dhcrelay4: add support for up/downstream interfaces

dhcrelay has the ability to monitor interfaces for requests
in a single direction only rather than listening to all
interfaces for requests.

Doing this allows one to suppress the duplication of having
the relay forward requests from the same network that the
DHCP server is on.

Signed-off-by: Brian J. Murrell <brian@interlinx.bc.ca>

net/isc-dhcp/files/dhcrelay4.init

@@ -25,6 +25,18 @@ start() {
 			append args "-i $ifname"
 		fi
 	done
+	config_get interfaces ipv4 upstream_interfaces
+	for net in $interfaces; do
+		if network_get_device ifname "$net"; then
+			append args "-iu $ifname"
+		fi
+	done
+	config_get interfaces ipv4 downstream_interfaces
+	for net in $interfaces; do
+		if network_get_device ifname "$net"; then
+			append args "-id $ifname"
+		fi
+	done
 
 	# link selection sub-option (RFC3527)
 	local link_selection