alt Hacker NewsBut... it doesn't do that. If incoming traffic isn't part of an established connection, NAT will just ignore it. It doesn't deny that traffic, it just lets it pass through to the router without translating the addresses in it.
The router will then do exactly the same thing it would've done if no NAT was involved at all: if the dest IP in the packet is the router itself then the router will accept or refuse the connection depending on whether anything is listening on the respective port, and if the dest IP is on the LAN then it will route it onto the LAN.
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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.
It depends on how you've configured the router. It's quite common to reject or drop ingress traffic received on an egress interface destined to a NATed network address. In fact, I would flag any configuration that didn't have that.