preinst is in ntop 3:4.1.0+dfsg1-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | #!/bin/sh
set -e
# Remove a no-longer used conffile
rm_conffile() {
    local PKGNAME="$1"
    local CONFFILE="$2"
    [ -e "$CONFFILE" ] || return 0
    local md5sum="$(md5sum $CONFFILE | sed -e 's/ .*//')"
    local old_md5sum="$(dpkg-query -W -f='${Conffiles}' $PKGNAME | \
            sed -n -e "\' $CONFFILE ' { s/ obsolete$//; s/.* //; p }")"
    if [ "$md5sum" != "$old_md5sum" ]; then
        echo "Obsolete conffile $CONFFILE has been modified by you."
        echo "Saving as $CONFFILE.dpkg-bak ..."
        mv -f "$CONFFILE" "$CONFFILE".dpkg-bak
    else
        echo "Removing obsolete conffile $CONFFILE ..."
        rm -f "$CONFFILE"
    fi
}
case "$1" in
install|upgrade)
    if dpkg --compare-versions "$2" le "3:3.3-14"; then
        rm_conffile ntop "/etc/ntop/AS-list.txt"
        rm_conffile ntop "/etc/ntop/etter.finger.os"
	rm_conffile ntop "/etc/ntop/oui.txt"
	rm_conffile ntop "/etc/ntop/p2c.opt.table"
    fi
esac
 |