Comment by kortilla | Hacker News Reader

Comment by kortilla | Hacker News Reader

Parent Full thread

I’m not sure you understand how NAT implementations work.

I recommend you setup a basic iptables NAT set of rules on linux to reflect what a home router does (masquerade) with ip forwarding enabled.

The inbound packets from the internet are to a public IP that belongs to the device performing NAT. If there is no entry in the conntrack table from a connection to translate that public IP to a private IP, there is nowhere it can be forwarded to. It’s already at the device that owns that destination IP.

Even if it spit the packet out a private interface (which makes no sense given the routing table), the destination IP would be a public IP that both cannot be ARPed for on the local network and will not be honored by any private device.

I'm reasonably sure I do. I've done that exact test literally half a dozen times now due to people telling me that I'm wrong, and each time I test it turns out that I'm right. What other conclusion am I supposed to come to?

I think it's someone else's turn to do the test now.

> The inbound packets from the internet are to a public IP that belongs to the device performing NAT.

If the inbound packets are addressed to the router, they get delivered to the router. But you cannot just declare that that's where they'll be addressed. You aren't in control of what packets show up to your router, you're only in control of how you process them after they show up.

you missed the part about an ACL. You’re whole shtick depends on a bad implementation of routing rules.

What you’re describing would happen if NAT were completely disabled. You’re just describing an open router

There's no inherent ACL in NAT, and adding one would just demonstrate that ACLs can block packets, which we already knew.

> What you’re describing would happen if NAT were completely disabled. You’re just describing an open router

Yep. It also happens when NAT is enabled. A router doing NAT is exactly the same thing as an open router -- it just has the additional property of editing outbound connections to appear to come from the IP of the router itself.

If NAT on its own blocked inbound connections, I would have seen that in my tests.

Reply on news.ycombinator.com