1. A method to control the flow of packets within a system that includes one or more computer networks comprising: storing policy rules in machine readable storage media that set forth attribute dependent conditions for communications among machines on the one or more networks; obtaining respective machine attributes and corresponding machine identifiers for respective machines on the one or more networks; transforming the policy rules to firewall rules that include machine identifiers of machines having attributes from among the obtained machine attributes that satisfy the attribute dependent policy rules; and storing the firewall rules in machine readable storage media.