SELinux by default uses access controls based on the file representation of communication primitives or the sockets that are used. In the case of TCP and UDP ports, administrators have some leeway in handling the controls through the semanage command without resorting to SELinux policy updates. Once we go into the realms of network-based communication, more advanced communication control can be accomplished through Linux netfilter support, using the SECMARK labeling, and through peer labeling.
In the case of SECMARK labeling, local firewall rules are used to map contexts to packets, which are then governed through SELinux policy. In the case of peer labeling, either the application context itself (in the case of labeled IPsec) or its sensitivity level (in the case of netfilter/CIPSO support) is used. This allows an almost application-to-application network flow control through SELinux policies.
In the next chapter, we will take a look at two platforms that use SELinux for their additional...