Spring Boot Cors behind reverse-proxy on non-default port

My application has a backend built with Spring Boot, and a frontend web application built with Angular and served by nginx. Nginx is configured to reverse-proxy requests to /api/ to the backend :

location /api/ {
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_pass http://back:8080;
}

If nginx is running on the default port (80), I can access my application at http://myserver/ without issues. XHR calls to http://myserver/api/some/REST/resource work perfectly.

If I change nginx to listen to some other port (randomly : 9043), then all XHR calls fail with an error "Invalid CORS request". Which is unexpected, because only my frontend application is making calls to the API, and thanks to the reverse-proxy this API is served on the same host as the javascript files.

For it to function, I need to add this to my Spring application :

@Configuration
public class CorsConfig {

    @Value("${url.base:''}")
    private String urlBase;

    @Value("${cors.allowed.origins:[]}")
    private String[] allowedOrigins;

    @Bean
    public WebMvcConfigurer corsConfigurer() {
        return new WebMvcConfigurerAdapter() {
            @Override
            public void addCorsMappings(CorsRegistry registry) {
                registry.addMapping(urlBase + "/**")
                        .allowedOrigins(allowedOrigins)
                        .allowedMethods("*");
            }
        };
    }
}

and then set cors.allowed.origins to http://myserver:9043/ in my configuration.

This works, but it is not practical if I'm going to :

• make the hostname and/or port dynamic (port is derived from the branch name during CI build, then the containers are deployed to a rancher cluster)
• hide the nginx behind a load-balancer (= another level of reverse-proxying)

Would there be a solution to fix this by doing any of the following :

• Have spring boot to ignore the port number when validating CORS requests ?
• Have nginx tweak the contents of the proxied request so that CORS validation by Spring succeeds ?

EDIT : some more details and example :

The services are dockerized :

• Frontend : nginx listens on port 80 inside the container, docker exposes it on the host as port 9043
• Backend : spring boot listens on 8080. The port is not exported by Docker, so it is only accessible from the frontend container, which has a link to the backend container.

It only works if the frontend is exposed to the outside world on port 80...

Add two http header in reverse request:

X-Forwarded-Proto

X-Forwarded-Port

to indecate the origin proto and ports (match the origin request).

see more: https://moi.vonos.net/java/spring-forward/