$ sudo apt-get update
$ sudo apt-get install kfreebsd-headers-9

kfreebsd-headers-9 is installed in your system.

Make ensure the kfreebsd-headers-9 package were installed using the commands given below,

$ sudo dpkg-query -l | grep kfreebsd-headers-9*

You will get with kfreebsd-headers-9 package name, version, architecture and description in a table

kfreebsd-headers-9 package basic information: