Contributing New Provider Modules

Note

The primary audience for this documentation are contributors that want to add support for new authentication providers. While you can follow it to implement your own custom providers it is much more advanced than using our built-in providers.

How Does Authentication Work?

The Backstage application can use various external authentication providers for authentication. An external provider is wrapped using an AuthProviderRouteHandlers interface for handling authentication. This interface consists of four methods. Each of these methods is hosted at an endpoint (by default) /api/auth/[provider]/method, where method performs a certain operation as follows:

  /auth/[provider]/start -> Initiate a login from the web page
  /auth/[provider]/handler/frame -> Handle a finished authentication operation
  /auth/[provider]/refresh -> Refresh the validity of a login
  /auth/[provider]/logout -> Log out a logged-in user

The flow is as follows:

A user attempts to sign in.
A popup window is opened, pointing to the auth endpoint. That endpoint does initial preparations and then re-directs the user to an external authenticator, still inside the popup.
The authenticator validates the user and returns the result of the validation (success OR failure), to the wrapper's endpoint (handler/frame).
The handler/frame rendered webpage will issue the appropriate response to the webpage that opened the popup window, and the popup is closed.
The user signs out by clicking on a UI interface and the webpage makes a request to logout the user.

Implementing Your Own Auth Wrapper

The core interface of any auth wrapper is the AuthProviderRouteHandlers interface. This interface has four methods corresponding to the API described in the initial section. Any auth wrapper will have to implement this interface.

When initiating a login, a pop-up window is created by the frontend, to allow the user to initiate a login. This login request is done to the /start endpoint which is handled by the start method.

The start method re-directs to the external auth provider who authenticates the request and re-directs the request to the /handler/frame endpoint, which is handled by the frameHandler method.

The frameHandler returns an HTML response, containing a script that does a postMessage to the frontend window, containing the result of the request. The WebMessageResponse type is the message sent by the postMessage to the frontend.

A postMessageResponse utility function wraps the logic of generating a postMessage response that ensures that CORS is successfully handled. This function takes an express.Response, a WebMessageResponse and the URL of the frontend (appOrigin) as parameters and return an HTML page with the script and the message.

Auth Environment Separation

The concept of an env is core to the way the auth backend works. It uses an env query parameter to identify the environment in which the application is running (development, staging, production, etc). Each runtime can simultaneously support multiple environments at the same time and the right handler for each request is identified and dispatched to, based on the env parameter.

OAuthEnvironmentHandler is a utility wrapper for an OAuthHandlers that implements the AuthProviderRouteHandlers interface while supporting multiple envs.

To instantiate OAuth providers (the same but for different environments), use OAuthEnvironmentHandler.mapConfig. It's a helper to iterate over a configuration object that is a map of environments to configurations. See one of the existing OAuth providers for an example of how it is used.

Given the following configuration:

development:
  clientId: abc
  clientSecret: secret
production:
  clientId: xyz
  clientSecret: supersecret

The OAuthEnvironmentHandler.mapConfig(config, envConfig => ...) call will split the config by the top level development and production keys, and pass on each block as envConfig.

For convenience, the AuthProviderFactory is a factory function that has to be implemented which can then generate a AuthProviderRouteHandlers for a given provider.

All of the supported providers provide an AuthProviderFactory that returns an OAuthEnvironmentHandler, capable of handling authentication for multiple environments.

Passport

We chose Passport as our authentication platform due to its comprehensive set of supported authentication strategies.

How to add a new strategy provider

Quick guide

1. Create a new auth provider module or add a proxy auth based provider depending on your needs.

2. Add the provider to the backend.

Create new auth provider module

In this example we will create auth module for a made up service named foobar.

Create a new module using yarn new, pick backend-module and provide auth-backend as the plugin ID and foobar-provider as the module ID.

