The binary rhnsd is used by Redhat to listen for connections from up2date servers. It is recommended that this is kept running because it not only will listen for security updates that we push down, but it also verifies a servers RHN credentials. If rhnsd is not running, your credentials will expire and you will not be able to use up2date to update your system packages. If this happens, open a ticket and we will re-register the service.