code: func addStubEdns0(m *dns.Msg) *dns.Msg {
  	option := m.IsEdns0()
  	// Add a custom EDNS0 option to the packet, so we can detect loops when 2 stubs are forwarding to each other.
  	if option != nil {
  		option.Option = append(option.Option, &dns.EDNS0_LOCAL{Code: ednsStubCode, Data: []byte{1}})
  		return m
  	}
  
  	m.Extra = append(m.Extra, ednsStub)
  	return m
  }
docs: // addStubEdns0 adds our special option to the message's OPT record.