You can use any port listet in /etc/services or the number
of the port. Also variables from the Aliases
definition can be used. Multiple entries are allowed in every input-field.
Each port in a ´Source´ field must have a corresponding entry
in the ´Destination´ field. Otherwise it will be ignored.
You can also block ports with adding a '!' in front of the port (without
blank between!) - see the default definition of ftp_passive for an example.