> With IPv6 the entire network is reachable outside by default.

The entire network might be routable, but it often isn't reachable. My router had a default deny rule, so everything in my network for sure wasn't reachable by default despite having IPv6 addressing.

If anything, I like firewalling in IPv6 far better than dealing with NATs. Just imagine having multiple boxes you'd like to reach by SSH or HTTPS from the outside. With NAT, you can only run one on a standard port. With IPv6, there's no need to NAT, everything can just use one of their many public IPv6 addresses, and then I can firewall to allow traffic to each of those boxes at the standard ports.

In fact, this gets even cooler. I can then have multiple services all bound to different IP addresses and have different firewall rules related to each of those services. There's so much more possible using IPv6 that you just practically can't do in IPv4, unless you just happened to have a /8 assigned to you back in the day.

Think about this: every device in my home network gets more IP addresses assigned to it than there are IP addresses in IPv4. I can have every container on my cluster have its own publicly routable IPv6 address, every application I run could theoretically have its own address and have its own network rules applied. And then I can look at my network edge and immediately identify any and all traffic flowing through that edge.

I can't wait until IPv4 is dead and I never have to deal with NAT issues again.

No.... Absolutely no...

NAT is absolutely not in any way a substitute for an actual firewall, despite the side effect of 'blocking' ports.

And how is "You have to think about which ports you want the NAT gateway to forward." any different from thinking about firewall rules?

And most consumer CPE devices (i.e. 'router' etc) are perfectly capable of running a firewall, and often do.

And any firewall that doesn't drop inbound traffic by default is not really much use at all.

And lastly, if you want you can still do NAT66 if you really must, or IPv6 network prefix translation, which is a slightly improved version.

>NAT is absolutely not in any way a substitute for an actual firewall, despite the side effect of 'blocking' ports.

This is one of those infosec tenets that is technically true but functionally unhelpful. Like correct-horse-battery-stable debates.

The claim is that IPv4 + NAT + bad firewall is better than IPv6 + bad firewall.

Yes, both are insufficient and inferior to a good firewall - but how confident are you that you never interact with a bad firewall?

What makes you think that, for critical systems, "IPv4 + NAT + bad firewall" is the default IPv4 deployment paradigm, rather than "IPV4 + bad firewall"?

Sure, big IaaS providers like AWS put you in a VPC by default. But most servers on the net are not hosted in an IaaS; they're hosted using a VPS or bare-metal hosting provider, or just coloed in a DC by their owner. And in all those cases, what that kind of deployment gets you, is a public IPv4 per VM/machine, that anyone on the Internet can march right up and talk to, where it's the responsibility of the machine itself to reject incoming packets (i.e. at the OS level with a kernel firewall.)

NAT on IPv4 is only really a default assumption for residential networks. Anywhere else, it's pretty much like the movie WarGames: even the mainframe has a phone number you can call. Staying on IPv4 isn't making anyone safe.

While I don't have any factual proof to refute your statements, in my personal experience almost every organization uses NAT & RFC1918 address space. The only client I can think of in my 20 years of experience that used a public IPv4 per VM/machine was the DoD, specifically, the U.S. Army.

From your very last statement, I think you've confused self hosting (like buying a VPS from Digital Ocean and hosting your own blag) and how the real world works (like going to Dell.com and ordering a new laptop). "The mainframe" these days is almost always behind a L4/L7 load balancer or other network device and very rarely directly addressable.

People assume that RFC1918 is not routable, but that's not the case... It's fully routable, but there is no global route. Have you ever tested routing to your RFC1918 address space from the ISP, or from a customer in the same neighborhood?

On some ISPs, all the customer routers in a given area are placed in a large legacy subnet, so if another customer adds a manual route to RFC1918 space using your router as next hop - the traffic will arrive on the WAN interface of your router. Some routers will actually route this traffic inside.

Have you ever tested this and verified that your router doesn't do this? Probably not, because most people haven't. They just assume that it can't, and get a nasty surprise if someone demonstrates that it can.

My company runs an API SaaS; my impressions come from a hobby I have of looking up the hosting providers behind our customer IPs as seen in our request logs (to find out what people think is a good idea for hosting a production web- or mobile-app service backend these days.)

By and large, our very-much "real world" customers are "self hosting" — usually on bare metal rather than a VPS, and usually with providers you've probably never heard of (ColoCrossing and ServerMania seem to come up fairly often among our US-based customers.) These hosting providers are all very much in the style of "you lease each machine as a separate contract; each machine gets one public IPv4 address included in the cost; private networking [i.e. an explicit VLAN] is an extra optional feature you can enable after the fact, and only works between higher-end machine types, rather than being a given, because our lower-end machines only have a single NIC in them [besides the one that's part of the BMC used for IPMI]."

