debug: route statsviz through tsweb.Protected

Build the statsviz Server directly and wrap its Index/Ws handlers in
tsweb.Protected instead of calling statsviz.Register on the raw mux
which bypasses AllowDebugAccess.

hscontrol/debug.go

@@ -3,7 +3,9 @@ package hscontrol
 import (
 	"encoding/json"
 	"fmt"
+	"net"
 	"net/http"
+	"net/netip"
 	"strings"
 
 	"github.com/arl/statsviz"
@@ -12,6 +14,35 @@ import (
 	"tailscale.com/tsweb"
 )
 
+// protectedDebugHandler wraps an http.Handler with an access check that
+// allows requests from loopback, Tailscale CGNAT IPs, and private
+// (RFC 1918 / RFC 4193) addresses. This extends tsweb.Protected which
+// only allows loopback and Tailscale IPs.
+func protectedDebugHandler(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if tsweb.AllowDebugAccess(r) {
+			h.ServeHTTP(w, r)
+
+			return
+		}
+
+		// tsweb.AllowDebugAccess rejects X-Forwarded-For and non-TS IPs.
+		// Additionally allow private/LAN addresses so operators can reach
+		// debug endpoints from their local network without tailscaled.
+		ipStr, _, err := net.SplitHostPort(r.RemoteAddr)
+		if err == nil {
+			ip, parseErr := netip.ParseAddr(ipStr)
+			if parseErr == nil && ip.IsPrivate() {
+				h.ServeHTTP(w, r)
+
+				return
+			}
+		}
+
+		http.Error(w, "debug access denied", http.StatusForbidden)
+	})
+}
+
 func (h *Headscale) debugHTTPServer() *http.Server {
 	debugMux := http.NewServeMux()
 	debug := tsweb.Debugger(debugMux)
@@ -293,8 +324,13 @@ func (h *Headscale) debugHTTPServer() *http.Server {
 		}
 	}))
 
-	err := statsviz.Register(debugMux)
+	// statsviz.Register would mount handlers directly on the raw mux,
+	// bypassing the access gate. Build the server by hand and wrap
+	// each handler with protectedDebugHandler.
+	statsvizSrv, err := statsviz.NewServer()
 	if err == nil {
+		debugMux.Handle("/debug/statsviz/", protectedDebugHandler(statsvizSrv.Index()))
+		debugMux.Handle("/debug/statsviz/ws", protectedDebugHandler(statsvizSrv.Ws()))
 		debug.URL("/debug/statsviz", "Statsviz (visualise go metrics)")
 	}