From: Mark Wooding <mdw@distorted.org.uk>
Date: Tue, 31 May 2011 20:48:01 +0000 (+0100)
Subject: classify.m4: Dislike multicast addresses as a source address.
X-Git-Url: https://git.distorted.org.uk/~mdw/firewall/commitdiff_plain/d52144713a3bd098bb1ef050277d8d502aaf06a8

classify.m4: Dislike multicast addresses as a source address.

I think they're not permitted.  They're certainly hard to deal with if
they are.
---

diff --git a/classify.m4 b/classify.m4
index c7e8d56..fcb008d 100644
--- a/classify.m4
+++ b/classify.m4
@@ -78,7 +78,9 @@ run iptables -t mangle -A in-classify -j RETURN \
 ## Since packets with source and destination addresses both local will go
 ## over the loopback interface, I shouldn't see a packet from me over any
 ## other interface.  Except that I will if I sent a broadcast or multicast.
-## Allow the broadcasts, and remember not to trust them.
+## Allow the broadcasts, and remember not to trust them.  There are no
+## broadcast addresses in IPv6 (only link-local multicast)m so we don't have
+## to worry about that.
 run iptables -t mangle -A local-source -j RETURN \
 	-m addrtype --dst-type BROADCAST
 run iptables -t mangle -A local-source -j RETURN \
@@ -93,6 +95,15 @@ for addr in $host_6addrs; do
 	  -s $addr
 done
 
+## It's not valid to have a multicast address as a packet source: multicast
+## routing is done away from the source, so a multicast address would make
+## this impossible to do.  So discard these packets.  Also discard class-E
+## IPv4 addresses, since they aren't assigned.
+run iptables -t mangle -A in-classify -g bad-source-address \
+	-s 224.0.0.0/3
+run ip6tables -t mangle -A in-classify -g bad-source-address \
+	-s ff00::/8
+
 m4_divert(41)m4_dnl
 ## Define the important networks.
 for pass in 1 2; do