What I assume is happening here isn't literal "self hosting" — these random non-IT-oriented customers wouldn't know the first thing about it — but rather that a given customer of ours has paid some "vertically-integrated IT consultancy" to both build and host their service for them; and said consultancy has chosen to use bare-metal hosting to host the resulting service, to minimize their own OpEx, and therefore maximize their margins. (In fact, I bet they're often packing several such customers onto a single box.)

---

Also, in a more professional capacity, I investigate the hosts behind IP addresses behind bulk-registration / DDoS attacks against our platform, in order to create signatures for them. Given the way some of these attacks seem to work, a large number of machines on the Internet — especially in Russia and [some parts of] Africa — seemingly aren't only un-NATed, but in fact have a public /24 or even /22 directly attached to a single box! (If traffic was originating from a random subset of a /24, it could just be someone spinning up a hundred VMs on top of some small colo's OpenStack deployment, sure. But tandem traffic from every IP in a /24, and only exactly said /24? That looks pretty much exactly like the sort of tandem IPv6 traffic that is generated when a box has a /48 or /56 assigned to it.)

Big universities (at least in my experience in the USA) are the other ones that would have a public IP address for every device, at least until rather recently. They were online very early and got allocated huge blocks of addresses, before anyone really imagined future scarcity.

In the mid-90's, every system at my university had a public IP address, including those on the campus residential networks. There were no firewalls. It was also a flat address space (/16, 255.255.0.0) for the whole campus! The 90's were certainly a different time.

> The claim is that IPv4 + NAT + bad firewall is better than IPv6 + bad firewall.

Even that is not true:

- It takes half-minute to scan an IPv4 public IP (NAT) for vulnerabilities.

- Good luck and have fun to scan a /64 for a potentially vulnerable machine. See you next century.

- And if it is not enough: most internet box support UPnP/NAT-PMP that allow any malware to get your NAT wild opened.

I use an old Parallax Propeller server as my DMZ, with instructions to log everything and answer "OK" to everything. It's funny what people try to do to it.

Why don't you write a blog post about this? I'm interested to see what will go on

Who's the monster that created NAT for IPv6 D:

Depending on the NAT implementation this can be incredibly naive. Many home routers will send ANY traffic incoming on a port to the NAT'd IP address, even if the sources don't line up.

So say Alice is behind a crappy NAT and wants to talk to Bob. Alice's router opens a port on its edge, lets say 1234, and sends traffic to Bob on port 80.

Let's say Charles knows Alice's IP address. Charles starts spamming Alice's router, eventually hitting port 1234 with bad data.

Alice's router is dumb. It sees traffic on port 1234, checks its NAT table, and sees that data is supposed to go to Alice. It happily rewrites that packet and passes it along to Alice. Now Alice is getting traffic from Bob *and* Charles. Uh oh!

Many game consoles are explicitly designed around this bad, broken behavior. You'll open a port to the matchmaking server and then the matchmaking server will tell people to connect to that IP address and port combination. Crappy home routers will happily route that data through its NAT configuration to the console despite the console never explicitly opening up traffic to those other parties. This is why some game consoles will complain about closed NAT versus open NAT.

> Alice's router is dumb. It sees traffic on port 1234, checks its NAT table, and sees that data is supposed to go to Alice.

While in principle that is possible, in practice almost all home routers are based on Linux, and Linux netfilter NAT implementation distinguish connections based on port and IP, not just port, so this would not work.

I think you would enjoy this article from Tailscale: https://tailscale.com/blog/how-nat-traversal-works/

The poke a hole to outside world to a random server, log the port allocated to you by your router and have someone else use this to connect to you is the basis of STUN protocol.

Home routers often greatly simplify the interface.

BT, one of the largest ISP's in the UK, only allow the configuration of destination IP and external/internal ports[0].

I've never expected my NAT to do anything other than map ports. I can see why the ability to map source IPs to different ports would be useful but relying on that as a security feature feels like a foot-gun. I wouldn't feel comfortable exposing an application that doesn't have some form of authentication and/or blacklisting.

[0] https://portforward.com/bt/home-hub-6/Port%20Forwarding.jpg

Any router running a poor NAT implementation (aka most of them) essentially has a built in firewall bypass for the right attacker.

A naive NAT implementation can allow an attacker to bypass the firewall.

Curious, could you expand on this?

