On Mon, Apr 01, 2013 at 12:31:05PM -0400, Jay Ashworth wrote:
From: "Jimmy Hess" <mysidia@gmail.com>
Ah, but did you actually test your guess on a reasonably large variety of NAT platforms?
He may not have, but now that I'm thinking (caffeine is a wonder drug), I have: I've worked on, for customers, nearly every brand of consumer router on the market, and all of them do outbound NAT the same way:
Pick up a DHCP address from the carrier, and use that as the source IP on all translated outbound packets.
The key issue at hand (I believe; please correct me if I'm wrong) is that "all translated outbound packets" may not equal "all outbound packets". Depending on how a NAT device is configured, it may pass some packets *untranslated*, and that allows a malicious actor behind the NAT device to still get their spoofed traffic out. To relate it back to something concrete, imagine this iptables rule in an otherwise "fully open" configuration: iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -o wan -j MASQUERADE Now, spoof a packet from behind this NAT box as coming from 192.0.2.12... what happens? It gets passed through the NAT box, *without* being NATed. Oops. Of course, it isn't hard to stop this sort of thing... iptables -A INPUT ! -s 192.168.1.0/24 -i lan -j DROP (or any number of other equivalent rules) The question is, how many in-common-use CPEs have considered this attack vector and are actually doing something functionally equivalent to the DROP rule above? I don't know, because I haven't tried it, but I'd only be surprised if the answer was "none" or "all of them". - Matt