postrm is in isc-dhcp-client 4.1.ESV-R4-0ubuntu5.10.
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  | #!/bin/sh -e
#
#
if [ "$1" = "purge" ]; then
	# Remove lease database
	rm -f /var/lib/dhcp/dhclient.leases*
	# Try to remove directory
	if [ -d /var/lib/dhcp ]; then
		rmdir --ignore-fail-on-non-empty /var/lib/dhcp/
	fi
	rmdir --ignore-fail-on-non-empty /etc/dhcp
fi
# Automatically added by dh_apparmor
if [ "$1" = "purge" ] && ! [ -e "/etc/apparmor.d/sbin.dhclient" ] ; then
    rm -f "/etc/apparmor.d/disable/sbin.dhclient" || true
    rm -f "/etc/apparmor.d/force-complain/sbin.dhclient" || true
    rm -f "/etc/apparmor.d/local/sbin.dhclient" || true
    rmdir /etc/apparmor.d/local 2>/dev/null || true
fi
# End automatically added section
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