With IPv6 the entire network is reachable outside by default. Granted I assume you can probably create a default DENY rule for inbound traffic and selectively open ports up as exceptions. Right?
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.
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.
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
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.
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.
A naive NAT implementation can allow an attacker to bypass the firewall.
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.
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
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.
NAT is not a substitute for a firewall.
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?
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.
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.
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.)
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.
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.
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 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.
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.
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).
That is usually not true. NAT punching is a thing for decades now.
- And is it any different from a stateful firewall on IPv6?
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.
> 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)?
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.
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.
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
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
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.
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.
That's what reasonable people would do for a V4 network too.
Only, and only, if you configured your router that way.
In both cases, it's absolutely the same:
IPv4 -> allowed NAT ports -> NAT network -> Everything else is dropped/denied
IPv4 -> allowed ports -> directly routed IPv4 network -> Everything else is dropped/denied
IPv6 -> allowed ports -> directly routed IPv6 network -> Everything else is dropped/denied
Of course if you are on someone's else network (typical for hosting when you aren't provided with your own v6 subnet, instead you have a bunch of addresses) then you should configure firewall on each your machine... which is what you need to do anyway?
any exceptions to this should be roasted (my twitter dm's are open)