NGINX not forwarding custom headers from authentication service to backend service

I’m using NGINX as a gateway to forward requests from some client to a backend service, with an authentication service in between. The authentication service do nothing for now just response 200 and sets a custom header (db_read_time) in the response. However, when I try to access my backend service, the "db_read_time" header is not being forwarded by NGINX.

I tried to log the header but it seems that nginx cant find it, in the nginx docs i see it says the upstream_http_db_read_time refer only to the last server response so i guess it only see the header from the backend and not the authentication for some reason.

I need a way to get an header from the authentication service and forward it to the backend service.

authentication service: `

@RequiredArgsConstructor
@RestController
public class AuthenticationController {

    private final JdbcTemplate jdbcTemplate;

    @GetMapping("/authenticate")
    public ResponseEntity<String> test(@RequestHeader("Authorization") String authorizationHeader) {
        return ResponseEntity.ok()
                .header("db_read_time", "123").body("placeholder");
    }
}

`

nginx conf: `

worker_processes  1;

events {
    worker_connections  1024;
}

http {
    include       mime.types;
    default_type  application/octet-stream;

    sendfile        on;
    keepalive_timeout  65;

    log_format custom '$remote_addr - $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" '
                    '"$upstream_http_db_read_time"';

    access_log logs/access.log custom;

    server {
        listen       80;
        server_name  localhost;

        location /api/ {
            auth_request /auth;

            proxy_pass http://localhost:5000;
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;

            proxy_set_header DB-Read-Time $upstream_http_db_read_time;
        }

        location = /auth {
            internal;
            proxy_pass http://localhost:5001/authenticate;
            proxy_set_header Content-Type application/x-www-form-urlencoded;
            proxy_set_header X-Original-URI $request_uri;
            proxy_set_header Authorization $http_authorization;
        }

        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   html;
        }
    }
}

`

nginx log: `

127.0.0.1 - student [04/Oct/2024:17:15:52 +0300] "GET /api/hello HTTP/1.1" 200 27 "-" "curl/8.1.2" "-" "-"

`