firewall: add rule for traceroute support

Running your firewall's "wan" zone in REJECT zone (1) exposes the
presence of the router, (2) depending on the sophistication of
fingerprinting tools might identify the OS and release running on
the firewall which then identifies known vulnerabilities with it
and (3) perhaps most importantly of all, your firewall can be
used in a DDoS reflection attack with spoofed traffic generating
ICMP Unreachables or TCP RST's to overwhelm a victim or saturate
his link.

This rule, when enabled, allows traceroute to work even when the
default input policy of the firewall for the wan zone has been
set to DROP.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

package/network/config/firewall/files/firewall.config

@@ -129,6 +129,19 @@ config rule
 	option proto		udp
 	option target		ACCEPT
 
+# allow interoperability with traceroute classic
+# note that traceroute uses a fixed port range, and depends on getting
+# back ICMP Unreachables.  if we're operating in DROP mode, it won't
+# work so we explicitly REJECT packets on these ports.
+config rule
+	option name		Support-UDP-Traceroute
+	option src		wan
+	option dest_port	33434:33689
+	option proto		udp
+	option family		ipv4
+	option target		REJECT
+	option enabled		false
+
 # include a file with users custom iptables rules
 config include
 	option path /etc/firewall.user