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Customize Authentication for Nginx Ingress Controller

This is a short content about implementing authentication with Nginx Ingress controller in a Kubernetes cluster.

Workflow

I have a service named user-rest-service that provides some user service. Now, I want to secure it by checking user requests for a JWT token that is valid with a service called jwt-rest-service. For valid we will check that the token has not expired by time.

If the JWT token is not expired then jwt-rest-service will return a Http 200 status. If JWT is expired it will return a Http 400 response code.

The following is the workflow with numbers reflecting the workflow sequence: The following is a mermaidjs diagram:

flowchart TD    
    A[user request] --> |"(1) http authorization bearer jwt in header"| B[Nignx Ingress]
    B -.-> |"(1.1) nginx ingress auth-url jwt validation"| C{jwt-rest-service}
    C -.-> |"(1.2) http 200 status"| B
    B -->|"(2) secured service"| D[user-rest-service] 
    C -.-> | http not 200 status, user received error| A

To achieve this you can deploy your service jwt-rest-service first that validates the JWT token. Then in the user-rest-service configuration your nginx ingress configuration must have the following annotation line:

annotations:
    nginx.ingress.kubernetes.io/auth-url: "https://$host/oauth2/auth"

This nginx ingress configuration will ensure that before Nginx serves your request to the user-rest-service it will forward to the auth-url endpoint to validate the request.

There are instances where you might want to return a custom header from your authentication service. The following example shows in Java a custom header returned in a handler code:

   return jwt.validate(serverRequest.headers().firstHeader("Authorization").replace(bearer, ""))
                .flatMap(map -> {
                    LOG.info("set subject in http header");
                   return ServerResponse.ok()
                            .contentType(MediaType.APPLICATION_JSON)
                           .headers(httpHeaders -> httpHeaders.set("authId", map.get("subject")))
                            .bodyValue(map);
                })
                .onErrorResume(throwable ->
                    ServerResponse.status(HttpStatus.UNAUTHORIZED).contentType(MediaType.APPLICATION_JSON)
                        .bodyValue(throwable.getMessage())
                );

In order for Nginx to pass the header to the service being intercepted you add the following additional annotation line to get that header forwarded into your service:

annotations:
    nginx.ingress.kubernetes.io/auth-url: http://jwt-rest-service-mychart.backend.svc.cluster.local:80/validate
    # pass authId header returned from jwt-rest-service validation of jwt token
    nginx.ingress.kubernetes.io/auth-response-headers: authId

This is a convenient way to implement a simple authentication service using Nginx ingress configuration.