/etc/dhcp/dhclient-enter-hooks.d/debug is in isc-dhcp-client 4.2.4-7ubuntu12.
This file is owned by root:root, with mode 0o644.
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 34 35 36 37 38  | #
# The purpose of this script is just to show the variables that are
# available to all the scripts in this directory. All these scripts
# are called from /etc/dhcp3/dhclient-script, which exports all the
# variables shown before. If you want to debug a problem with your DHCP
# setup you can enable this script and take a look at
# /tmp/dhclient-script.debug.
# To enable this script set the following variable to "yes"
RUN="no"
if [ "$RUN" = "yes" ]; then
	echo $(date): entering ${0%/*}, dumping variables. \
		>> /tmp/dhclient-script.debug
	# loop over the 4 possible prefixes: (empty), cur_, new_, old_
	for prefix in '' 'cur_' 'new_' 'old_'; do
		# loop over the DHCP variables passed to dhclient-script
		for basevar in reason interface medium alias_ip_address \
			       ip_address host_name network_number subnet_mask \
			       broadcast_address routers static_routes \
			       rfc3442_classless_static_routes \
			       domain_name domain_search domain_name_servers \
			       netbios_name_servers netbios_scope \
			       ntp_servers \
			       ip6_address ip6_prefix ip6_prefixlen \
			       dhcp6_domain_search dhcp6_name_servers ; do
			var="${prefix}${basevar}"
			# show only variables with values set
			if [ -n "${!var}" ]; then
				echo "$var='${!var}'" >> /tmp/dhclient-script.debug
			fi
		done
	done
	echo '--------------------------' >> /tmp/dhclient-script.debug
fi
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