IPFilter can be installed as a runtime-loadable kernel module or directly incorporated into the operating system kernel, depending on the specifics of each kernel and user preferences. The software's documentation recommends the module approach, if possible.
Notes
^The full license text can be found from IPFILTER.LICENCE file of its source directory. The author later also licensed version 5.1.2 under the previous BSD-like license for use in FreeBSD project.[1]
^"Features". illumos. 2019-10-02. Retrieved 2019-11-10. illumos uses ipfilter for firewalling. Using ipfilter you can create firewalls not just for the host system, but also for zones and hardware virtualized systems.