'Write the DNS message header.
Writing the DNS message header is done after all sections
have been rendered, but before the optional TSIG signature
is added.'

def write_header(self):

    self.output.seek(0)
    self.output.write(struct.pack('!HHHHHH', self.id, self.flags, self.counts[0], self.counts[1], self.counts[2], self.counts[3]))
    self.output.seek(0, 2)
