ratelimit: key on rightmost X-Forwarded-For entry (fixes #85)

internal/api/ratelimit.go

@@ -48,8 +48,28 @@ func (l *limiter) allow(key string, rate, burst float64) bool {
 	return true
 }
 
-// clientIP extracts the request IP (no reverse proxy header by default).
+// clientIP extracts the client IP for rate-limit keying (#85).
+//
+// Trust boundary: palette runs behind exactly ONE trusted reverse proxy
+// (Traefik in the k3s pod network). Traefik APPENDS the real client IP to
+// X-Forwarded-For, so the RIGHTMOST entry is the last value the trusted
+// proxy observed and is unspoofable by the client (a client-supplied fake
+// entry only lands on the LEFT and is ignored). This matches chi's
+// middleware.RealIP semantics for a single trusted proxy hop.
+//
+// Direct connections (no XFF header) fall back to RemoteAddr. Directly
+// reachable deployments must NOT expose the app to untrusted networks
+// without a proxy in front, or attackers could forge the rightmost entry.
 func clientIP(r *http.Request) string {
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		if i := strings.LastIndex(xff, ","); i >= 0 {
+			return strings.TrimSpace(xff[i+1:])
+		}
+		return strings.TrimSpace(xff)
+	}
+	if xr := r.Header.Get("X-Real-Ip"); xr != "" {
+		return strings.TrimSpace(xr)
+	}
 	host := r.RemoteAddr
 	if i := strings.LastIndex(host, ":"); i > 0 {
 		host = host[:i]