Both the ndc and the rndc utilities use a control channel to send commands to and retrieve information from a name server. However, there are differences between the utilities.
In BIND 8, ndc can use AF_UNIX domain sockets (UNIX control channel) or TCP/IP sockets (inet control channel). By default, ndc does not need any support in /etc/named.conf, because BIND 8 servers use a UNIX domain socket with a path (/var/run/ndc.d/ndc) compiled into in.named.
For BIND 9, however, rndc only uses an authenticated TCP/IP inet control channel and so is not backward compatible with BIND 8. There is no UNIX domain socket support for control channels in BIND 9 servers.
When using rndc, you need to specify a 'key' clause to communicate with the name server. It is mandatory that the BIND 9 server and the rndc client share the same key (defined both in /etc/named.conf and /etc/rndc.conf). Using the BIND 8 controls entry in BIND 9 will result in an error message.
Some command options have changed from the ndc to the rndc implementation. This includes the -c option, which has a different syntax in BIND 9. Therefore, to specify the control channel in BIND 9, use rndc -s <server> -p <port>.