How to use this image

All examples in this guide use the public image. If you've mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/linkerd-metrics-api:<tag>
• Mirrored image: <your-namespace>/dhi-linkerd-metrics-api:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

Start a linkerd-metrics-api image

Linkerd starts this image for you. The Linkerd viz Helm chart runs it in the metrics-api deployment and passes the Linkerd namespace, the cluster domain and the Prometheus URL as arguments. The metrics API serves its API on port 8085 and its admin endpoints, /ping, /ready and /metrics, on port 9995.

Use the image with the Linkerd viz Helm chart

Install the Linkerd control plane as described in the Linkerd installation guide⁠, then install the viz extension and pass the image to the chart:

$ helm repo add linkerd https://helm.linkerd.io/edge
$ helm install linkerd-viz linkerd/linkerd-viz \
  --create-namespace --namespace linkerd-viz \
  --set metricsAPI.image.registry=dhi.io \
  --set metricsAPI.image.name=linkerd-metrics-api \
  --set metricsAPI.image.tag=<tag>

Without metricsAPI.image.tag the chart uses its own Linkerd release version, for example edge-26.10.1, which is also a tag of this image. Add --set imagePullSecrets[0].name=<secret> when the cluster pulls from dhi.io with a pull secret.

Check the packaged release

The metrics API prints the Linkerd release it belongs to:

$ docker run --rm dhi.io/linkerd-metrics-api:<tag> -version
edge-26.10.1

Common linkerd-metrics-api use cases

Use an external Prometheus with basic auth

Point the metrics API at your own Prometheus and give it the credentials in a secret with user and password keys, created in the viz namespace before the chart is installed:

$ kubectl create namespace linkerd-viz
$ kubectl create secret generic prometheus-creds --namespace linkerd-viz \
  --from-literal=user=<user> --from-literal=password=<password>
$ helm install linkerd-viz linkerd/linkerd-viz \
  --namespace linkerd-viz \
  --set metricsAPI.image.registry=dhi.io \
  --set metricsAPI.image.name=linkerd-metrics-api \
  --set metricsAPI.image.tag=<tag> \
  --set prometheus.enabled=false \
  --set prometheusUrl=http://prometheus.example.com:9090 \
  --set prometheusCredsSecret=prometheus-creds

The image ships the CA certificate bundle at /etc/ssl/certs/ca-certificates.crt, the same path as the upstream image.

Query traffic metrics of meshed workloads

The linkerd viz CLI commands and the Linkerd dashboard read their metrics from the metrics API. Once the viz extension runs with this image, show the traffic stats of the deployments in a namespace:

$ linkerd viz stat deploy --namespace <namespace>

Run the metrics API outside a cluster

The metrics API needs a Kubernetes cluster. Outside a cluster it stops with an error:

$ docker run --rm dhi.io/linkerd-metrics-api:<tag> -prometheus-url=http://prometheus:9090

Non-hardened images vs. Docker Hardened Images

• Tags carry the Linkerd release the image ships in, for example edge-26.10.1, and the release number, for example 26.10.1. The upstream image is tagged with the Linkerd release only. The 26 tag moves to each new edge release of 2026 and the 26.10 tag to each one of October 2026. Upstream edge releases can include breaking changes, so pin edge-26.10.1 or 26.10.1 to stay on one release.
• The image runs as the nonroot user, UID 65532, where the upstream image runs as root unless the pod sets a user. The Linkerd viz chart sets runAsUser for the metrics API pod, so Linkerd deployments are not affected.
• The image entrypoint is /usr/local/bin/linkerd2-metrics-api. The upstream path /metrics-api is a symbolic link to the binary.
• -version prints the Linkerd release, for example edge-26.10.1, from the LINKERD_CONTAINER_VERSION_OVERRIDE environment variable. The upstream image does not set the variable and prints dev-undefined.
• The upstream image ships /linkerd/LICENSE. This image does not.
• The runtime variants contain no shell. Use the dev variants, or Docker Debug, to get one.

Image variants

Docker Hardened Images come in different variants depending on their intended use. Image variants are identified by their tag.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as a nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the tag name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

To view the image variants and get more information about them, select the Tags tab for this repository, and then select a tag.

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshooting migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.