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RE: Inappropriate TCP Resets Considered Harmful
From: Ben Nagy <ben.nagy () marconi com au>
Date: Tue, 15 May 2001 16:01:36 +1000

<an asside>
I have been following this thread with great interest, as 
this cuts to the
core of a common scheme of firewall configurations.

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

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.

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.

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

-----Original Message-----
From: Ben Nagy [mailto:ben.nagy () marconi com au]
Sent: Monday, 14 May 2001 9:43 AM
To: 'Darren Reed'

Why is a retry bad? If I were writing firewall (heaven 
forbid!) I'd treat
ECN packets either by silently discarding them or by sending 
an ICMP error.

This is one area in which I disagree. One network scanning 
option is to send
a packet with the high-bit tcp flags set.

You mean IP Options, right?

How can I tell if 
this packet is
ECN or scanning?

You can't. 

Oh wait, that question was rhetorical, right? 8)


[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? 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.

      Crispin Harris
      DeMorgan Information Security Specialists


Ben Nagy
Network Security Specialist
Marconi Services Australia Pty Ltd
Mb: +61 414 411 520  PGP Key ID: 0x1A86E30
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