The following table lists the Linux kernel versions and the corresponding ipfwadm versions:

Older 2.1.x kernels will probably work with ipfwadm 2.3.0. Kernels 2.1.102 and above and the upcoming 2.2.x kernels will not work with ipfwadm. For these kernels, you need the new ipchains utility. Read all about it on the ipchains home page which also refers to an extensive HOWTO, including a list of differences between ipfwadm and ipchains.

Ipfwadm 2.3.0 does not handle all features of the most recent 2.0.x kernels (such as ICMP masquerading).

Look at the psuedo-file /proc/version to find the version of your Linux kernel. If you have ipfwadm already running, ipfwadm -h will show its version.