This file is indexed.

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.

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#!/bin/sh

set -e 

if [ "$1" != "upgrade" ]; then
	dpkg-divert --package isc-dhcp-client-ddns --add --rename \
		--divert /sbin/dhclient-noddns /sbin/dhclient
fi