IPv6 Firewall on CPEs - Default on or off

Lorenzo Colitti lorenzo at google.com
Wed Nov 28 12:36:18 CET 2012

On Wed, Nov 28, 2012 at 1:43 AM, Benedikt Stockebrand <
me at benedikt-stockebrand.de> wrote:

> Using a default setting with a "diode configuration" (I don't really like
> to use the term "firewall" here) and an option that allows users to change
> that behaviour sounds like the safest option to me---for both the
> customer and the ISP.

But the one-way configuration is ham-fisted and stupid and ends up being
overkill in many cases.

Suppose you see a packet inbound to port tcp/445 (or 139, or whatever the
Windows ports are). How likely is that to be an attack? Probably fairly
likely. It seems reasonable to block it - after all, if the user is trying
to set up Windows file sharing across the Internet, they either know what
they're doing or it won't work.

On the other hand, say your CPE sees a UDP packet from port 61209 to port
48993, from a flow it hasn't seen before. Is it an attack? Or is there just
a missing state entry because it's part of a peer-to-peer protocol (e.g.,
video chat) - or if the router lost state due to a reboot, timeout, state
overflow, or anything else?

I would argue that that packet is extremely unlikely to be an unsolicited
attack. The chances of an attacker successfully mounting an attack like
that without any prior knowledge are basically zero - there are way better
avenues of attack than that. The packet is much more likely to be a video
chat, peer-to-peer or RTSP packet that the user actually wants. but the
one-way configuration blocks it anyway, because "zomg it comes from outside
drop it!!1".

If you will, it's a bit like pouring reinforced concrete on the floor of
your garage just in case someone steals your car by digging a tunnel into
the garage - sure, it prevents that particular mode of theft, but who's
going to do that, really?

Always remember that if the user is running a binary that listens on port
udp/48993, then you have already lost - because all that binary needs to do
is the extra, trivial step of setting up a rendezvous point (heck, even a
teredo tunnel) to allow unsolicited incoming traffic anyway. And what does
the firewall buy you? Nothing, really.

Well, perhaps not quite nothing. It does allows you to CYA. ("We give users
firewalls! Our connection is super safe! It's not our fault they get
hacked!!!"). That may be enough for many, and certainly in certain
countries (e.g., the USA), it's very important. But it's not really a good
reason, I think. We should be able to do better than that.
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