First they will have to change their policy of only providing one IPv4 address per ONT connection. Then they will have to convince me to disable NAT on my router, disable the DHCP server on my router, and bridge the WAN port with the LAN block.
Meanwhile in IPv6 land the ISP provided router that my relative has came configured by default to hand out globally routable addresses from the ISP provided /64. Thankfully it also had a stateful firewall enabled by default so there was no difference in practice.
If your public IP from your ISP is 12.13.14.15, and your internal block is 192.168.0.0/24, then your ISP can send a packet to 12.13.14.15 destined for 192.168.0.7, and without a firewall your router will happily forward it. An attacker who can convince intervening routers to send traffic destined for 192.168.0.7 to 12.13.14.15 (and these attacks do exist, particularly over UDP) can also do that.
Okay, so not only do you have to create a bogus packet, you have to convince every piece of equipment in between you and the end user to collude with it, in the hopes that the final router is so woefully misconfigured as to act upon it?
Send packets to the device? A NAT is in it's most basic form a mapping from one IP/port set to another IP/port set describable by some function "f" and its inverse "g". The common home user case has the firewall detect a flow from inside the network and modify "f" and "g" to allow this flow. Without the firewall, and assuming you want your devices to talk to the internet in some way, the NAT would forward (with modifications) traffic based on "f" and "g" to all your devices.
First they will have to change their policy of only providing one IPv4 address per ONT connection. Then they will have to convince me to disable NAT on my router, disable the DHCP server on my router, and bridge the WAN port with the LAN block.
Meanwhile in IPv6 land the ISP provided router that my relative has came configured by default to hand out globally routable addresses from the ISP provided /64. Thankfully it also had a stateful firewall enabled by default so there was no difference in practice.
How do they manage that?
If your public IP from your ISP is 12.13.14.15, and your internal block is 192.168.0.0/24, then your ISP can send a packet to 12.13.14.15 destined for 192.168.0.7, and without a firewall your router will happily forward it. An attacker who can convince intervening routers to send traffic destined for 192.168.0.7 to 12.13.14.15 (and these attacks do exist, particularly over UDP) can also do that.
Okay, so not only do you have to create a bogus packet, you have to convince every piece of equipment in between you and the end user to collude with it, in the hopes that the final router is so woefully misconfigured as to act upon it?
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Send packets to the device? A NAT is in it's most basic form a mapping from one IP/port set to another IP/port set describable by some function "f" and its inverse "g". The common home user case has the firewall detect a flow from inside the network and modify "f" and "g" to allow this flow. Without the firewall, and assuming you want your devices to talk to the internet in some way, the NAT would forward (with modifications) traffic based on "f" and "g" to all your devices.