Great Circle Associates Firewalls
(June 1997)

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Subject: Re: Stateful Packet Filters vs. Proxies
From: Ryan Russell/SYBASE <Ryan . Russell @ sybase . com>
Date: 6 Jun 97 20:04:23 EDT
To: "Kelly E. Gibbs" <kgibbs @ best . com>
Cc: Ryan Russell/SYBASE <Ryan . Russell @ sybase . com>, Bill Stout <stoutb @ pios . com>, firewalls <firewalls @ GreatCircle . COM>

Well, the NAT I'm talking about specifically (IP
NAT products like the ones from Checkpoint
and Cisco, and probably others) work at layer 4.
They need to understand TCP and so-forth.
One could write one that works strictly at layer 3,
but for many IP protocols it wouldn't work very well,
and certainly wouldn't work for many-to-few NAT

Anything above layer 4 would probably be called 
an application gateway, or maybe a proxy :)

Layer 2 NAT implementations would probably be called
translational bridges.

I can't think of a layer 1 standard that includes anything one
could call an address...


---------- Previous Message ----------
To: Ryan.Russell
cc: stoutb, firewalls
From: kgibbs @ ("Kelly E. Gibbs") @ smtp
Date: 06/06/97 06:51:55 PM
Subject: Re: Stateful Packet Filters vs. Proxies

At what level does the NAT occur in the OSI model?  So far I've heard 2 and
4... whats the right answer?

On 6 Jun 1997, Ryan Russell/SYBASE wrote:

> Criticisms are welcome.
> I was thinking today that I should say more about
> application-specific proxies vs. generic proxies.
> Socks is a good example of a generic proxy (at
> least I believe that is the case) and is indeed 
> roughly equivalent to a packet filter, which is one
> of my points.  At least you agree on that.
> One of the things I'd like to have to better educate
> myself is a list of proxies that do a good job of 
> understanding and interacting with the protocols.
> But, I think my point still stands, that if you have
> more than a couple of protocols you have to 
> proxy, and you want to utilize non-generic proxies,
> then you will end up using unrelated proxy software
> for some of the protocols (Your examples of
> RealAudio and SQLNet are good ones) and they will
> have different feature sets, security models, and yes,
> bugs.  Translate bugs to mean potential holes or
> weaknesses.  
> The converse of that could be:  If you have n proxies
> that all understand and interact with the protocol
> in a meaningful way, and they all have 0 bugs,
> and they work exactly how you want, isn't that 
> better than a layer 4 SPF?  Yup.
> The above situation is a case where a SPF, as a
> unified security program, may be a better choice.
> It would be extreamly difficult to pick the cutoff
> point (i.e. if you have 5 proxies, just go to SPF) given
> that you probably lose some control over the protocols
> that you had before, but are now dealing with less holes, etc..
> BTW, doesn't the proxy for SQLNet exist because the
> protocol is complex, not to increase correctness of the
> data? I.e. a non-stateful packet filter would have to leave
> too many ports "open"?
> As for the last part of your note:  I'll go back and
> re-read how I worded my conclusions, but what I mean
> to say is that generic proxies are equivalent to
> stateful packet filters.  
> I also conclude that NAT is darn close to a generic proxy 
> and a SPF, if you are using many-to-few translation
> and the NAT device doesn't allow the outside to initiate 
> connections, which is probably a side-effect of most 
> implmentations of many-to-few NAT.
> Thanks for the feedback.
>      Ryan
> ---------- Previous Message ----------
> To: Ryan.Russell, firewalls
> cc: 
> From: stoutb @
 pios .
 com (Bill Stout) @ smtp
> Date: 06/06/97 11:15:43 AM
> Subject: Re: Stateful Packet Filters vs. Proxies
> Forgive my criticisms:
> The paper is founded on some incorrect assumptions.  
> It groups application specific proxies with generic proxies.  Generic or
> 'plug-gw' proxies are not desireable because they don't filter application
> commands, and are viewed as nearly as weak as packet filtering.  Application
> specific proxies are aware (to varying levels) of application commands.
> A proxy server typically comprises of application specific proxies, and does
> not comprise of only generic proxies.  Generic proxies are avoided at all
> costs, at least until management wants 'something added'.  Occasionally
> generic proxies are used as last resort, then replaced, for example
> RealAudio and SQLnet were initially filtered with plug-gw proxies until
> application (RealAudio/SQLnet) specific proxies were released.
> The paper then continues to compare generic proxy functions with packet 
> filters and concludes they are the same.  A discussion on NAT ensues which 
> is not an equivalent technology to either.  
> Bill Stout
> At 12:29 AM 6/6/97 -0400, Ryan Russell/SYBASE wrote:
> >Well, I finally got around to writing down my arguments
> >on the above subject.  Check it out at:
> >
> >
> >
> >Warning:  It's lengthy.
> >
> >Comments welcome.  
> >
> >    Ryan
> >
> _____________________________________________________________________________
> Bill Stout       (Systems Engineer/Consultant)         stoutb @
 pios .
> Pioneer Standard (Computer Systems & Components)
> San Jose, CA     (Location of 1 of 52 U.S. offices)    (408) 954-9100
> *My opinions do not reflect that of the company, and visa-versa, thankfully.*

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