fix: avoid closing proxy relay listener before validating remote source

server/internal/proxy/session.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"log"
 	"net"
+	"os"
 	"sync"
 	"time"
 
@@ -86,22 +87,34 @@ func (s *Session) Serve(stream AgentStream) error {
 		_ = l.SetDeadline(time.Now().Add(s.timeout))
 	}
 
-	conn, err := s.listener.Accept()
-	if err != nil {
-		s.setReason("guacd_timeout")
-		return fmt.Errorf("waiting for guacd: %w", err)
-	}
-	// One connection only: nothing else may claim this port.
-	s.Close("")
+	// Accept until a connection from an allowed source arrives or the
+	// rendezvous deadline (set once, above) expires. A rejected connection is
+	// closed and the loop retries within the same deadline; the listener is
+	// only closed once we have a valid connection, the deadline expires, or
+	// Accept fails for a genuine (non-timeout) reason.
+	for {
+		conn, err := s.listener.Accept()
+		if err != nil {
+			var netErr net.Error
+			if errors.Is(err, os.ErrDeadlineExceeded) || (errors.As(err, &netErr) && netErr.Timeout()) {
+				s.setReason("guacd_timeout")
+			} else {
+				s.setReason("accept_failed")
+			}
+			return fmt.Errorf("waiting for guacd: %w", err)
+		}
 
-	if !allowedRemote(conn.RemoteAddr().String(), s.allowed) {
-		_ = conn.Close()
-		s.setReason("foreign_source")
-		return fmt.Errorf("relay connection from disallowed source %s", conn.RemoteAddr())
-	}
-	defer conn.Close()
+		if !allowedRemote(conn.RemoteAddr().String(), s.allowed) {
+			_ = conn.Close()
+			continue
+		}
 
-	return s.relay(conn, stream)
+		// One connection only: nothing else may claim this port.
+		s.Close("")
+		defer conn.Close()
+
+		return s.relay(conn, stream)
+	}
 }
 
 func (s *Session) relay(conn net.Conn, stream AgentStream) error {