Host tags

Host tags are keywords that can be assigned to hosts in order to structure and organize them, for example by importance, IP address family, or the way the host receives its data. For instance, host tags will be inherited via the folder structure in Checkmk and can be selected as a condition in rules for targeting hosts.

Host tags are useful not only for performing a configuration, but also for the monitoring. For example, there is a filter for host tags in views and the Virtual host tree snap-in can arrange your folders in a tree diagram based on the tags.

Likewise, on the command line, for many commands with the @foo syntax you can select all hosts with the foo tag.

To ensure sure that everything makes sense, you should create your own host tag structure that best fits your environment and which matches the other options in the structuring hosts procedure. Once set up, subsequent changes to host tags are possible, but should be avoided — since renaming the ID of a tag will require manual rework in most cases.

But before we show you how to define your own host tags in the Setup, let’s first clarify some terms.

You can get started configuring the host tags via Setup > Hosts > Tags:

Checkmk sets up multiple host tag groups during an installation:

You can customize predefined tag groups as long as they are not marked as built-in (in the Actions column). The built-in tag groups are required internally by Checkmk when generating configurations and are therefore not modifiable. In contrast, changes in Criticality or Network Segment are not problematic. These are provided as examples only.

You start the creation of custom host tags on the Tag groups page, which again can be accessed via Setup > Hosts > Tags.

Before you can create host tags, you must first create the host tag group that will contain the tags. Creating a new tag group is done with the Add tag group button, which opens the Basic settings box with the following fields:

The Tag group ID is used internally as the ID for the tag group. It must be unique and cannot be changed later. The usual rules for permitted characters apply (letters, digits, underscore only).

The Title is used everywhere in the GUI where the tag group is concerned. Since this is a display only text, it can be changed at any time without affecting the existing configuration.

You can leave the Topic empty. Your tag group will then be displayed together with the supplied Criticality and Networking Segment groups in the Custom attributes box of the host properties. You can also create your own topics and use these to consolidate your tag groups neatly together.

Most importantly, the next box — Tag choices — is where you specify, one by one, all of the host tags for the new tag group:

For this the Tag ID must be unique within the group.

The order, which you can change as usual with the button, has not only an optical function: The first tag in the list is the default value! This means that all hosts that do not have an explicit setting for this tag group are automatically set to this value.

Under Auxiliary tags, you can assign auxiliary tags to each host tag to be automatically added to the host when the host tag is selected.

You create a checkbox tag in the same way, by creating a tag group but containing only one tag:

In the host properties, this tag will then be displayed like this:

In addition to the predefined host tag groups, Checkmk also sets up matching auxiliary tags, which are listed under the groups on the Tag groups page.

You can create new auxiliary tags with Add aux tag.

With the fixed ID and a descriptive title, all of the settings required for an auxiliary tag will be defined. The assignment of auxiliary tags to host tags is done in the tag groups.

Modifying an existing tag group configuration may look like a simple operation at first glance, unfortunately this is not always the case, as it can have a major impact on your existing configuration.

Changes that only affect the display or only add new options are not problematic and do not affect the existing hosts and rules:

Any other changes may affect existing folders, hosts and rules that use the affected tags. When doing so, Checkmk does not simply prohibit such changes, but tries to modify your existing configuration for you, so that everything will again make sense. The exact results will depend on the type of operation.

Tip: Checkmk can show you which host and auxiliary tags are currently in use in folders, hosts and rules: To do this, on the Tag groups page, select the Tags > Tag usage menu item.

Hosts are usually organized into folders in Checkmk. You can display the resulting hierarchy as a tree view in the sidebar at the bottom of the snap-ins Tree of Folders and from there call the default view for hosts filtered per branch. The snap-in adds filtering options for topics and options for alternative views for this tree.

You can also create such a tree view from host tags and thus map a 'virtual' hierarchy — using the Virtual host tree snap-in. In addition to the host tags, you may also include the folder structure in such trees, whereby both the number of virtual trees and the respective branches are unlimited.

Let’s assume you use the three tag groups Criticality, Data center and Operating systems for your hosts. You will then get a selection by system at the top tree level, by data center below that, and finally by operating system. Each hierarchy level takes you directly to a view of all hosts with these tags.

To create a Virtual host tree, first add the snap-in to it via the button at the bottom of the sidebar:

Click on the link in the text that points to the missing configuration, or manually access the page in the global settings via Setup > General > Global Settings > User interface > Virtual host trees:

Next, create a new tree with the Create new virtual host tree configuration button:

First assign an ID and a Title to the tree. You can optionally exclude the display of empty tree branches by checking Exclude empty tag choices. Then add the desired tag groups in the desired order via Add new element. You can also include the folder hierarchy via Folder tree. You can change the order for the hierarchy as usual with the button.

After saving, the snap-in will now show the selected hierarchy as a tree structure:

The branches and leaves of the tree are the host tags from the tag groups selected in the configuration. The numbers in parentheses alongside the 'leaves' show how many hosts have those tags.