When the pfilter packet filtering system is turned on, the default OSCAR settings allow any network communication between the machines in the cluster, and allow ssh and http access to the cluster main machine from the outside.
Communication between cluster machines and the outside network are limited to outgoing connections only. Incoming network connections to cluster machines are blocked.
To allow outside network connections to ports on the cluster machines, special rules will have to be added to the pfilter configuration. See your cluster administrator for help on this.