Checkmk: what the GPL-2.0 Community edition actually gives you

The problem Checkmk solves, and the line between free and paid

Checkmk collects state from servers, network gear, containers and cloud resources and turns it into a single monitoring view. The README describes it as covering physical, virtual, containerized and cloud environments, with agent-based and agentless monitoring and support for hundreds of official and community-maintained plugins. That is a wide claim, and the edition list is what gives it shape.

The Community edition is the GPL-2.0 one. According to the README it contains the core monitoring engine, the web UI, agent-based and agentless monitoring, and the plugin catalogue. The README positions it for "smaller environments with basic requirements in terms of automation, dashboarding, and support." That sentence is doing real work: it is the vendor telling you where the free edition stops being comfortable.

Everything above Community is commercial. Pro adds distributed monitoring, built-in dashboards, automated agent management, LDAP and REST API integrations, and professional support. Ultimate adds Kubernetes, AWS, Azure and GCP monitoring, OpenTelemetry application metrics, push agents and host auto-registration. Cloud is the SaaS deployment of the same product. Ultimate with Multi-Tenancy targets managed service providers with isolated access controls and MSP reporting. Read that list as the adoption boundary, not as an upsell: a team that needs distributed monitoring across sites is not going to get there on Community alone.

How the monitoring engine, agents and plugins fit together

The repository layout shows a monitoring system split along the same lines the README describes. At the top level sit active_checks/ and agents/, alongside cmk/, buildscripts/ and a bazel/ tree with MODULE.bazel and BUILD files. That is a monorepo: the server-side engine, the agent code and the build tooling live in one repository rather than in separate projects.

The agents/ directory is not Python-only. Cargo.toml declares a Rust workspace whose members include packages/cmk-agent-ctl, packages/check-http, packages/check-cert, packages/mk-oracle, packages/mk-sql and agents/wnx/extensions/robotmk_ext. So the agent-side tooling and several check plugins are compiled Rust binaries, while the monitoring core is Python. The Cargo.toml comment on the cmk-agent-ctl release profile sets opt-level = "z" to optimize for size, which fits an agent that has to be pushed to many hosts.

On the Python side, pyproject.toml configures ruff with target-version = "py314" and line-length = 100, and enables preview rules with explicit-preview-rules = true so only named preview rules apply. It also configures bandit with a small skip list, each entry carrying a ticket reference and a stated reason, for example skipping the hardcoded-bind-all-interfaces check on the grounds of a high false-positive rate. requirements.txt is generated by uv via a Bazel target and pins every package with sha256 hashes. None of this affects how you run Checkmk, but it tells you what kind of codebase you are adopting: heavily pinned, linted, and built through Bazel rather than pip and setuptools.

Installing Checkmk Community on Linux and reaching the web UI

The README does not give a single install command. It points at three distribution-specific documentation pages (RedHat, SUSE Linux Enterprise Server, and Debian and Ubuntu) and at a separate Docker page, and says packages live on the downloads page. So the first step is choosing your distribution and following that page, not copying a command from the repository.

If you prefer containers, the README states that all editions can be installed as Docker containers, with full instructions on the Docker documentation page. The repository does not contain a docker-compose file for a quick start, so treat the container path as documented elsewhere.

Building from source is explicitly discouraged. The README says it is "highly recommended to use the prebuilt Checkmk packages" and warns that the Dockerfiles under buildscripts/infrastructure/build-nodes/ rely heavily on the project's own infrastructure and "won't build from scratch on your machine." If you do want to try, the README gives exactly two targets:

make rpm

DEBFULLNAME="Mr. Buildmaster" DEBEMAIL="[email protected]" make deb

The README says the result should be packages named check-mk-[edition]-[version].[deb|rpm] in your current directory. Note the environment variables in the DEB invocation: without a name and email, the Debian build tooling has nothing to put in the changelog.

Once installed, the README points to a beginners guide at docs.checkmk.com rather than describing first login itself. The repository does not document default credentials, ports or the URL path, and the README is silent on all three, so take those from the beginners guide. For the agent side, the README lists no installation commands either; agents/ and the Rust workspace in Cargo.toml show the agent code exists, but the README defers agent rollout to the documentation.

