code: func (s *Stack) SetForwarding(enable bool) {
  	// TODO(igudger, bgeffon): Expose via /proc/sys/net/ipv4/ip_forward.
  	s.mu.Lock()
  	s.forwarding = enable
  	s.mu.Unlock()
  }
docs: // SetForwarding enables or disables the packet forwarding between NICs.