$ sudo apt-get update
$ sudo apt-get install kfreebsd-headers-8

kfreebsd-headers-8 is installed in your system.

Make ensure the kfreebsd-headers-8 package were installed using the commands given below,

$ sudo dpkg-query -l | grep kfreebsd-headers-8*

You will get with kfreebsd-headers-8 package name, version, architecture and description in a table

kfreebsd-headers-8 package basic information: