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The up2date clients behind a firewall are configured to communicate via either http or https, so you would need to open internally port 80 and 443 for the up2date clients to connect to the RHN Proxy server. The RHN Proxy will then make outgoing connections to xmlrpc.rhn.redhat.com - so you would need to allow outgoing port 80 and 443 access from the RHN Proxy server to xmlrpc.rhn.redhat.com.

Depending on how locked down and if this is being done with iptables/ipchains locally on the server, the following would also apply:

Apache redirects/queries requests to the local squid proxy running on port 8080 via localhost (127.0.0.1) and also communicates with the rhn_auth_cache daemon listening on port 9999 via localhost. So you would need to ensure that these localhost connections are also allowed.