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Docker dynamic ports

Docker dynamically assigns host machine ports that map into container application ports.

Inside the Devcontainer

Info

The Devcontainer can bind to a single container’s port(s) and present those to your localhost machine via VS Code. Other services started along with the Devcontainer are not visible in VS Code. See Outside the Devcontainer for how to find information on those.

Once started with F5, the benefits Django application runs on port 8000 inside the Devcontainer. To find the localhost address, look on the PORTS tab in VS Code’s Terminal window. The Local Address corresponding to the record where 8000 is in the Port column is where the site is accessible on your host machine.

Replace 0.0.0.0 with localhost and use the same port number shown in the Local Address column. This is highlighted by the red box in the image below:

Screenshot showing a Local Address for a container application

Outside the Devcontainer

When running a docker compose ... command, or in other scenarios outside of the Devcontainer, there are multiple ways to find the http://localhost port corresponding to the service in question.

Docker Desktop

The Docker Desktop application shows information about running containers and services/groups, including information about bound ports. In most cases, the application provides a button to launch a container/service directly in your browser when a port binding is available.

In the Containers / Apps tab, expand the service group if needed to find the container in question, where you should see labels indicating the container is RUNNING and bound to PORT: XYZ.

Hover over the container in question, and click the Open in Browser button to launch the app in your web browser.

Screenshot showing the "Open in Browser" button for a service in Docker Desktop

Docker CLI commands

Using the docker command line interface, you can find the bound port(s) of running containers.

docker ps -f name=<service>

e.g. for the docs service:

docker ps -f name=docs

This prints output like the following:

CONTAINER ID   IMAGE                 COMMAND                  CREATED         STATUS         PORTS                     NAMES
0d5b2e1fb910   benefits_client:dev   "mkdocs serve --dev-…"   2 minutes ago   Up 2 minutes   0.0.0.0:62093->8000/tcp   benefits_docs_1

Looking at the PORTS column:

PORTS
0.0.0.0:62093->8000/tcp

We can see that locally, port 62093 is bound to the container port 8000.

In this case, entering http://localhost:62093 in the web browser navigates to the docs site homepage.