You are correct that RMI, by default does use portmap and therefore random, non-root ports.
However that does not suit many production environments.
To lock it down you specify the port and remoteObjectPort as shown below (note this is in the sample ehcache.xml which ships with ehcache or is available straight off the home page, if you were looking for it)
All properties are optional. They are:
* hostName - the hostName of the host the listener is running on. Specify
where the host is multihomed and you want to control the interface over which cluster
messages are received. Defaults to the host name of the default interface if not
* port - the port the RMI Registry listener listens on. This defaults to a free port if not specified.
* remoteObjectPort - the port number on which the remote objects bound in the registry receive calls.
This defaults to a free port if not specified.
The other thing you want to specify is the multicast IP and port: