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nginx-auth

This is a tool that allows users to use Tailscale Whois authentication with NGINX as a reverse proxy. This allows users that already have a bunch of services hosted on an internal NGINX server to point those domains to the Tailscale IP of the NGINX server and then seamlessly use Tailscale for authentication.

Many thanks to @zrail on Twitter for introducing the basic idea and offering some sample code. This program is based on that sample code with security enhancements. Namely:

This listens over a UNIX socket instead of a TCP socket, to prevent leakage to the network
This uses systemd socket activation so that systemd owns the socket and can then lock down the service to the bare minimum required to do its job without having to worry about dropping permissions
This provides additional information in HTTP response headers that can be useful for integrating with various services

Configuration

In order to protect a service with this tool, do the following in the respective server block:

Create an authentication location with the internal flag set:

location /auth {
  internal;

  proxy_pass http://unix:/run/tailscale.nginx-auth.sock;
  proxy_pass_request_body off;

  proxy_set_header Host $http_host;
  proxy_set_header Remote-Addr $remote_addr;
  proxy_set_header Remote-Port $remote_port;
  proxy_set_header Original-URI $request_uri;
}

Then add the following to the location / block:

auth_request /auth;
auth_request_set $auth_user $upstream_http_tailscale_user;
auth_request_set $auth_name $upstream_http_tailscale_name;
auth_request_set $auth_login $upstream_http_tailscale_login;
auth_request_set $auth_tailnet $upstream_http_tailscale_tailnet;
auth_request_set $auth_profile_picture $upstream_http_tailscale_profile_picture;

proxy_set_header X-Webauth-User "$auth_user";
proxy_set_header X-Webauth-Name "$auth_name";
proxy_set_header X-Webauth-Login "$auth_login";
proxy_set_header X-Webauth-Tailnet "$auth_tailnet";
proxy_set_header X-Webauth-Profile-Picture "$auth_profile_picture";

When this configuration is used with a Go HTTP handler such as this:

http.HandlerFunc(func (w http.ResponseWriter, r *http.Request) {
	e := json.NewEncoder(w)
	e.SetIndent("", "  ")
	e.Encode(r.Header)
})

You will get output like this:

{
  "Accept": [
    "*/*"
  ],
  "Connection": [
    "upgrade"
  ],
  "User-Agent": [
    "curl/7.82.0"
  ],
  "X-Webauth-Login": [
    "Xe"
  ],
  "X-Webauth-Name": [
    "Xe Iaso"
  ],
  "X-Webauth-Profile-Picture": [
    "https://avatars.githubusercontent.com/u/529003?v=4"
  ],
  "X-Webauth-Tailnet": [
    "cetacean.org.github"
  ]
  "X-Webauth-User": [
    "Xe@github"
  ]
}

Headers

The authentication service provides the following headers to decorate your proxied requests:

Header	Example Value	Description
`Tailscale-User`	`azurediamond@hunter2.net`	The Tailscale username the remote machine is logged in as in user@host form
`Tailscale-Login`	`azurediamond`	The user portion of the Tailscale username the remote machine is logged in as
`Tailscale-Name`	`Azure Diamond`	The "real name" of the Tailscale user the machine is logged in as
`Tailscale-Profile-Picture`	`https://i.kym-cdn.com/photos/images/newsfeed/001/065/963/ae0.png`	The profile picture provided by the Identity Provider your tailnet uses
`Tailscale-Tailnet`	`hunter2.net`	The tailnet name

Most of the time you can set X-Webauth-User to the contents of the Tailscale-User header, but some services may not accept a username with an @ symbol in it. If this is the case, set X-Webauth-User to the Tailscale-Login header.

The Tailscale-Tailnet header can help you identify which tailnet the session is coming from. If you are using node sharing, this can help you make sure that you aren't giving administrative access to people outside your tailnet.

Allow Requests From Only One Tailnet

If you want to prevent node sharing from allowing users to access a service, add the Expected-Tailnet header to your auth request:

location /auth {
  # ...
  proxy_set_header Expected-Tailnet "tailnet012345.ts.net";
}

If a user from a different tailnet tries to use that service, this will return a generic "forbidden" error page:

<html>
<head><title>403 Forbidden</title></head>
<body>
<center><h1>403 Forbidden</h1></center>
<hr><center>nginx/1.18.0 (Ubuntu)</center>
</body>
</html>

You can get the tailnet name from the admin panel.

Building

Install cmd/mkpkg:

cd .. && go install ./mkpkg

Then run ./mkdeb.sh. It will emit a .deb and .rpm package for amd64 machines (Linux uname flag: x86_64). You can add these to your deployment methods as you see fit.

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