What the Community edition does not include

The clearest limitation is written into the edition table. Distributed monitoring, built-in dashboards, automated agent management, LDAP and REST API integrations are Pro features. Kubernetes, AWS, Azure and GCP monitoring, OpenTelemetry metrics, push agents and host auto-registration are Ultimate features. If your environment is ephemeral and cloud-native, the README is effectively telling you that Community is the wrong edition before you install it.

Support is the second boundary. The README lists professional support only for Pro and above, and describes Community as suited to basic requirements in terms of support. There is a SECURITY.md in the repository, but the README does not describe a support SLA or a commercial escalation path for Community users.

Building is a third. The source tree is Bazel-based, with MODULE.bazel, bazel_variables.bzl, bazel_downloader.cfg and a pinned requirements.txt generated through Bazel. The README's warning that the build Dockerfiles depend on project infrastructure is the practical consequence: this is not a project you fork and rebuild casually to patch a check.

The README is also silent on upgrade and rollback procedures. It mentions new releases on the website but documents no upgrade command, no downgrade path, and no version compatibility matrix between agent and server. That silence matters more than usual for a monitoring system, where the agent and the monitoring site have to agree with each other.

Checkmk versus Zabbix, and how the approaches differ

The most common comparison people search for is Checkmk against Zabbix, and the README gives you one concrete axis to judge it on: edition structure. Checkmk ships a GPL-2.0 core with named commercial tiers, and the README is explicit about which capabilities sit in which tier. That makes the free/paid decision legible before you install anything. A project that puts everything in one open-source edition makes the opposite trade: no feature gate, but also no commercial support contract attached to specific capabilities.

A second difference is implementation language, which the repository makes visible. Checkmk's core is Python, with agent-side tooling and several checks written in Rust and built through a Bazel monorepo. That is a modern toolchain, and it also means contributing a check may require understanding both the Python engine and the Rust workspace in Cargo.toml. Choose accordingly: if you want to write checks in the same language as the server, this split is friction; if you want compiled, size-optimized agents, it is the reason the agent exists in that form.

The comparison against Nagios that also shows up in search is harder to answer from this material. The README never mentions Nagios, so any claim about migration paths or check compatibility would be invented. What the README does support is the plugin story: hundreds of official and community-maintained plugins, with Community including them. Whether a specific Nagios plugin maps to a Checkmk one is not something the repository states.

Licence, contributions and what the GPL-2.0 core implies

Checkmk Community is licensed under the GNU GPL v2, and the repository carries COPYING, AUTHORS and CONTRIBUTING.md at the top level. The licence applies to the Community edition; the README presents Pro, Ultimate, Cloud and Ultimate with Multi-Tenancy as separate commercial products with their own terms, and it does not describe how the commercial editions relate to the GPL code. That is a question for the vendor, not for the repository.

For most operators the practical effect is simple: you can run Community without paying for a licence, and the README markets it as completely free and open-source. If you plan to redistribute a modified Checkmk, or to embed it in something you sell, read COPYING and CONTRIBUTING.md in the repository rather than relying on this summary. Nothing here is legal advice, and the README does not address redistribution scenarios at all.

Contribution is governed by CONTRIBUTING.md, which the README links before anything else in its contributing section. The repository also carries AGENTS.md and CLAUDE.md at the top level, along with a .claude/ directory, which suggests AI-assisted development tooling is part of the project's workflow. That is unusual enough to be worth knowing before you open a pull request and wonder why the conventions look different from a typical Python project.

Editorial conclusion

Adopt Checkmk Community if you run a Linux estate, want agent-based checks without paying for a licence, and are willing to install from DEB or RPM on a supported distribution. Do not adopt it expecting distributed monitoring, built-in dashboards, automated agent management, LDAP or REST API integrations, or Kubernetes and cloud auto-registration: the README places all of those in the Pro, Ultimate or Cloud editions. Before committing, verify two things on the docs site rather than here: that your distribution and version appear on the RedHat, SUSE or Debian and Ubuntu install pages, and whether the plugin you need ships with the Community edition or only with a commercial one.

Frequently asked questions

Official sources

1. Checkmk/checkmk on GitHub
2. Issues
3. License: GPL-2.0
4. Project website
5. README