Arguing against using public IP space

Owen DeLong owen at
Wed Nov 16 17:04:28 CST 2011

On Nov 16, 2011, at 10:58 AM, Jay Ashworth wrote:

> ----- Original Message -----
>> From: "Owen DeLong" <owen at>
>> In this case, a router with NAT is slightly more likely to fail closed than
>> a router without NAT.
> "Slightly"?  Continuing to assume here, as we have been, that the network
> behind a NAT is *unroutable*, then a NAT router has, IME, *many* more obvious
> possible failure modes which will make the internal network inaccessible from
> outside than modes which cause the opposite.

What matters is not the number of failure modes, but, the probability of
occurrence. Additionally, I have no reason to assume as you have been
that private addresses behind the NAT are for some reason "unroutable".
I would argue that this (sometimes) fallacious assumption may be one
of the key dependencies in your argument.

> If you're an attacker, targeting a behind-NAT box from the outside, then
> if the NAT's working, you can hit directly any ports that are forwarded to it.


> If not, then you have to a) know the private IP of the box and b) be able to
> get packets to the last upstream hop with source routing on them and c) the
> box has to have failed (or been configured or built) in such a way as to 
> *listen* to source-routing.  Those layers may have varying thicknesses, but 
> there *are* at least 3 more of them, *on top of* "did it fail in a way where
> it's listening at all?".


a) I have to either know, detect, or guess at the private IP of the box (maybe).
b) I have to get packets to the last upstream hop. Source routing is just one tool
that could be used to do so. There are other possibilities.
c) The box doesn't have to listen to source routing. It has to accept packets
destined to it's exterior MAC address with an interior IP address (or other
address that causes it to meaningfully forward things inside).

>>                       However, a firewall without NAT is more likely
>> to fail closed than a router with or without NAT and equally likely to
>> a firewall with NAT.
> If it's a firewall that meets your definition of the word, as opposed to,
> say, a shorewall box, a smoothwall box, a pf box, or any of the other 3 or
> 4 dozen packaged linux based firewall routers of which there are *lots* out
> there.  Probably the most common failure more on those is "iptables accidentally
> cleared; box routes all packets".
Actually, if you configure iptables on those boxes correctly and turn off
ip forwarding (requiring instead that iptables actually do the forwarding,
which is possible), clearing iptables does not cause it to forward all

> That's one failure to get to that point, insted of 2, 3 or 4.  And since it's
> human-based a lot of the time, it's probably even more likely.
I would argue it's at least two. First you had to configure iptables and/or
the OS incorrectly in order for clearing iptables to cause that problem.
Then you had to clear iptables.

>>                     In other words, NAT doesn't really improve anything,
>> but, the difference between the common failure modes of a firewall
>> vs. a router are worthy of consideration. The infinitesimal advantage
>> of NAT if you use a router instead of a firewall to perform the duties
>> of a firewall is dramatically overshadowed by the costs and damage
>> done by NAT.
> Costs already sunk, IME.  Damage is a question-begging term here.

For IPv4, that argument can perhaps be made. Since the question was
originally about the desirability of bringing NAT forward into IPv6, I don't
think that argument actually holds water.

>> OTOH, routers, being designed primarily to forward packets and having
>> security appliance features added as a secondary capability will, in
>> many cases, address most of these failures by passing packets which
>> would not be permitted if properly configured and/or functioning.
> Yup.  What I've been saying (or implying) right along.  So, in networks,
> or in seats, take your pick, does anyone have any deployment numbers on 
> router-based firewalls vs the other sort, whatever we're calling them?

I have no idea. I think based on my observations at a variety of companies
it is a relatively even split, but, I will point out that the companies that pay
any attention to security at all do, by and large, use firewalls (my definition)
rather than attempt to press routers into that role. In fact, many use both
a router with ACLs and/or stateful inspection at the ISP handoff in addition
to a firewall behind it before you get to anything not intended to be on the
public internet.

>> Yes, they are identical and NAT makes no meaningful difference
>> to the chances that undesired packets will be forwarded in the event
>> of a catastrophic failure outside of these more common failure modes.
> I guess we're going to have to agree to disagree here; our respective 
> clients will decide what their opinions on that are.

I suspect so.


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