Kong gateway how to retrieve client cert and set to header

Question

I am using Kubernetes and kong ingress controller. I have set up a client, Kong gateway, and server. The client connects Kong gateway with mTLS, Kong gateway connects the server with plain HTTP with a header contains the client cert pem.

In nginx i can simplely add this config.

server{
    location / {
        proxy_set_header ssl-client-cert $ssl_client_escaped_cert;
    }
}

In kong gateway, i try to set helm values.yaml

env.nginx_proxy_proxy_set_header: "ssl-client-cert $ssl_client_escaped_cert"

But in the container /etc/kong_prefix/nginx-kong.conf, it looks like this and the header is not sent out.

server {
    proxy_set_header ssl-client-cert $ssl_client_escaped_cert;
}

I try to use plugin request-transformer but it considers $ssl_client_escaped_cert as a plain string.

apiVersion: configuration.konghq.com/v1
kind: KongClusterPlugin
metadata:
  name: kong-plugin-client-cert-header
  annotations:
    kubernetes.io/ingress.class: kong
  labels:
    global: "true"
config: 
  add:
    headers:
    - ssl-client-cert: $ssl_client_escaped_cert;
plugin: request-transformer

How can I get the client cert and send to server as header?

Answer 1

You can use the Kong Function plugin to do custom transformation. From that plugin you can use all Nginx variables available:

• https://docs.konghq.com/hub/kong-inc/serverless-functions

For example, your plugin can be configured with something like:

apiVersion: configuration.konghq.com/v1
kind: KongPlugin
metadata:
  name: kong-plugin-client-cert-header
config:
  access:
  - |
    kong.service.request.set_header('ssl-client-cert', ('--' or ngx.var.ssl_client_escaped_cert))
plugin: post-function