preinst is in isc-dhcp-client-ddns 4.3.5-3+deb9u1.
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 | #!/bin/sh
set -e
if [ "$1" != "upgrade" ]; then
dpkg-divert --package isc-dhcp-client-ddns --add --rename \
--divert /sbin/dhclient-noddns /sbin/dhclient
fi
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