clinch/docs/forward-auth.md

Forward Authentication

References:

• https://www.reddit.com/r/selfhosted/comments/1hybe81/i_wanted_to_implement_my_own_forward_auth_proxy/
• https://www.kevinsimper.dk/posts/implementing-a-forward_auth-proxy-tips-and-details

Overview

Forward authentication allows a reverse proxy (like Caddy, Nginx, Traefik) to delegate authentication decisions to a separate service. Clinch implements this pattern to provide SSO for multiple applications.

Key Implementation Details

Tip 1: Forward URL Configuration ✅

Clinch includes the original destination URL in the redirect parameters:

login_params = {
  rd: original_url,    # redirect destination
  rm: request.method   # request method
}
login_url = "#{base_url}/signin?#{login_params.to_query}"

Example: https://clinch.aapamilne.com/signin?rd=https://metube.aapamilne.com/&rm=GET

Tip 2: Root Domain Cookies ✅

Clinch sets authentication cookies on the root domain to enable cross-subdomain authentication:

def extract_root_domain(host)
  # clinch.aapamilne.com -> .aapamilne.com
  # app.example.co.uk -> .example.co.uk
  # localhost -> nil (no domain restriction)
end

cookies.signed.permanent[:session_id] = {
  value: session.id,
  httponly: true,
  same_site: :lax,
  secure: Rails.env.production?,
  domain: ".aapamilne.com"  # Available to all subdomains
}

This allows the same session cookie to work across:

• clinch.aapamilne.com (auth service)
• metube.aapamilne.com (protected app)
• sonarr.aapamilne.com (protected app)

Authelia Analysis

Implementation Comparison

Authelia Approach (from analysis of tmp/authelia/):

• Returns 302 Found or 303 See Other with Location header
• Direct browser redirects (bypasses some proxy logic)
• Uses StatusFound (302) or StatusSeeOther (303)

Clinch Current Implementation:

• Returns 302 Found directly to login URL (matching Authelia)
• Includes rd (redirect destination) and rm (request method) parameters
• Uses root domain cookies for cross-subdomain authentication

How Clinch Forward Auth Works

Authentication Flow

1. User visits https://metube.aapamilne.com/
2. Caddy forwards to http://clinch:9000/api/verify?rd=https://clinch.aapamilne.com
3. Clinch checks session:
   • If authenticated: Returns 200 OK with user headers
   • If not authenticated: Returns 302 Found to login URL with redirect parameters
4. Browser follows redirect to Clinch login page
5. User logs in → gets redirected back to original MEtube URL
6. Caddy tries again → succeeds and forwards to MEtube

Response Headers

Successful Authentication (200 OK):

Remote-User: user@example.com
Remote-Email: user@example.com
Remote-Groups: media-managers,users
Remote-Admin: false

Redirect to Login (302 Found):

Location: https://clinch.aapamilne.com/signin?rd=https://metube.aapamilne.com/&rm=GET

Caddy Configuration

# Clinch SSO (main authentication server)
clinch.aapamilne.com {
    reverse_proxy clinch:9000
}

# MEtube (protected by Clinch)
metube.aapamilne.com {
    forward_auth clinch:9000 {
        uri /api/verify?rd=https://clinch.aapamilne.com
        copy_headers Remote-User Remote-Email Remote-Groups Remote-Admin
    }

    handle {
        reverse_proxy * {
            to http://192.168.2.223:8081
            header_up X-Real-IP {remote_host}
        }
    }
}

Key Files

• Forward Auth Controller: app/controllers/api/forward_auth_controller.rb
• Authentication Logic: app/controllers/concerns/authentication.rb
• Caddy Examples: docs/caddy-example.md
• Authelia Analysis: docs/authelia-forward-auth.md

Testing

# Test forward auth endpoint directly
curl -v http://localhost:9000/api/verify?rd=https://clinch.aapamilne.com

# Should return 302 redirect to login page
# Or 200 OK if you have a valid session cookie

Troubleshooting

Common Issues

1. Authentication Loop: Check that cookies are set on the root domain
2. Session Not Shared: Verify extract_root_domain is working correctly
3. Caddy Connection: Ensure clinch:9000 resolves from your Caddy container

Debug Logging

Enable debug logging in forward_auth_controller.rb to see:

• Headers received from Caddy
• Domain extraction results
• Redirect URLs being generated

Rails.logger.info "ForwardAuth Headers: Host=#{host}, X-Forwarded-Host=#{original_host}"
Rails.logger.info "Setting 302 redirect to: #{login_url}"