def opt_secondary(self, ip_domain):
        """Act as secondary for the specified domain, performing
        zone transfers from the specified IP (IP/domain)
        """
        args = ip_domain.split('/', 1)
        if len(args) != 2:
            raise usage.UsageError("Argument must be of the form IP[:port]/domain")
        address = args[0].split(':')
        if len(address) == 1:
            address = (address[0], dns.PORT)
        else:
            try:
                port = int(address[1])
            except ValueError:
                raise usage.UsageError(
                    "Specify an integer port number, not %r" % (address[1],))
            address = (address[0], port)
        self.secondaries.append((address, [args[1]]))
Act as secondary for the specified domain, performing
        zone transfers from the specified IP (IP/domain)
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/names/tap.py