You can see the list of kernel parameters using the command:
$ sysctl -a
I looked on a CentOS 6.x system along with a Ubuntu 12.04 system and neither had that parameter too.
$ sysctl -a | grep forwarding
I wonder if it's a BSD only parameter?This forum thread shows it as just being a pfSense & BSD option, net.inet.ip.fastforwarding performance tweak
I haven't found it referenced anywhere except in connection to pfsense. Googling for "net.inet.ip.fastforwarding" only shows BSD related links.
This tutorial, FreeBSD Tuning and Optimization also shows it as just a BSD option.
net.inet.ip
This entire tree doesn't even exist in Linux, it would appear to be BSD only Kernel Parameters. Here's a post that covers the rest of them, titled: FreeBSD net.inet.ip Sysctls Explained. Linux uses net.ipv4.inet
.