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The Natural Philosopher writes:
> On 01/01/2026 11:34, Richard Kettlewell wrote:
>> The Natural Philosopher writes:
>>> On 31/12/2025 20:18, Richard Kettlewell wrote:
>>>> NAT does not offer any protection. The reason that a typical domestic
>>>> NAT-equipped router protects you from inbound connections is that it
>>>> has a firewall as well. (Getting a packet addressed to your internal
>>>> addresses to your external interface is inconvenient for many
>>>> attackers, for sure, but straightforward for your ISP or anyone who
>>>> can hack or coerce them.)
>>>
>>> How?
>>> Genuine question.
>> Same as routing any other packet. Make sure there?s an appropriate
>> routing table entry for the customer addresses on the ISP?s
>> customer-facing router (and whatever intermediate routers there are
>> between that and the attack source), then call socket/connect/write.
>> The question is then what the customer router does with it.
>> * If it follows the strong end system then the packet is discarded
>> before NAT even comes into the question.
>> Linux follows the weak end system model by default, so this
>> possibility doesn?t apply to Linux-based router unless someone has
>> taken the trouble to change its behavior somehow.
>> * If there?s a basically competent firewall on the customer router
>> then
>> the packet is discard by that.
>> * If there?s a NAT then it gets to look at the packet, but it won?t
>> match any of the rules that enable translation, so it will not be
>> modified at this stage.
>
> Ah. I misunderstood your original post. Sure the ISP can send an
> internally addressed packet to your router. If it wants to lose its
> customer base.
* The police turn up with a warrant and inform you that if you don?t
help them break into a certain customer?s network, you will go to
prison.
* The mafia turn up with a gun, and inform you that if you don?t help
them break into a certain customer?s network, you will be shot.
* A teenager who follows full-disclosure exploits the latest zero day to
break into the ISP?s network, and from there to as many customers as
they can reach.
* An ISP staff member suspects that a friend who happens to be a
customer is having an affair with their wife. Overwhelmed by jealousy
they decide to attempt to hack the customer?s computers.
You can probably imagine more scenarios.
> But will that get forwarded along?
>
>> * All that?s now left is normal routing, so the packet passes on to its
>> destination on the customer network.
>> https://www.greenend.org.uk/rjk/tech/nat.html has a worked example.
>>
> I can't believe that my router would accept arbitrary packets with
> an internal destination address on its external facing port...
It probbaly doesn?t: it might use the strong end system model, and if
not it might have a firewall preventing it. But whatever its
configuration, it?s not NAT that stops it.
> if the destination is not in its tables, it will be automatically
> discarded...
The internal network destination address _is_ in its routing table.
That?s how it sends legitimate packets back to the internal network
(e.g. when an internal host pings the router).
On my router:
$ ip route show|grep 172.17.207.0
172.17.207.0/24 dev br0 proto kernel scope link src 172.17.207.1
(172.17.207.0 is my internal network.)
--
https://www.greenend.org.uk/rjk/
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