How to configure Spring OpenAPI UI behind Kong - spring-boot

Question

I have a Spring Boot application and I'm using Kong as an API Gateway.
I would like to document my app's REST API with SpringDoc OpenAPI.
Everything works great locally, when I run my Spring Boot app as a standalone, but I'm facing a problem when accessing the Swagger/OpenAPI UI behind Kong.
This is my kong.yml:
services:
- name: foo
url: http://localhost:9000/foo
routes:
- name: foo-route
paths:
- /local/api/foo
methods:
- GET
- POST
- PUT
- OPTIONS
- DELETE
- PATCH
- HEAD
Assume that kong is on port 8000, and my Spring App is on port 9000.
When I hit Kong at http://localhost:8000/local/api/foo/swagger-ui.html, I get redirected to http://localhost:8000/foo/swagger-ui/index.html?configUrl=/foo/v3/api-docs/swagger-config, which is the wrong path.
How can I fix this issue?

Answer 1

Found a solution!
On the Spring Boot side, since I'm using a version higher than 2.2, all is needed is the following bean:
#Bean
ForwardedHeaderFilter forwardedHeaderFilter() {
return new ForwardedHeaderFilter();
}
and this configuration:
server:
forward-headers-strategy: framework
On the Kong side, since I'm using an outdated version (2.0.3), I need to add the following plugin to my kong.yml configuration file:
plugins:
- name: request-transformer
service: foo
config:
add:
headers:
- x-forwarded-prefix:/local/api/foo
from my understanding, this is not needed in newer Kong versions: as long as the host has a trusted ip, the header x-forwarded-prefix will be added automatically.