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Harbormaster is a webhook handler for validating JWT tokens sent to the Kubernetes API. This gives the flexibility to add extra information to the user object for Kubernetes.

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Harbormaster can be configured using command line flags or environment variables. A list of those is below:

  -r, --ca-cert string               ca certificate to use for root trust. This is required. [HARBORMASTER_CA_CERT] (default "/usr/share/harbormaster/ca.pem")
  -e, --cache-expiry duration        how long before a cache entry is considered invalid [HARBORMASTER_CACHE_EXPIRY] (default 1h0m0s)
  -m, --cache-max-memory int         amount of memory (in MB) to cap the cache at [HARBORMASTER_CACHE_MAX_MEMORY] (default 1024)
  -c, --cert string                  certificate to use for https. This is required. [HARBORMASTER_CERT] (default "/usr/share/harbormaster/harbormaster.pem")
  -i, --client-id string             OIDC provider client ID. [HARBORMASTER_CLIENT_ID]
      --graph-client-id string       MS Graph application client ID to use [HARBORMASTER_GRAPH_CLIENT_ID]
      --graph-client-secret string   MS Graph application client secret to use [HARBORMASTER_GRAPH_CLIENT_SECRET]
      --graph-tenant-name string     MS Graph application tenant name to use (generally <company> [HARBORMASTER_GRAPH_TENANT_NAME]
  -g, --group-claim string           claim to use for user's groups [HARBORMASTER_GROUP_CLAIM] (default "groups")
      --health-port int              port to listen on for health checks. [HARBORMASTER_HEALTH_PORT] (default 8000)
  -h, --help                         help for harbormaster
  -u, --issuer-url string            OIDC token issuer URL. [HARBORMASTER_ISSUER_URL] (default "")
  -k, --key string                   keyfile for the certificate. This is required. [HARBORMASTER_KEY] (default "/usr/share/harbormaster/harbormaster.key")
  -v, --log-level string             log level to use. Must be one of: 'DEBUG', 'INFO', 'WARN', or 'ERROR'. [HARBORMASTER_LOG_LEVEL] (default "WARN")
  -p, --port int                     port harbormaster should listen on. [HARBORMASTER_PORT] (default 9000)
  -n, --username-claim string        claim to use as the username [HARBORMASTER_USERNAME_CLAIM] (default "email")

Harbormaster relies on TLS authentication to validate the connection. We recommend using the same CA that is used for the Kubernetes API server, and, if possible, the same API server key for setting up HTTPS. HTTPS is required for the webhook and we have no plans to support authentication webhooks without encryption.

Group getters

Harbormaster supports getting additional user groups from other sources. Currently, only one source is implemented. However, additional sources can be added by implementing the groups.Getter interface. The groups sent back to Kubernetes will be any groups contained in the JWT combined with any groups from a group getter. If no group getter configuration is specified, Harbormaster will only use the groups from the JWT.

For performance, Harbormaster caches group lookups. The amount of time a cache entry is valid can be adjusted with a command line flag.

Microsoft Graph

You can optionally specify a Microsoft Graph API client ID, secret, and tenant name. If you specify these flags or environment variables, Harbormaster will fetch groups from MS Graph.


The username claim specifies which username is used when querying the MS graph API. So if it isn't an email address, the lookup in the MS Graph API may fail.

Docker images

We build Docker images for Harbormaster that are hosted on Docker Hub. Every commit to master pushes a new build tagged as canary for those who want to use the bleeding edge. Otherwise, the version of Harbormaster will be used to tag the image.


Development should be fairly straightforward for anyone who has built Go-based project. There are only 2 dependencies: a valid Go installation and the new dep tool (that will soon be added into the main Go toolchain). Once you have those installed, you will need to clone this project into the proper place in your GOPATH:

mkdir -p $GOPATH/src/
cd $GOPATH/src/
git clone
cd harbormaster

To get started, run make bootstrap to install project dependencies. Once that is done, you can run make test to run the package tests and make build to build a local binary at ./harbormaster.

How it works

Most of the functionality is broken out into different internal packages that can only be used by Harbormaster. There are also a few external packages. The internal packages are located at ./internal/pkg and a description of each follows.

Package Purpose
handler Contains http handler functions and related helpers to handle and decode the TokenReview requests
validator Contains all necessary functions for validating and caching token data

The external packages currently consist only of code related to group getters:

Package Purpose
graph A package for logging into and accessing user group information from the MS Graph API
groups A simple package containing the Getter interface that must be implemented by any group getter

The workflow can be a bit confusing at first glance. It is broken down below for easy reference:

  1. Harbormaster receives a JSON encoded TokenReview object from the API server
  2. The TokenReview is deserialized into a native Kubernetes TokenReview object
  3. The token is retrieved from the object and validated using an OIDC library
  4. A map of claims is retrieved from the token for use in constructing the response to the API server
  5. If a group getter is configured, these additional steps occur
    1. The validator checks if a list of user groups associated with the username exists
    2. If the cache entry exists, it uses those groups. Otherwise it uses the getter to fetch a group list and caches it
  6. Using the username and group list, a new TokenReview object is created and serialized to JSON before sending back to the API server

Releases and Docker images

To build binaries for a release, run the make release command. This will build binaries for Windows, Linux, and MacOS in parallel and tag them with a version using git describe --always --tags. You can also manually specify a version using the VERSION environment variable.

If you wish to build a docker image, you can use make image to build one. Essentially, all that is happening is a make release and then it wraps those binaries in a Docker image. Once again, VERSION will be used for tagging the image.

To push an image, run make image-push. You must be logged in to Docker Hub in order to run this step. If you used VERSION in any of the previous steps, you will need to set it here as well.


We keep track of outstanding issues and enhancement in the issues page. You can follow along with existing issues or request new features there. However, the first priorities on our list are below:

  • Integration Tests
  • Performance optimizations
  • Distributed cache instead of a local cache



Harbormaster is a webhook handler for the Kubernetes API.








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