I'm trying to install Cisco's vpn client, and it requests that I provide the directory containing linux kernel source code. Specifically, it reads:
In order to build the VPN kernel module, you must have the kernel headers for the version of the kernel you are running.
For RedHat 6.x users these files are installed in /usr/src/linux by default For RedHat 7.x users these files are installed in /usr/src/linux-2.4 by default For Suse 7.3 users these files are installed in /usr/src/linux-2.4.10.SuSE by default
Directory containing linux kernel source code 
where the last line is the prompt and the content in square brackets (in this case nothing) is the default directory.
I'm running crunchbang linux and I cannot find the directory containing the linux kernel source code. Where is the directory containing the linux kernel source code? Or, how can I find it?
Here's is what the manual, entitled VPN Client User Guide for Linux and Solaris has to say about these kernel source requirements:
To install the VPN Client, you must have the kernel source that was used to build the kernel that is running on the system. If the system is using a kernel that came as part of the Linux distribution, or a custom built kernel, the kernel code can be obtained in different ways:
• For users running kernels that came with their distribution—You must install the corresponding kernel-source rpm. The vpn_install script should be able to automatically find the kernel source.
• For users running a custom-built kernel—You must use the same copy of the kernel source that was used to build the kernel you are running. Unpacking the source code for the version of the kernel you are using is insufficient. There are several files generated when the kernel is compiled that the VPN Client uses. These files must exactly match with the kernel you are running. Otherwise, the VPN Client installation might fail.
Since I think I'm in the first case, I ran
sudo apt-get install rpm (without knowing anything about rpm), and tried the installation again, hoping that the correct value would now be defaulted, but it wasn't and hence the installation still failed. What is the "kernel-source rpm" and how do I install it?
As suggested, I tried to install
linux-headers-something, but there were many auto-completions of
sudo apt-get install linux-headers specifically, they were:
linux-headers-2.6.32-5-486 linux-headers-2.6.32-5-686 linux-headers-2.6.32-5-686-bigmem linux-headers-2.6.32-5-all linux-headers-2.6.32-5-all-i386 linux-headers-2.6.32-5-amd64 linux-headers-2.6.32-5-common linux-headers-2.6.32-5-common-openvz linux-headers-2.6.32-5-common-vserver linux-headers-2.6.32-5-common-xen linux-headers-2.6.32-5-openvz-686 linux-headers-2.6.32-5-vserver-686 linux-headers-2.6.32-5-vserver-686-bigmem linux-headers-2.6.32-5-xen-686 linux-headers-2.6.36-2.dmz.5-liquorix-686 linux-headers-2.6-486 linux-headers-2.6-686 linux-headers-2.6-686-bigmem linux-headers-2.6-amd64 linux-headers-2.6-liquorix-686 linux-headers-2.6-openvz-686 linux-headers-2.6-vserver-686 linux-headers-2.6-vserver-686-bigmem linux-headers-2.6-xen-686
How can I determine which one I should install?