I gave an example just a few comments above this. Alice never wanted Charles' traffic, the firewall should not have let it through. But because the NAT is dumb, and the firewall rules are often tied to the NAT on these crappy home routers, it's allowed. So now because Alice wanted to talk to Bob, she opened a port to the world that she never wanted opened as wide.

Thanks! (you added this afterwards, right? Or it's just me being tired and skipping this)

This is straight up untrue. The only thing NAT does is change the apparent source address of outbound connections. Inbound connections aren't outbound connections, so it does nothing to them.

NAT is not a substitute for a firewall.

those of us who want to have the same port on different computers available to the internet might see that as a bad thing

Oh you don't need a firewall then? I guess accessing a routers web interface from the WAN is a-okay

My shitty cable modem which is also a router does not expose its web interface to the world by default.

I don't understand why you'd need a firewall if

- you trust devices on your network (yes, big if, but even then: the only reachable ports of a machine from the outside are those explicitly open to the outside, most stuff listens to 127.0.0.1 anyway)

- you only configure your NAT to forward ports you would open on your firewall

My shitty router also firewalls incoming IPv6 connections by default, unless I manually allow them per-device, so I don't get your point.

My point is lzaaz's one https://news.ycombinator.com/item?id=33897568

I didn't think of my cable modem as a firewall. Maybe technically it has one to provide the feature of blocking access to its web interface from the world, or maybe it just listens to the right network. I don't know, but for all intent and purposes, setting up a firewall myself does not seem necessary.

To be fair, I was also a bit annoyed by staringback's phrasing.

My router's httpd listens on the LAN not the WAN unless I tell it to. This is unrelated to what I said.

> It’s the firewall rules that always creep me out. The nice thing about NAT is open ports on your internal network are hidden to the outside world by default. You have to think about which ports you want the NAT gateway to forward.

Have you never had more than a handful of IPv4 addresses? IPv6 works the same in this regard as a router IPv4 network e.g. universities, large/old enterprises etc. NAT started as a workaround to make the available public address space last longer.

The address (and port) translation wasn't intended as a security feature on its own. These days lots of protocols automatically deal with NAT and mostly manage to establish bidirectional communication over UDP or TCP through NATs. I rather deal with stateful firewalls and public IPv6 addresses end to end instead of gluing the segments of flows between IPv4 translators back together.

NAT wasn’t started as a way to make the address space last longer. A few decades ago you could get hundreds of thousands of IP’s by filling out a form with ARIN without any serious justification if you wanted them. I worked at an ISP and IP’s weren’t a scarce resource.

NAT started because having a network didn’t mean you were necessarily participating in the Internet. Globally unique addresses weren’t that important. At some point you had this decentralized situation where local networks wanted to bridge their users address space to the Internet without renumbering everything and thus NAT was born.

> Granted I assume you can probably create a default DENY rule for inbound traffic and selectively open ports up as exceptions. Right?

Sure, of course. That’s how firewalls work for IPv4 as well— you have an implicit deny rule at the end, and then allow rules come before it.

Every consumer and professional router I've seen comes with a deny rule for incoming traffic by default, unless the device is configured as a "router router", like inside an ISP.

NAT has many problems because people rely on it for security. For example, many shitty IoT devices and even consoles (looking at you, Nintendo Switch) tell you to put their device in the DMZ to make them work.

The norm for IPv6 in practice is that you've got your firewall on and need to make exceptions for ports you want open, just like on IPv4, except that with IPv6 you don't need some kind of interactive state machine attackers can confuse and abuse running inside your router's kernel (ALG).

> The nice thing about NAT is open ports on your internal network are hidden to the outside world by default.

That is usually not true. NAT punching is a thing for decades now.

- NAT punching does require cooperation of programs on the protected machines, though, no? How is it different from them inviting traffic in any other way (like, requesting a page via https from an infected server could hijack the client on the protected machine if the client has security holes in the right places; the https client is willing to get traffic here, just as some VoIP program is willing to receive a call)?

- And is it any different from a stateful firewall on IPv6?

> NAT punching does require cooperation of programs on the protected machines

As does listening to a port.

> And is it any different from a stateful firewall on IPv6?

Hum, not much. And that's the point, all of those are basically the same. NAT doesn't give you much security, and NAT without a firewall usually gives you less security than a firewall, since NAT usually is configured for connectivity, and a firewall for protection.

> > NAT punching does require cooperation of programs on the protected machines

> As does listening to a port.