Make sure that the module has the appropriate passport provider as a dependency.

cd plugins/auth-backend-backend-module-foobar-provider
yarn add passport-provider-a
yarn add @types/passport-provider-a

Adding an OAuth based provider

We're then creating a new module that can extend the Auth backend using the authProvidersExtensionPoint.

plugins/auth-backend-foobar-provider/src/module.ts
import { createBackendModule } from '@backstage/backend-plugin-api';
import {
  authProvidersExtensionPoint,
  commonSignInResolvers,
  createOAuthProviderFactory,
} from '@backstage/plugin-auth-node';
import { providerAuthenticator } from './authenticator';

/** @public */
export const authModuleFoobarProvider = createBackendModule({
  pluginId: 'auth',
  moduleId: 'foobar',
  register(reg) {
    reg.registerInit({
      deps: {
        providers: authProvidersExtensionPoint,
      },
      async init({ providers }) {
        providers.registerProvider({
          providerId: 'foobar',
          factory: createOAuthProviderFactory({
            authenticator: providerAuthenticator,
            signInResolverFactories: {
              ...commonSignInResolvers,
            },
          }),
        });
      },
    });
  },
});

Now let's implement the actual authenticator for our provider using Strategy from a passport package. The authenticator is responsible for creating the passport strategy and handling the authentication flow using secrets from the config file.

plugins/auth-backend-foobar-provider/src/authenticator.ts
import { Strategy as ProviderStrategy } from 'passport-provider-a';
import {
  createOAuthAuthenticator,
  PassportOAuthAuthenticatorHelper,
  PassportOAuthDoneCallback,
  PassportProfile,
} from '@backstage/plugin-auth-node';

/** @public */
export const providerAuthenticator = createOAuthAuthenticator({
  defaultProfileTransform:
    PassportOAuthAuthenticatorHelper.defaultProfileTransform,
  scopes: {
    // Scopes required by the provider
    required: ['openid', 'email', 'profile', 'offline_access'],
  },
  initialize({ callbackUrl, config }) {
    const clientId = config.getString('clientId');
    const clientSecret = config.getString('clientSecret');

    return PassportOAuthAuthenticatorHelper.from(
      new ProviderStrategy(
        {
          clientID: clientId,
          clientSecret: clientSecret,
          // ... other options
        },
        (
          accessToken: string,
          refreshToken: string,
          params: any,
          fullProfile: PassportProfile,
          done: PassportOAuthDoneCallback,
        ) => {
          done(
            undefined,
            { fullProfile, params, accessToken },
            { refreshToken },
          );
        },
      ),
    );
  },

  async start(input, helper) {
    return helper.start(input);
  },

  async authenticate(input, helper) {
    return helper.authenticate(input);
  },

  async refresh(input, helper) {
    return helper.refresh(input);
  },
});

Here are some examples of authenticators that are already implemented in the codebase:

Creating proxy auth based provider

A proxy auth provider is a provider that uses another provider to authenticate for example Google IAP or AWS ALB, please note that those providers are already supported by Backstage.

The implementation is similar to the OAuth provider, but the authenticator function is different. There are already some examples on how to implement a proxy provider in the codebase, for example auth-backend-module-gcp-iap-provider and auth-backend-module-aws-alb-provider

Verify Callback

Strategies require what is known as a verify callback. The purpose of a verify callback is to find the user that possesses a set of credentials. When Passport authenticates a request, it parses the credentials contained in the request. It then invokes the verify callback with those credentials as arguments [...]. If the credentials are valid, the verify callback invokes done to supply Passport with the user that authenticated.

If the credentials are not valid (for example, if the password is incorrect), done should be invoked with false instead of a user to indicate an authentication failure.

http://www.passportjs.org/docs/configure/

Add the provider to the backend

The process for adding the new module is the same as for any other type of module or backend plugin.

If this provider is internal to your installation the import path that you add to packages/backend/src/index.ts would be something like:

backend.add(import('@internal/plugin-auth-backend-module-foobar-provider'));

But if this module is contributed directly to Backstage the module would be imported as

backend.add(import('@backstage/plugin-auth-backend-module-foobar-provider'));

By doing this auth-backend automatically adds these endpoints:

router.get('/auth/providerA/start');
router.get('/auth/providerA/handler/frame');
router.post('/auth/providerA/handler/frame');
router.post('/auth/providerA/logout');
router.get('/auth/providerA/refresh'); // if supported
router.post('/auth/providerA/refresh'); // if supported

As you can see each endpoint is prefixed with both /auth and its provider name.

Test the new provider

You can curl -i localhost:7007/api/auth/providerA/start and which should provide a 302 redirect with a Location header. Paste the URL from that header into a web browser and you should be able to trigger the authorization flow.

How Does Authentication Work?​

Implementing Your Own Auth Wrapper​

Auth Environment Separation​

Passport​

How to add a new strategy provider​

Quick guide​

Create new auth provider module​

Adding an OAuth based provider​

Creating proxy auth based provider​

Verify Callback​

Add the provider to the backend​

Test the new provider​