[globular-toast]

Yeah, I keep meaning to write something about this. I've definitely noticed people wary of IPv6 because their machines get "real" IP addresses rather than the "safe" RFC1918 ones. Of course, having a real IP address is precisely the point of IPv6.

It's like we've been collectively trained to think of RFC1918 as "safe" and forgotten what a firewall is. It's one of those "a little knowledge is a dangerous thing" things.

[sshine]

In a world where people think NAT addresses are safe because you don’t need to know anything else about firewalls, IPv6 _is_ fundamentally less secure.

[throw0101a]

> In a world where people think NAT addresses are safe because […]

The vast, vast majority of people do not know what NAT is: ask your mom, aunt, uncle, grandma, cousin(s), etc. They simply have a 'magic box' (often from the ISP) that "connects to Internet". People connect to it (now mostly via Wifi) and they are "on the Internet".

They do not know about IPv4 or IPv6 (or ARP, or DHCP, or SLAAC).

As long as the magic box is statefully inspecting traffic, which is done for IPv4-NAT, and for IPv6 firewalls, it makes no practical difference which address family you are using from a security perspective.

The rending of garments over having a globally routable IPv6 address (but not globally reachable, because of SPI) on your home is just silliness.

If you think NAT addresses are safe because… of any reason whatsoever really… simply shows a lack of network understanding. You might as well be talking to a Flat Earther about orbital mechanics.

[mqus]

> which is done for IPv4-NAT, and for IPv6 firewalls

Are internet routers that do ipv4 NAT usually also doing an IPv6 firewall (meaning they only let incoming connections in if they are explicitly allowed by some configuration)? Maybe thats the point where the insecurity comes from. A Home NAT cannot work any other way(it fails "safely"), a firewall being absent usually means everything just gets through.

[globular-toast]

All the ones I've had have had a firewall by default for IPv4 and IPv6, yes. If ISPs are shipping stuff without a firewall by default I'd consider that incompetence given people don't understand this stuff and shitty IoT devices exist.

I do wonder how real the problem is, though. How are people going to discover a random IPv6 device on the internet? Even if you knew some /64 is residential it's still impractical to scan and find anything there (18 quintillion possible addresses). If you scanned an address per millisecond it would take 10^8 years, or about 1/8 the age of the earth, to scan a /64.

Are we just not able to think in such big numbers?

[throw0101a]

> Are internet routers that do ipv4 NAT usually also doing an IPv6 firewall (meaning they only let incoming connections in if they are explicitly allowed by some configuration)?

Consider the counter-factual: can you list any home routers/CPEs that do not do SPI, regardless of protocol? If someone found such a thing, IMHO there would be a CVE issued quite quickly for it.

And not just residential stuff: $WORK upgraded firewalls earlier in 2025, and in the rules table of the device(s) there is an entry at the bottom that says "Implicit deny all" (for all protocols).

So my question to NAT/IPv6 Truthers is: what are the devices that allow IPv6 connections without SPI?

And even if such a thing exists, a single IPv6 /64 subnet is as large as four billion (2^32) IPv4 Internets (2^32 addresses): good luck trying to find a host to hit in that space (RFC 7721).

[thayne]

There is one practical difference. IPv6 without a NAT exposes information about different devices inside the private network. A NAT (whether ipv4 or ipv6) will obfuscate how many devices are on the network. Whether that is desirable depends on the circumstances.

[throw0101a]

> A NAT (whether ipv4 or ipv6) will obfuscate how many devices are on the network. Whether that is desirable depends on the circumstances.

"Revisiting IoT Fingerprinting behind a NAT":

* https://par.nsf.gov/servlets/purl/10332218

"Study on OS Fingerprinting and NAT/Tethering based on DNS Log Analysis":

* https://www.irtf.org/raim-2015-papers/raim-2015-paper21.pdf

Also:

> […] In this paper, we design an efficient and scalable system via spatial-temporal traffic fingerprinting from an ISP’s perspective in consideration of practical issues like learning- testing asymmetry. Our system can accurately identify typical IoT devices in a network, with the additional capability of identifying what devices are hidden behind NAT and the number of each type of device that share the same IP address. […]

* https://www.thucloud.com/zhenhua/papers/TON'22%20Hidden_IoT....

Thinking you're hiding things because you're behind a NAT is security theatre.

[labcomputer]

> IPv6 without a NAT exposes information about different devices inside the private network.

In practice this has not been true for over 20 years.

IPv6 devices on SLAAC networks (which is to say, almost all of them) regularly rotate their IPv6 address. The protocol also explicitly encourages (actually, requires) hosts to have more than one IPv6 address active at any given time.

You are also making a wrong assumption that the externally visible address and port ranges chosen by the NAT device do not make the identity of internal devices easily guessable.

[zamadatix]

In both cases the only consumer security comes from "the home router defaults to being a stateful firewall". The only difference between the two is whether it also defaults to doing NAT with that state, which is not what was making IPv4 secure for people unaware either.