Setup > Agents > Windows, Linux, Solaris, AIX > Agent rules > Agent Plugins:

Since agent plug-ins are executable programs, you can run them manually for testing and diagnostic purposes. However, there are plug-ins that need certain environment variables set by the agent to find their configuration file, for example. If necessary, set them manually if they are needed in the script or program.

There are two good reasons to continue using Nagios plug-ins under Checkmk. If you have migrated your monitoring from a Nagios based solution to Checkmk, you can continue to use older check plug-ins for which there is no Checkmk equivalent yet. In many cases these are self-written plug-ins in Perl or shell.

The second reason is true end-to-end monitoring. Let’s assume you have your Checkmk server, a web server and a database server distributed over a large data center. In such a case, the database server response times measured from the Checkmk server are not very meaningful. It is far more important to know these values for the connection between the web server and the database server.

The Checkmk agent provides a simple mechanism to meet these two requirements: MK’s Remote Plugin Executor or MRPE for short. The name is deliberately an analogy to the NRPE of Nagios, which performs the same task there.

The MRPE is built into the agent and is controlled by various configuration files.

As an alternative to configuring directly on a host in the user-specific configuration file, you can also define your MRPE plug-ins directly in the Setup menu. To do this, use the Setup > Agents > Windows, Linux, Solaris, AIX > Agent > Agent rules > Execute MRPE checks rule set. The necessary entry is then automatically created in the Agent Bakery configuration file.