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/etc/dhcp/dhclient-exit-hooks.d/rfc3442-classless-routes is in isc-dhcp-client 4.3.5-3+deb9u1.

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The actual contents of the file can be viewed below.

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# set classless routes based on the format specified in RFC3442
# e.g.:
#   new_rfc3442_classless_static_routes='24 192 168 10 192 168 1 1 8 10 10 17 66 41'
# specifies the routes:
#   192.168.10.0/24 via 192.168.1.1
#   10.0.0.0/8 via 10.10.17.66.41

RUN="yes"


if [ "$RUN" = "yes" ]; then
	if [ -n "$new_rfc3442_classless_static_routes" ]; then
		if [ "$reason" = "BOUND" ] || [ "$reason" = "REBOOT" ]; then

			set -- $new_rfc3442_classless_static_routes

			while [ $# -gt 0 ]; do
				net_length=$1
				via_arg=''

				case $net_length in
					32|31|30|29|28|27|26|25)
						if [ $# -lt 9 ]; then
							return 1
						fi
						net_address="${2}.${3}.${4}.${5}"
						gateway="${6}.${7}.${8}.${9}"
						shift 9
						;;
					24|23|22|21|20|19|18|17)
						if [ $# -lt 8 ]; then
							return 1
						fi
						net_address="${2}.${3}.${4}.0"
						gateway="${5}.${6}.${7}.${8}"
						shift 8
						;;
					16|15|14|13|12|11|10|9)
						if [ $# -lt 7 ]; then
							return 1
						fi
						net_address="${2}.${3}.0.0"
						gateway="${4}.${5}.${6}.${7}"
						shift 7
						;;
					8|7|6|5|4|3|2|1)
						if [ $# -lt 6 ]; then
							return 1
						fi
						net_address="${2}.0.0.0"
						gateway="${3}.${4}.${5}.${6}"
						shift 6
						;;
					0)	# default route
						if [ $# -lt 5 ]; then
							return 1
						fi
						net_address="0.0.0.0"
						gateway="${2}.${3}.${4}.${5}"
						shift 5
						;;
					*)	# error
						return 1
						;;
				esac

				# take care of link-local routes
				if [ "${gateway}" != '0.0.0.0' ]; then
					via_arg="via ${gateway}"
				fi

				# set route (ip detects host routes automatically)
				ip -4 route add "${net_address}/${net_length}" \
					${via_arg} dev "${interface}" >/dev/null 2>&1
			done
		fi
	fi
fi