docs: oidc faq resolution (#5352)

Signed-off-by: James Elliott <james-d-elliott@users.noreply.github.com>

docs/content/en/integration/openid-connect/frequently-asked-questions.md

@@ -12,7 +12,11 @@ weight: 615
 toc: true
 ---
 
-## How do I generate client secrets?
+### Questions
+
+The following section lists individual questions.
+
+### How do I generate client secrets?
 
 We strongly recommend the following guidelines for generating client secrets:
 
@@ -35,7 +39,7 @@ party / client application encoding the characters correctly as it uses the
 
 [Generating a Random Password Hash]: ../../reference/guides/generating-secure-values.md#generating-a-random-password-hash
 
-### Plaintext
+#### Plaintext
 
 Authelia *technically* supports storing the plaintext secret in the configuration. This will likely be completely
 unavailable in the future as it was a mistake to implement it like this in the first place. While some other OpenID
@@ -58,7 +62,7 @@ Plaintext is either denoted by the `$plaintext$` prefix where everything after t
 the secret does not start with the `$` character it's considered as a plaintext secret for the time being but is
 deprecated as is the `$plaintext$` prefix.
 
-## Why isn't my application able to retrieve the token even though I've consented?
+### Why isn't my application able to retrieve the token even though I've consented?
 
 The most common cause for this issue is when the affected application can not make requests to the Token [Endpoint].
 This becomes obvious when the log level is set to `debug` or `trace` and a presence of requests to the Authorization
@@ -71,4 +75,77 @@ to the Token [Endpoint].
 All causes should be clearly logged by the client application, and all errors that do not match this scenario are
 clearly logged by Authelia. It's not possible for us to log requests that never occur however.
 
+One potential solution to this is detailed in the [Solution: Configure DNS Appropriately](#configure-dns-appropriately)
+section. This section also details how to identity if you're affected.
+
+### Why doesn't the discovery endpoint return the correct issuer and endpoint URL's?
+
+The most common cause for this is if the `X-Forwarded-Proto` and `X-Forwarded-Host` / `Host` headers do not match the
+fully qualified URL of the provider. This can be because of requesting from the Authelia port directly i.e. without going
+through your proxy or due to a poorly configured proxy.
+
+If you've configured Authelia alongside a proxy and are making a request directly to Authelia you need to perform the
+request via the proxy. If you're avoiding the proxy due to a DNS limitation see
+[Solution: Configure DNS Appropriately](#configure-dns-appropriately) section.
+
+## Solutions
+
+The following section details solutions for multiple of the questions above.
+
+### Configure DNS Appropriately
+
+In order to make requests to Authelia an application must be able to resolve it. It's important in all instances to
+check if the application with the issue can resolve the correct IP address for Authelia between each step of the
+process, and this check also can be used to clearly identity if this is the most likely underlying cause for an issue
+you're facing.
+
+##### Bare-Metal
+
+1. If you're running an internal DNS server ensure an A record exists for the FQDN of Authelia with the value being the
+   IP of the server responsible for handling requests for Authelia.
+2. If you're not running an internal DNS server then do check the following:
+   1. Ensure the external DNS server(s) have the same A record as described above.
+   2. Ensure that that your NAT-hairpin is configured correctly.
+   3. If all else fails add a hosts file entry to work around this issue.
+
+##### Docker
+
+1. Ensure both the application with the issue shares a network in common with the proxy container.
+2. Ensure an alias for the FQDN of Authelia is present for the proxy container:
+   - If using `docker compose` see the
+     [network aliases](https://docs.docker.com/compose/compose-file/compose-file-v3/#aliases) documentation
+     reference for more information.
+   - If using `docker run` see the `--network-alias` option of the [docker run](https://docs.docker.com/engine/reference/commandline/run/)
+     reference for more information.
+
+Examples (assuming your Authelia Root URL is `https://auth.example.com`):
+
+```yaml
+version: "3.8"
+services:
+  application:
+    ## Mandatory that the application is on the same network as the proxy.
+    networks:
+      proxy: {}
+  proxy:
+    networks:
+      ## Mandatory that the proxy is on the same network as the application, and that it has this alias.
+      proxy:
+        aliases:
+          - auth.example.com
+  authelia:
+    networks:
+      proxy: {}
+networks:
+  proxy:
+    ## An external network can be created manually and shared between multiple compose files. This is NOT mandatory.
+    external: true
+    name: proxy-net
+```
+
+```console
+docker run -d --name proxy --network proxy --network-alias auth.example.com <other proxy arguments>
+docker run -d --name application --network proxy <other application arguments>
+```
+
 [Endpoint]: ./introduction.md#discoverable-endpoints