create or alter trigger CONNECT_CHECK
active on connect position 0
as
declare variable RC bigint;
begin
  if (exists (select * from MON$ATTACHMENTS a 
              where a.MON$ATTACHMENT_ID = current_connection 
                and a.MON$REMOTE_PROTOCOL <> a.MON$REMOTE_PROTOCOL <> 'XNET'))       
    then exception ERROR 'Недопустимая строка подключения';
end

select * from MON$ATTACHMENTS a where a.MON$ATTACHMENT_ID = current_connection and a.MON$REMOTE_PROTOCOL <> 'XNET'