Re: CERT vulnerability note VU# 539363

[Daniel Hartmeier <daniel () benzedrine cx>]

On Wed, Oct 16, 2002 at 09:36:06AM -0400, Paul D. Robertson wrote:

> If you're hosting public resources behind the same firewall that's 
> protecting everything else in your enterprise, you've probably made a 
> questionable architectural decision.  If you're keeping state on say 
> inbound SMTP traffic, the question is "Why?"  If the 'Net as a whole can 
> connect to something, the state itself isn't going to do much good.  If 
> you're trying to rewrite sequence numbers because of a host that talks to 
> the public with high predeictability, again you're probably made a 
> questionable architectural decision.

Keeping state can have performance benefits. Depending on your rule set,
associating a packet with a state entry is cheaper than evaluating the
rules. Keeping state does not 'just' increase the quality of filter
decisions.

> Public-talking hosts should be protectable with simple non-stateful packet 
> filtering rules- *especially* those which allow the untrusted side to 
> initiate connections.

In my experience, allowing to specify a maximum for the number of states
created by a filter rule is very useful in this case (if you want to
keep state on all connections, and everything passes through the same
firewall). While an attacker can exhaust the individual maxima for
incoming connections to different services, other kinds of connections
(like outgoing connections, or connections the attacker can't establish)
are not affected.

Daniel
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