[Dagger2]

And when people talk about "PAT", they're actually talking about a form of NAT that doesn't block connections.

Here's how you do "PAT" on Linux: `iptables -t nat -A POSTROUTING -o wan0 -j MASQUERADE`. Notice how it's limited to outbound connections ("-o wan0")? That means it doesn't apply to inbound connections, and thus doesn't have any effect on the behavior of inbound connections.

If it doesn't have any effect on the behavior of inbound connections, then how could it possibly block inbound connections?

(The typical consumer configuration pairs "PAT" with a firewall, and the firewall does block inbound connections. It's also typical to pair it with RFC1918 addresses, which doesn't block connections but does make it much harder for most people to make the relevant connections in the first place. None of that changes the fact that "PAT" doesn't block connections.)

[icedchai]

So in other words, even without a firewall, it still provides some level of security. If your attacker can't route to your target's addresses because they are private RFC1918, they're "blocked" for all practical purposes. Yes, I know they're not technically blocked... but the typical attacker 10 hops away isn't going to know...

[zAy0LfpBZLC8mAC]

On the other hand, this might also give other people a false sense of security. Most people who tell you that "NAT provides security" think that NAT somehow drops packets, and if your network is actually targeted, this myth might well be the reason why someone ends up downloading all your files by connecting to your file server through your NAT gateway.

[Dagger2]

No... it doesn't drop connections, so it doesn't provide any security.

[icedchai]

In practical terms, it still provides some (low) level of security. If an attacker can't get IP packets to your machine because it's on an un-routable address, they can't attack it. If your attacker is getting "cooperation" from your ISP to route to it, you have bigger things to worry about it.

Obviously you should really use a firewall...

[Dagger2]

It won't prevent an attacker from getting IP packets to your machine. How could it do that, when it only acts on outbound connections and its only act is to change the apparent source address of those connections?

[icedchai]

Did you miss "because it's on an un-routable address" part? If there's no route to your machine from an attacker, they can't attack you.

[Dagger2]

The discussion was about the behavior of PAT, and PAT has no influence on whether or not an attacker has a route to you.