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by blibble 1057 days ago
it already has stateful firewall

so that's: source ip, dest ip, protocol, source port, dest port, connection state (say 16 bytes total)

doing NAT too is what, 3 more bytes per connection (8 bits for an offset into an IP table and 16 bits for the translated port)
2 comments
dijit 1057 days ago
NAT and Stateful firewalling are commonly bundled together (especially on home systems) but I would not go so far as to say “NAT has a stateful firewall”-

I hear such takes all the time and its really frustrating; usually in threads regarding IPv6, incidentally it is usually programmers who think they understand everything about networks because they know how tcp operates.

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blibble 1057 days ago
> but I would not go so far as to say “NAT has a stateful firewall”-

> I hear such takes all the time and its really frustrating

maybe you'd be less frustrated if you understood what people were saying, because I didn't say that

AWS already do 1:1 NAT and there's additionally a stateful firewall, which necessitates connection state tracking

adding the extra few bytes to do port translation shouldn't vastly increase the memory required

> incidentally it is usually programmers who think they understand everything about networks because they know how tcp operates.

from someone who has written a commercial packet filter: in terms of complexity, TCP blows the preceding layers of the stack out of the water

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lxgr 1057 days ago
In almost all NAT implementations, public-side ports are dynamically assigned, which implies that inbound connections aren't possible (unless port forwarding is explicitly configured).

Is that really conceptually so different from a stateful firewall allowing inbound packets only for established connections/flows?

"NATs are good because otherwise people wouldn't have any firewalls" is a tired take, yes, but I don't see the point being needlessly pedantic about the semantics of NAT vs. stateful firewalls when in this case, the effect is the same: No inbound packets without prior outbound packets (or a connection establishment for TCP).

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p1mrx 1057 days ago
Generally an ISP does not have a stateful firewall prior to deploying CGNAT.
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