full-disclosure - Stateful Packet Inspection

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From: shash at etisalat-nis.ae (Shashank Rai)
Subject: Stateful Packet Inspection

On Sun, 2004-08-01 at 20:19, Goetz Von Berlichingen wrote:
>    The original message has some merit with respect to netfilter - the 
> Linux kernel firewall is capable of looking at headers only.  This does 
> allow some stateful packet inspection - one can discriminate against 
> incoming connection attempts with --syn, for instance.  This isn't 
> really stateful, however, since the firewall does not retain any 
> knowledge of the state of a connection.  
Not exactly correct... netfilter uses "connection tracking" helper
module to keep a track of connections. "--established" can be used to
define rules for the same. As Rusty says on the netfilter.org website
"netfilter, iptables and the connection tracking as well as the NAT
subsystem together build the whole framework."

> iptables is pretty much useless 
> agains covert channels such as Loki, Q, or any of the various tunneling 
> packages.

A "packet filter" is not supposed to look into this kind of stuff.
Covert channels communication using DNS, ICMP or whatever can even pass
through Cisco PIX (another packet level filter) too. u'll need one of
the "deep packet inspection" (real fancy name) firewalls ;)

>    The problem with stateful inspection is that it so easily leads to 
> self-denial of service.  An attacker need only make enough legitimate 
> connections to overflow the firewall's capability.  At that point, the 
> firewall either crashes or quits stateful inspection. 
Depends on which firewall you are using and it's features. Cisco PIX and
Checkpoint can be configured to start dropping connections after a
certain number of "established" or half open connections have been
reached and same with netfilter. None of these firewalls crash or "quit
statefull inspection" upon reaching these limits. And i wouldn't call it
self DoS. The idea of is to protect the resource behind the firewall....
and eventually everything has a limit and a cap. A bigger fish can
always eat a smaller one. 

> Goetz


-- 
shashank

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