Configure Spring Gateway with Spring Authorization Server - spring

Question

I have Spring Gateway application with the following Gradle dependencies:
implementation group: 'org.springframework.cloud', name: 'spring-cloud-starter-gateway'
implementation 'com.netflix.eureka:eureka-core'
implementation 'org.springframework.cloud:spring-cloud-starter-netflix-eureka-client'
implementation 'org.springframework.boot:spring-boot-starter-actuator'
Route configuration:
#Bean
public RouteLocator routes(RouteLocatorBuilder builder, LoggingGatewayFilterFactory loggingFactory) {
return builder.routes()
.route("service_route_clients_summary", r -> r.path("/management/home/clients/summary")
.filters(f -> f.rewritePath("/api/management/home/clients/summary", "/management/home/clients/summary")
.filter(loggingFactory.apply(new LoggingGatewayFilterFactory.Config("My Custom Message", true, true))))
.uri("lb://merchant-hub-admin-service:8000/management/home/clients/summary"))
.build();
}
I want to forward /api/management/home/clients/summary to internal link /management/home/clients/summary after successful authorization with Spring Authorization Server. Is it possible to use Spring Authorization server without exposing it to outside?
What is the proper way to implement this?
Code example: https://github.com/rcbandit111/Spring_Cloud_Gateway_POC

Answer 1

Is it possible to use Spring Authorization server without exposing it to outside?
The Spring Authorization Server is implementing the OAuth2 protocol. If we look at the specs, we can see that it's naturally a client, which wants to authenticate itself: https://datatracker.ietf.org/doc/html/rfc6749#section-1.2. In your setup, it seems like that client is not the gateway itself, but lets say an app. So if that app should use the Spring Authorization Server, it would authenticate with it (most of the time, that means acquiring a token), before the first interaction with the gateway happens.
So to answer that question, if you want to use OAuth2 as it was meant to be used, you would need a direct connection between the app and the Spring Authorization Server, which means, it needs to be exposed to the outside. This makes perfect sense, since OAuth2 is meant to be used not with a single service, but with lots of different services, e.g. to allow single-sign-on.
What is the proper way to implement this?
You could setup your own Spring Authorization Server, implement your custom Authorization Code Grant logic, so that a client can authenticate itself and acquire a token. This could for example be a JSON Web Token (JWT). This can (should) be independent from your gateway and other services.
To authorize requests at the gateway, there are different ways. You could implement a logic yourself, which reads a JWT from the request and authorize it with the Spring Authorization Server or another identity provider, which you might have.
You can also use Spring Cloud Security with the Spring Authorization Server, which is described here: https://spring.io/blog/2019/08/16/securing-services-with-spring-cloud-gateway. This is definitely a proper way to implement it.
One more thing: as you can see, OAuth2 comes with a price, being that it is not trivial to understand and configure. You must understand it in detail, before you deploy such a setup. On the other hand, its a solid standard and you can have feature like single-sign-on out of the box. So if you only need authorization for single or multiple apps to a single service, there are probably easier ways to gain a token and secure an app and OAuth2 is maybe overkill. But OAuth2 really pays off, if you have lots of services and lots of apps which should be authenticated with a central (sometimes company-wide) solution.