Comment by kortilla
19 days ago
That makes no sense. If a packet comes into a public IP to a session that doesn’t exist, there is nowhere to forward the packet onto. The public IP belongs to the router.
If the packet was going to a private RFC 1918 address, there wouldn’t be a way to get it to the router in the first place from the internet.
There's always somewhere to forward a packet to. The router looks at the dest IP field in the packet header, and that's where it goes.
> If the packet was going to a private RFC 1918 address, there wouldn’t be a way to get it to the router in the first place from the internet.
This is generally going to be true, but it's not relevant to how NAT behaves when it receives inbound connections.
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.
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