Listening on a port is for incoming connections, exactly the kind that we're blocking with either a (stateful) firewall or NAT. Listening on a port is a declaration of a program (a server) to communicate with whichever counter party can connect to this port (until the server program decides to close the connection), and limiting that reach is the topic here.

NAT hole punching is more like an outgoing connection in that the client agrees to communicate with a particular single counter party for each punch. That's why I made the comparison to opening an https connection to a server. The risks look basically the same to me (the client has to trust the server in the https case, and the client has to trust the intermediating server in the NAT hole punching case to intermediate with the right client; admittedly it additionally has to trust the other peer (e.g. that its compressed data doesn't try to break decoders), but in cases where it communicates with another party via a server the situation may be the same again (unless the server re-encodes the data and does that securely)).

> And that's the point, all of those are basically the same.

That's a relief for me to hear, as I started to doubt myself whether I am missing something ("why is it not OK to use NAT to block incoming connections?").

> since NAT usually is configured for connectivity, and a firewall for protection.

Sure, a firewall can add additional restrictions. I have always understood NAT's protection to be limited to prohibiting incoming connections (unless when adding port forwarding), while allowing outgoing connections including NAT hole punching.

I'm also talking in the context of configuring a Linux machine via iptables (where you configure both NAT and other firewalling rules). Maybe you're thinking more in terms of consumer "NAT" vs. consumer "firewalling" devices and their respective capabilities. Or maybe this whole "don't consider NAT to be a security feature" movement is just to pull people towards IPv6 by saying they don't need NAT to be as secure (or better if they configure additional restrictions)?

The point here is that the movement against IPv6 for security reasons is disingenuous or even outright dishonest. Those security reasons don't exist.

Personally, I have never seen any argument for IPv6 based on security (except for some very fringe ones about address enumeration). But if anybody makes one to you, well, it would be disingenuous, or maybe even dishonest too. There is no security-based argument either way.

> I have always understood NAT's protection to be limited to prohibiting incoming connections

It doesn't actually do this. NAT rewrites the source address of outbound connections. Inbound connections aren't outbound connections so it does nothing to them, which means it doesn't prohibit them.

That is why you don't need NAT for security: it doesn't give any in the first place.

> which means it doesn't prohibit them

OK. I want to dig down into this. Let's say I have a router `R`, which I'm running NAT and optionally other iptables rules on. I've got a client machine `C` sitting in a private network "behind" `R`. `R` is connected to the internet via a gateway `G`. `A` is some machine out there owned by an attacker. There's a vulnerable TCP service running on `C` listening on *:1313.

       A
       |
    internet
       |
       G
       | 4.3.2.1
       |
       | eth_public 4.3.2.77
       R
       | eth_private 10.0.0.1
       |
       | 10.0.0.2
       C

`A` can't connect to 10.0.0.2:1313 since it's not routable from their position. Thus, the fact that NAT on its own doesn't prohibit traffic to `C` doesn't matter in this scenario, practically `A` still can't reach it. So far so good?

The only issue I can see is that if `A` can hack `G`, because `G` doesn't have to depend on routing to reach `R`, it can send traffic to `R` with a target address of 10.0.0.2, which `R` then forwards to `C`. I haven't verified that this works (don't have enough devices with me). Is this what you're after? Fair point.

If I'd add the following rule to `G`, `C` would be safe even if `G` is hacked[*]:

   iptables -A FORWARD -i eth_public -d 10.0.0.0/16 -j REJECT

[*] Of course that requires that any outgoing connections that `C` makes are not vulnerable against the possible packet manipulation from `G`.

Am I missing anything?

Edit: simplified the rule

PS. I'd welcome a good pointer (book or other) on network security and also IPv6; I'm a software developer, and only occasionally dealing with networks.

That's basically it. In that network, G can connect to C just fine. You need the firewall rule to block inbound connections, because NAT just does nothing to them.

I don't have any good learning resources for this stuff, sorry. I mostly picked it all up by running it on my home network and Googling for stuff when I hit something I didn't get.

> Granted I assume you can probably create a default DENY rule for inbound traffic and selectively open ports up as exceptions. Right?

That's what reasonable people would do for a V4 network too.

I just started University, and as a result I have had my first experience of Internet without NAT. The firewall rules provided are simply On or Off, which has been very strange to me, and it doesn’t seem all that secure.

My own ISP provided router is by default setup to deny all inbound traffic on IPv6. I'm surprised it's not the default everywhere.

My ISP doesn’t support IPv6 at all, unless I want to voluntarily go behind a CGNAT.

it's the default behaviour by most cpe, correct

any exceptions to this should be roasted (my twitter dm's are open)