Google is your Friend

Googling for "libpcap n800" leads you to gronmayer's excellent site:
http://www.gronmayer.com/it/index.ph...&system=maemo3
There you will find all sorts of repositories for the N800. Enter "libpcap" into the search window (Be sure to click on your correct OS version first) and you will find a list of the repositories that have libpcap in them. If you are doing all this on the Nokia, then you can just click on the green "install" arrow and the repository will be added into the Application Manager for you. If not, you will need to manually enter the URL into Application Manager; select Tools/Application catalogue from the menu.

Once that's done you can install the package normally. You will need to be in Red Pill mode to see libpcap, I'm fairly sure.

HTH
Walt