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Re: IPS vs. Firewalls (why vs. ?)
From: "Marcus J. Ranum" <mjr () ranum com>
Date: Fri, 03 Feb 2006 12:32:39 -0500

Gabriele Buratti wrote:
3) new technologies:
- reassemble the fragments in a separate space, do the checks, then if
ok send the fragments (no session rewriting).
- focus on the "strange things won't be forwarded", rather than
signatures: faster, sharp, you can use the marketing wizard's "0-day
protection" word :)
- decode recursively to stop blended attacks
- don't use a proxy: check on the fly and if test is passed then forward the packet (so no session rewrites and no 
dangerous listening ports)

I seem to remember a discussion similar to this back in 1992...  Of course
then it was proxy-versus-stateful firewalls. :)

What you're doing is your confusing features with implementation and
are talking about the performance results of implementation as if it
has something to do with the value of the features. The original
implementation of proxies, as separate processes atop an O/S
kernel, was because it was convenient to implement and the O/S
interprocess memory protection was a useful feature. If you extend
that line of thinking a bit, making each process use socket-level
abstractions leverages the O/S' IP stack effectively to do reassembly
and error correction. Again, those are useful attributes of an O/S
kernel and so we used them. But, if you think about it for a bit,
there's little difference (other than implementation details) if you had
a device that did full TCP state-tracking, packet sequencing,
IP checksumming,  etc -- most of the features of an IP stack -- in
silicon in the packet-shuffling loop of a switch. As long as it's
doing the _right_ error checking and doing it _correctly_ it's
irrelevant from a security standpoint whether it's implemented as
a separate process or in some state machine someplace else.
What matters is, and always will be, the error checking and
security processing that's done and how rigorous it happens to
be. What shouldn't come as a surprise to anyone is that the
amount of CPU power it takes to do IP processing in an IP
stack is about comparable to the amount of CPU power it
takes to do the same IP processing in a state engine in a
subroutine.

The way to get performance gains is, simply, to
cut corners. Do you want your IP processing to get 1/3
faster? Don't check the checksums! There are a number of
commercially significant firewalls from the old days that
did _exactly_ this and gained significant market-share by
being demonstrably faster than their competitors. And, you
know what? It worked fine because at that time the hackers
didn't have readily accessible packet-crafting tools and DOS
attacks weren't fashionable yet.

Now, we come to the meat of the matter - the security industry's
customers have demonstrated that they are always willing to
trade for the perception of performance over the perception of
security.(*) Of course, nobody really understands what checks,
internally, their product of choice makes. Indeed, most vendors
don't, either. They simply hope it will be the right combination
of checks to defuse the current and (hopefully!) the next attack.

So, what you're saying is that your product is doing a blend of
checks and is doing them as fast as it can. That's great!!! The
fundamental issue always boils down to whether your product
inherently implements default permit or default deny; what it
does with the stuff that it will, inevitably, encounter that is
outside of its knowledgebase. My concern with IPS is not the
implementation but the idea - the very premise of IPS is to
permit everything but try to shoot down that which is discovered
(on the fly, I may add) to be bad. Fundamentally, that's a stupid
idea. So, perhaps you have a good implementation of a stupid
idea; good for you. I've been saddened to see that nobody has
done to the design table and tried to make a fast implementation
of a good idea, instead.

mjr.
(* There are a few outliers - hi Dave!) 

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