docs(oidc): tailscale integration (#5287)

Adding Tailscale configuration documentation.

docs/content/en/integration/openid-connect/tailscale/index.md

@@ -0,0 +1,99 @@
+---
+title: "Tailscale"
+description: "Using Authelia as the Tailscale OpenID Connect Provider."
+lead: ""
+date: 2023-04-20T08:53:36.993Z
+draft: false
+images: []
+menu:
+  integration:
+    parent: "openid-connect"
+weight: 620
+toc: true
+community: true
+---
+
+## Tested Versions
+
+* [Authelia]
+  * [v4.37.5](https://github.com/authelia/authelia/releases/tag/v4.37.5)
+* [Tailscale] - Note: Version not important, since configuration is via the web UI
+  * [1.38.4](https://github.com/tailscale/tailscale/releases/tag/v1.38.4)
+
+## Before You Begin
+
+{{% oidc-common %}}
+
+### Assumptions
+
+This example makes the following assumptions:
+
+* __Domain Root URL:__ `https://example.com`
+* __Authelia Root URL:__ `https://auth.example.com`
+* __Authelia Account:__ `user@example.com`
+* __Client ID:__ `tailscale`
+* __Client Secret:__ `insecure_secret`
+
+
+## Configuration
+The configuration in Authelia is straightforwarded: Tailscale is just another `identity_provider/oidc` entry. Complicating things is the necessary WebFinger reply for your domain - see the following [Application](#application) section.
+
+
+### Application
+
+To configure [Tailscale] to utilize Authelia as an [OpenID Connect 1.0] Provider, you will need a public WebFinger reply for your domain (see [RFC 7033](https://www.rfc-editor.org/rfc/rfc7033#section-3.1)) and point it to Authelia. The steps necessary are outlined in the Tailscale documentation on [Custom OIDC providers](https://tailscale.com/kb/1240/sso-custom-oidc/). This WebFinger reply is not generated by Authelia, so your external webserver hosted at the root of your domain will need to generate the reponse (Check [See also](#see-also) for example implementations). The following steps are necessary to get Tailscale working with Authelia:
+
+1. Your domain will need to reply to a WebFinger request for your Authelia account
+2. Your domain root is `example.com` and the Authelia account in question is `user@example.com`, the WebFinger request will be: `https://example.com/.well-known/webfinger/?resource=acct:user@example.com`
+3. The WebFinger request will need to be answered with the following example reply:  
+```
+{
+ "subject" : "acct:user@example.com",
+ "links" :
+ [
+   {
+     "rel" : "http://openid.net/specs/connect/1.0/issuer",
+     "href" : "https://auth.example.com"
+   }
+ ]
+}
+```
+4. For any other users that you want to add to Tailscale, you will need to to provide similar WebFinger replies (e.g. for `user2@example.com` or `user3@example.com`)
+5. Once you have the WebFinger reply set up and your [Authelia OpenID Connect Discovery endpoint](https://www.authelia.com/integration/openid-connect/introduction/#well-known-discovery-endpoints) is working (e.g. `https://auth.example.com/.well-known/openid-configuration`), you can sign up for a **new Tailnet** (currently migration isn't supported) via the link: [Sign up with OIDC](https://login.tailscale.com/start/oidc) where you will see the following screen:  
+{{< figure src="tailscale_signup_1.png" alt="Tailscale Signup Screen 1" width="300" >}}
+6. After clicking on **Get OIDC Issuer**, Tailscale will fetch the WebFinger reply via `https://example.com/.well-known/webfinger/?resource=acct:user@example.com` and follow the set `href` to `https://auth.example.com/.well-known/openid-configuration`.  
+**Note:** make sure that the `href` URL matches the `issuer` URL returned from the Authelia OIDC dicsovery endpoint
+7. On the next screen you will need to add your client ID & secret configured in Authelia to finish the OIDC provider registration in [Tailscale].
+
+
+### Authelia
+
+The following YAML configuration is an example __Authelia__
+[client configuration](../../../configuration/identity-providers/open-id-connect.md#clients) for use with [Tailscale] which
+will operate with the above example:
+
+```yaml
+identity_providers:
+  oidc:
+    ## The other portions of the mandatory OpenID Connect 1.0 configuration go here.
+    ## See: https://www.authelia.com/c/oidc
+    clients:
+    - id: tailscale
+      description: Tailscale SSO
+      secret: '$pbkdf2-sha512$310000$c8p78n7pUMln0jzvd4aK4Q$JNRBzwAo0ek5qKn50cFzzvE9RXV88h1wJn5KGiHrD0YKtZaR/nCb2CJPOsKaPK0hjf.9yHxzQGZziziccp6Yng'  # The digest of 'insecure_secret'.
+      redirect_uris:
+        - https://login.tailscale.com/a/oauth_response
+      scopes:
+        - openid
+        - email
+        - profile
+```
+
+## See Also
+
+- [Tailscale] [Custom OIDC Provider Knowledge Base entry](https://tailscale.com/kb/1240/sso-custom-oidc/):
+- [RFC 7033, Identity Provider Discovery for OpenID Connect](https://www.rfc-editor.org/rfc/rfc7033#section-3.1)
+- [WebFinger example implementations](https://webfinger.net/code/)
+
+[Tailscale]: https://tailscale.com
+[OpenID Connect 1.0]: ../../openid-connect/introduction.md