Linux netfilter and SECMARK support

The approach with TCP and UDP ports has a few downsides. One of them is that there is no knowledge of the target host, so you cannot govern where an application can connect to. There is also no way of limiting daemons from binding on any interface: in a multi-homed situation, we might want to make sure that a daemon only binds on the interface facing the internal network and not the Internet-facing one, or vice versa.

In the past, SELinux allowed support for this binding issue through the interface and node labels: a domain could only be allowed to bind to one interface and not on any other, or even on a particular address (referred to as the node). This support had its flaws though, and has been largely deprecated in favor of SECMARK filtering.

Introducing netfilter

Before explaining SECMARK and how administrators can control it, let's first take a quick look at Linux's netfilter subsystem, which is the de facto standard for local firewall capabilities on...