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/selenium-hub:<tag>
• Mirrored image: <your-namespace>/dhi-selenium-hub:<tag>

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

What's included in this selenium-hub image

This Docker Hardened Selenium Hub image runs the Hub role of a Selenium Grid. The Hub is the central coordinator that receives WebDriver session requests from test clients and routes them to registered browser Nodes. It exposes the Grid UI and the WebDriver endpoint on port 4444, and the Event Bus on ports 4442 (publish) and 4443 (subscribe) for Node registration.

The image ships the selenium-server JAR (the same artifact used by the official Selenium project) together with an entrypoint script at /usr/local/bin/selenium-hub. That script is a port of upstream selenium/hub's start-selenium-grid-hub.sh: it maps the same SE_* environment variables (host/port, timeouts, basic auth, TLS, tracing, JVM options) onto selenium-server hub flags, so an existing selenium/hub configuration carries over. The one functional difference is that the upstream Prometheus metrics-exporter sidecar (port 9615) is not bundled — see Non-hardened images vs. Docker Hardened Images⁠.

Run the selenium-hub container

To start a Hub and expose all three ports:

docker run -d \
  --name selenium-hub \
  -p 4442:4442 \
  -p 4443:4443 \
  -p 4444:4444 \
  dhi.io/selenium-hub:<tag>

The Grid UI is available at http://localhost:4444/ui⁠ and the WebDriver endpoint at http://localhost:4444/wd/hub⁠ once the Hub is ready.

Start a standalone Hub waiting for nodes

Start the Hub and verify it is listening before registering any Nodes. The Hub begins accepting Node registrations immediately; the Grid UI shows zero sessions until at least one Node connects.

docker run -d \
  --name selenium-hub \
  -p 4442:4442 \
  -p 4443:4443 \
  -p 4444:4444 \
  dhi.io/selenium-hub:<tag>

Check Grid status using the built-in status endpoint (no shell required inside the container — query from the host):

curl -s http://localhost:4444/status | python3 -m json.tool

The Hub is operational as soon as /status responds with HTTP 200. Note that "ready" stays false until at least one Node registers — a Hub with no Nodes is healthy and accepting registrations, it simply has no browser slots to serve sessions yet. Once a Node connects, "ready" flips to true.

Hub and Node with Docker Compose

The following compose.yml starts a Hub alongside a Chromium Node. The Node connects back to the Hub over the shared Docker network using the SE_EVENT_BUS_* variables.

services:
  selenium-hub:
    image: dhi.io/selenium-hub:<tag>
    container_name: selenium-hub
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

  chromium-node:
    image: selenium/node-chromium:latest
    container_name: chromium-node
    depends_on:
      - selenium-hub
    environment:
      SE_EVENT_BUS_HOST: selenium-hub
      SE_EVENT_BUS_PUBLISH_PORT: "4442"
      SE_EVENT_BUS_SUBSCRIBE_PORT: "4443"

Start both services:

docker compose up -d

Point your WebDriver client at http://localhost:4444/wd/hub (or http://localhost:4444 for W3C-style clients). The Grid UI at http://localhost:4444/ui⁠ shows the registered Node and available browser slots.

Hub with basic auth and tuned timeouts

Use SE_ROUTER_USERNAME and SE_ROUTER_PASSWORD to enable HTTP basic authentication on the Grid UI and all /session API endpoints. Adjust SE_SESSION_REQUEST_TIMEOUT to control how long the Hub queues a session request before rejecting it.

docker run -d \
  --name selenium-hub \
  -p 4442:4442 \
  -p 4443:4443 \
  -p 4444:4444 \
  -e SE_ROUTER_USERNAME=admin \
  -e SE_ROUTER_PASSWORD=secret \
  -e SE_SESSION_REQUEST_TIMEOUT=120 \
  -e SE_SESSION_RETRY_INTERVAL=5 \
  dhi.io/selenium-hub:<tag>

Configuration reference

The entrypoint script maps the following environment variables to Hub CLI flags. All variables are optional; the defaults shown are the values baked into the image.

Advanced configuration

For advanced deployments — including TLS/HTTPS termination, fully-distributed mode (separate Router, Distributor, SessionQueue, and EventBus components), OpenTelemetry tracing configuration, and Kubernetes or Helm chart deployment — refer to the official Selenium Grid documentation at https://www.selenium.dev/documentation/grid/⁠.

Non-hardened images vs. Docker Hardened Images

The upstream selenium/hub image differs from this hardened image in the following ways:

The two removed pieces are easy to recover when you need them:

• Prometheus metrics. The :9615 exporter is a standalone sidecar, not part of selenium-server. Run a Grid metrics exporter as its own container pointed at this Hub's GraphQL endpoint, http://<hub>:4444/graphql, which exposes full Grid state (the Hub's live status is also available as JSON at /status).
• Process supervision. The Hub JVM runs as PID 1 and shuts down cleanly on SIGTERM. Because there is no in-container supervisor to restart a crashed process, rely on your orchestrator's restart policy (docker run --restart=on-failure, Kubernetes restartPolicy: Always) — the container-native equivalent of upstream's supervisord auto-restart.

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

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 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.