On Wed, 12 Jan 2011 12:04:01 EST, William Herrin said:
In a client (rather than server) scenario, the picture is different. Depending on the specific "NAT" technology in use, the firewall may be incapable of selecting a target for unsolicited communications inbound from the public Internet. In fact, it may be theoretically impossible for it to do so. In those scenarios, the presence of NAT in the equation makes a large class of direct attacks on the interior host impractical, requiring the attacker to fall back on other methods like attempting to breach the firewall itself or indirectly polluting the responses to communication initiated by the internal host.
Note that the presence of a firewall with a 'default deny' rule for inbound packets provides the same level of impracticality. And given the fact that Windows has had a reasonably sane host-based firewall since XP SP2, and the truly huge number of compromised PC's that sit behind a NAT on a DSL or cablemodem, it's pretty obvious that the presence of NAT is doing approximately *zero* to actually slow down the miscreants. 140 million compromised PC's, most of them behind a NAT, can't be wrong. :)