Added docs about connecting to an API with authentication (#47)

docs/api-auth.md

@@ -0,0 +1,75 @@
+# How to connect to authenticate to an API
+
+Kubernetes allows you to secure the Thundernetes' GameServer API [using an Ingress](https://playfab.github.io/thundernetes/howtos/serviceingress.html), if you do this you will need a way to provide the certificates from this client. Currently the Thundernetes UI is conceived as a simple React application, and because of this it's not safe to store credentials in the app. A very easy to implement alternative is to use a proxy that contains the credentials, in the following sections we show an example for deploying a reverse proxy using the official [Nginx Docker image](https://hub.docker.com/_/nginx).
+
+## Architecture
+
+The idea is to use a proxy between the UI and Thunernetes' API, so the UI makes the requests to the proxy, and the proxy to the Thundernetes clusters, this way the credential can be stored in the proxy without them being exposed through the browser. You can also use a single proxy for multiple clusters, using Nginx to act as a reverse proxy.
+
+![Graphic describing the architecture of Thundernetes UI using a proxy](./thundernetes_ui_proxy.png "Graphic describing the architecture of Thundernetes UI using a proxy")
+
+## How to configure your own proxy with mTLS
+
+All we need to do is to write a configuration file for Nginx and a Dockerfile to create a container with the credentials. Lets create a configuration file called ```default.conf``` and add the following:
+
+```nginx
+server{
+  listen 80;
+
+  location /tls/ {
+    proxy_pass                    https://{your_endpoint_with_mTLS}/;
+    proxy_ssl_certificate         /etc/ssl/private/client.crt;
+    proxy_ssl_certificate_key     /etc/ssl/private/client.key;
+    proxy_ssl_trusted_certificate /etc/ssl/private/ca.crt;
+    proxy_ssl_verify              on;
+  }
+
+}
+```
+
+This file uses ```proxy_pass``` to define that every request to ```http://{proxy_ip}/tls/``` will be forwarded to ```https://{your_endpoint_with_mTLS}/```, the trailing forward slashes are necessary so the rest of the URI is forwarded too. It also defines the key pair that the client, this proxy, uses to authenticate, for this ```proxy_ssl_certificate``` and ```proxy_ssl_certificate_key``` are used. Finally, you use ```proxy_ssl_trusted_certificate``` to define a trusted Certificate Authority (CA), so every certificate signed with that will be trusted, and also ```proxy_ssl_verify``` to toggle the validation for the certificates that come from the server.
+
+Now create a Dockerfile with the following:
+
+```Dockerfile
+FROM nginx:1.23-alpine
+RUN rm /etc/nginx/conf.d/default.conf
+COPY default.conf /etc/nginx/conf.d/
+COPY client.crt /etc/ssl/private/
+COPY client.key /etc/ssl/private/
+COPY ca.crt /etc/ssl/private/
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]
+
+```
+
+In this you pull the Nginx Docker image, add the configuration, and the certificates. Now you can build the image:
+
+```
+docker build -t nginx-proxy:0.1 .
+```
+
+And run it:
+
+```
+docker run -d -p 80:80 nginx-proxy:0.1
+```
+
+## Connect Thundernetes UI to the proxy
+
+Simply add the proxy IP, with the correct URI, to the config file before starting the UI:
+
+```js
+var clusters = {
+  "with_tls": {
+    "api": "http://{proxy_IP}/api/v1/",
+    "allocate": "http://{allocate_api}:5000/api/v1/allocate",
+  }
+}
+```
+
+> **_NOTE_**: This tutorial only shows how to connect to the GameServer API with mTLS, but the steps are exactly the same if you added mTLS to the allocate endpoint.
+
+## Other authentication methods
+
+Nginx also allows you to add headers using the ```proxy_set_header``` directive, so any other type of authentication that uses headers to send credentials are easily supported by this method.