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RE: Inappropriate TCP Resets Considered Harmful
From: Crispin Harris <Harris_C () DeMorgan com au>
Date: Thu, 17 May 2001 10:01:13 +1000

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-----Original Message-----
From: Ben Nagy [mailto:ben.nagy () marconi com au]
Sent: Tuesday, 15 May 2001 4:02 PM
Subject: RE: [fw-wiz] Inappropriate TCP Resets Considered Harmful

[Crispin]
One family of opinion states that the firewall should provide 
an absolute
minimum of information regarding its configuration and state.

[Ben] 
Being able to have your firewall fingerprinted is probably 
not optimal, but
not an overriding concern, IMO. Going too far down that path leads to
"Security by Obscurity" sophistry.

I don't believe that obscurity is a valid security concept, however
"information hiding" (obscuring) CAN be used to increase security.
(Otherwise pre-shared secrets wouldn't work...)

[Crispin]
From a security point of view, I believe that it is perfectly 
valid for a
firewall to deny or reject any traffic that is not 
_PRE-APPROVED_. i.e. if
the firewall receives ECN traffic, and the organisation has 
not said "We
want to allow ECN", then the firewall administrator would be 
negligent if
this traffic was not dropped.

[Ben]
I agree. This seems to be a common opinion among firewall 
people. 

And not just firewall people. Talk to any trained Security Officer, and they
will tell you that the Primary Tenet of Operational Security (in a security
sensitive situation) is "Prevent that which is not explicitly allowed".
[Deem the standard disclaimer about the inverse situation included]

That would tend to lead me to assume that the only reason that 
ECN works for such a large percentage of hosts is because many 
firewalls so not adequately enforce RFC compliance in the TCP 
stream, not because the administrators have taken a lenient 
security stance.

Some-Some. My experience would tend to say that for "Highly-Competant
Security Professionals", that is the case, however I have found that the
vast majority of Firewall Administrators are NOT Security Professionals....

[Crispin votes for TCP RST as a response to ECN-TCP packets]
(Mind you, the argument changes when talking non-TCP :-)

OK - what's your pick for non-TCP? 

The RST mechanism is only appropriate when dealing with TCP streams. My
basic argument doesn't change: "I believe that session/connection/packet
rejections should be handled in the same way regardless of whether it was a
'Deny by rule', 'Inappropriate Options', or 'Host/Service not available'.

UDP doesn't have a RST mechanism. 
        ICMP Port Unreachable
ICMP doesn't require a RST mechanism. 
        
IP other protocols
        <?help? Don't have an answer for this>

That's going to be relevant, as well, and variation in the 
handling of ECN for other IP protocols is almost certainly
going to lead to fingerprinting heaven.

What a wonderful thought. <wry grimace>
 
Regards,
        Crispin Harris
        DeMorgan Information Security Specialists

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