[GauntletWizard]

Because it was never intended as such and does not necessarily need to add any. The fact that many NAT implementations do add some security (by dropping inbound connections by default) is a side effect. I've seen NAT implementations that get it precisely wrong (consumer routers that set up .2 as the default DMZ), but that's still entirely valid.

[noja]

Whether or not NAT was designed with security in mind doesn't matter.

Using NAT increases security simply by having deny by default.

[vy8vWJlco]

But that is not a feature of NAT, that is a feature of a firewall (for example, it is possible to route incoming packets via the WAN as well as masquerade outgoing ones from the LAN - most people wouldn't even know their pants are down). It is a coincidence that home routers sometimes provide both leading people to conflate their firewall with their NAT system - but if a firewall is what is wanted (and is arguably the only valuable component), NAT is not the thing to ask for. Conflating NAT with firewalls also promotes the idea that NAT has a place in any network with abundant addresses. IMHO, it does not.

[nitrogen]

NAT will always have a place, because not everyone wants to expose a uniquely routable address for every device they own (probably based on a device's MAC address) to the world.

[IgorPartola]

The simple and much more flexible answer to this is to have a firewall rule on the edge router that simply drops all packets to and from the hosts you do not want visible. Note that this is more flexible in that if you want some hosts visible and some not, you can do that. You cannot do that with NAT: you only have one port 80 and several servers for example.

In general, use the right tool for the job. NAT provides some blunt security features incidentally. It does not, for example, prevent your server from making connections to the outside world. The firewall is what is meant to be used to restrict traffic. That is its only job and it does that well.

[vy8vWJlco]

Can you give an example that isn't based on security (since the NAT isn't providing that)? I can only think of short-term (undesirable) transition or load-balancing mechanisms - all of which have better long-term solutions (anycast, multicast, and dynamic service discovery, etc)...

[nitrogen]

What's with the widespread fixation against attributing any positive security attributes to Network Address Translation? It's like the old warning about NAT not being enough to protect a private network got cargo culted into a universal anti-NAT maxim.

What would you call it when a router at the edge of a private network presents a single IP to the world no matter how many devices are behind it, for privacy reasons?

[vy8vWJlco]

"What's with the widespread fixation against attributing any positive security attributes to Network Address Translation"

It probably has something to do with NAT being a huge obstacle later, when needs change. For example, while it might make sense at one point to have multiple hosts appear as one, it is at the expense of direct addressability, and the workaround - having a unique address+port combination - makes less sense than having a unique directly-routable address.

"What would you call it when a router at the edge of a private network presents a single IP to the world no matter how many devices are behind it, for privacy reasons?"

The wrong tool for the job. :)

The privacy is gained not by translation but by blocking direct connections (which is a feature of a firewall - not of NAT). While there is some value in NAT's ability to falsify the origin of data - ie, to take credit for non-local flows - but if those applications ever grow they will be fighting to escape the single address of NAT and are thus only suitable in the short term. Ideally, NAT would go unused because every element is uniquely addressable and fully independent... Most people would not consider someone else who continually takes credit for their or someone else's work to be a feature, and so it is with NAT.

[kiallmacinnes]

No, using a firewall increases security. It just so happens that most routers which do NAT, also have a firewall. You are, in my opinion, confusing two functions provided by one device.

I'm willing to bet that any IPv6 capable router also has a firewall.

[dspillett]

That doesn't increase security unless your baseline is broken. All NAT is potentially replacing from a security perspective is a single default drop (or default reject) rule, which should have been there to start with.

While it could be argued that NAT adds an extra layer to security-in-depth by making it harder to accidentally open things up by missing out the default drop/reject rule, but I'd argue that all the faf that NAT can create by making it difficult to arrange point-to-point connections where they are actually desirable is not worth that little bit of protection against failing to configure the firewall correctly.