Webmin uses ports 10000-100010 for RPC communication. When setting up a bind slave, Webmin uses these extra ports for communication.
What ports does Webmin RPC use>
Webmin has two RPC modes - slow mode, that only uses the same HTTP port the webserver listens on (typically 10000), and fast mode which uses ports 10000 on up. The upper bound depends on the number of concurrent RPC operations, but opening the range 10000 to 10010 should be enough when configuring the firewall between two Webmin servers.
Webmin uses ports 10000-100010 for RPC communication. When setting up a bind slave, Webmin uses these extra ports for communication.
http://www.webmin.com/faq.html