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Part 3 - Using Basic Authentication with a Secured Registry in Linux

From Part 2 we have a registry running in a Docker container, which we can securely access over HTTPS from any machine in our network. We used a self-signed certificate, which has security implications, but you could buy an SSL from a CA instead, and use that for your registry. With secure communication in place, we can set up user authentication.

Usernames and Passwords

The registry server and the Docker client support basic authentication over HTTPS. The server uses a file with a collection of usernames and encrypted passwords. The file uses Apache's htpasswd.

Create the password file with an entry for user "moby" with password "gordon";

$ mkdir auth
$ sudo docker run --entrypoint htpasswd registry:latest -Bbn moby gordon > auth/htpasswd

The options are:

  • --entrypoint Overwrite the default ENTRYPOINT of the image <<<<<<< HEAD
  • -B to force bcrypt vs default md5 =======
  • -B Use bcrypt encryption (required)

master

  • -b run in batch mode
  • -n display results

We can verify the entries have been written by checking the file contents - which shows the user names in plain text and a cipher text password:

$ cat auth/htpasswd
moby:$2y$05$Geu2Z4LN0QDpUJBHvP5JVOsKOLH/XPoJBqISv1D8Aeh6LVGvjWWVC

Running an Authenticated Secure Registry

Adding authentication to the registry is a similar process to adding SSL - we need to run the registry with access to the htpasswd file on the host, and configure authentication using environment variables.

As before, we'll remove the existing container and run a new one with authentication configured:

$ sudo docker kill registry
$ sudo docker rm registry
$ sudo docker run -d -p 5000:5000 --name registry \
  --restart unless-stopped \
  -v $(pwd)/registry-data:/var/lib/registry \
  -v $(pwd)/certs:/certs \
  -v $(pwd)/auth:/auth \
  -e REGISTRY_HTTP_TLS_CERTIFICATE=/certs/domain.crt \
  -e REGISTRY_HTTP_TLS_KEY=/certs/domain.key \
  -e REGISTRY_AUTH=htpasswd \
  -e "REGISTRY_AUTH_HTPASSWD_REALM=Registry Realm" \
  -e "REGISTRY_AUTH_HTPASSWD_PATH=/auth/htpasswd" \
  registry

The options for this container are:

  • -v $(pwd)/auth:/auth - mount the local auth folder into the container, so the registry server can access htpasswd file;
  • -e REGISTRY_AUTH=htpasswd - use the registry's htpasswd authentication method;
  • -e REGISTRY_AUTH_HTPASSWD_REALM='Registry Realm' - specify the authentication realm;
  • -e REGISTRY_AUTH_HTPASSWD_PATH=/auth/htpasswd - specify the location of the htpasswd file.

Now the registry is using secure transport and user authentication.

Authenticating with the Registry

With basic authentication, users cannot push or pull from the registry unless they are authenticated. If you try and pull an image without authenticating, you will get an error:

$ sudo docker pull localhost:5000/hello-world
Using default tag: latest
Error response from daemon: Get https://localhost:5000/v2/hello-world/manifests/latest: no basic auth credentials

The result is the same for valid and invalid image names, so you can't even check a repository exists without authenticating. Logging in to the registry is the same docker login command you use for Docker Store, specifying the registry hostname:

$ sudo docker login registry.local:5000
Username: moby
Password:
Login Succeeded

If you use the wrong password or a username that doesn't exist, you get a 401 error message:

Error response from daemon: login attempt to https://registry.local:5000/v2/ failed with status: 401 Unauthorized

Now you're authenticated, you can push and pull as before:

$ sudo docker pull localhost:5000/hello-world
Using default tag: latest
latest: Pulling from hello-world
Digest: sha256:961497c5ca49dc217a6275d4d64b5e4681dd3b2712d94974b8ce4762675720b4
Status: Image is up to date for registry.local:5000/hello-world:latest

Note. The open-source registry does not support the same authorization model as Docker Store or Docker Trusted Registry. Once you are logged in to the registry, you can push and pull from any repository, there is no restriction to limit specific users to specific repositories.

Using Docker Compose to Start the Registry

Typing in all the options to start the registry can become tedious. An easier and simpler way is to use Docker Compose. Here's an example of a docker-compose.yml file that will start the registry.

registry:
  restart: always
  image: registry:2
  ports:
    - 5000:5000
  environment:
    REGISTRY_HTTP_TLS_CERTIFICATE: /certs/domain.crt
    REGISTRY_HTTP_TLS_KEY: /certs/domain.key
    REGISTRY_AUTH: htpasswd
    REGISTRY_AUTH_HTPASSWD_PATH: /auth/htpasswd
    REGISTRY_AUTH_HTPASSWD_REALM: Registry Realm
  volumes:
    - /path/registry-data:/var/lib/registry
    - /path/certs:/certs
    - /path/auth:/auth

To start the registry, type:

$ sudo docker-compose up

Conclusion

Docker Registry is a free, open-source application for storing and accessing Docker images. You can run the registry in a container on your own network, or in a virtual network in the cloud, to host private images with secure access. For Linux hosts, there is an official registry image on Docker Store.

We've covered all the options, from running an insecure registry, through adding SSL to encrypt traffic, and finally adding basic authentication to restrict access. By now you know how to set up a usable registry in your own environment, and you've also used some key Docker patterns - using containers as build agents and to run basic commands, without having to install software on your host machines.

There is still more you can do with Docker Registry - using a different storage driver so the image data is saved to reliable share storage, and setting up your registry as a caching proxy for Docker Store are good next steps.