Feature Request: Enhanced DoH Client Identification via Custom HTTP Header

[vdias]

Summary
Allow AdGuard Home to read a custom HTTP header (e.g., X-Client-ID or X-Forwarded-Client-ID) in DoH requests when behind a reverse proxy/relay. This would enable easy per-client identification without requiring a full second AdGuard Home instance on the local network.

Motivation
When running AdGuard Home (AGH) as a public DoH endpoint behind a generic reverse proxy (like Nginx, Caddy, or dnsproxy), all traffic from a single network is logged as one client (the public IP of the network).

Current methods for per-client identification are complicated:

1. Running a second AGH instance: This is high-overhead and redundant.
2. Using the /dns-query/ path: This requires complex, per-device configuration and is often impossible on simple clients (e.g., smart TVs).

A client relay (like dnsproxy or a sophisticated router) can easily insert a custom HTTP header containing the local client's IP or a generated ID before forwarding to the public DoH server. Implementing a feature to read and trust this header would simplify secure, per-client identification for a large segment of the self-hosting community, replicating the seamless client visibility offered by services like NextDNS.

Proposed Solution
Add a configuration option to AdGuardHome.yaml that specifies a custom HTTP header field to check for client identification on proxied DoH requests:

1. Configuration Change:

# AdGuardHome.yaml
dns:
  # ... other settings ...
  # New setting to specify a custom HTTP header for client ID
  doh_client_id_header: 'X-Forwarded-Client-ID'

2. Behavior:

• If doh_client_id_header is set, AGH checks for the value of that header in the incoming DoH request.
• If the header is present and the value corresponds to a configured AGH Client ID, the request is logged and processed for that client.
• If the header is missing or the value is unrecognized, the request falls back to standard client identification (IP/subnet).

This feature would offer a clean, low-overhead, standards-compliant way to get per-client logging.