[jeroenhd]

Every guide I see tells you to disable Upnp (or IGD, the part of UPNP that lets you open ports), for good reason. It's a protocol that just disables the security you thought you had before.

The reason Xboxes need port forwarding in the first place is that IPv4 relies on NAT. The unreliability and unpredictability of NAT means remote devices won't know what ports to talk to or if those ports will even be mapped to the right device. IPv6 removes that problem all together! It alleviates the need for 99% of the port forwarding cases that UPNP provides, assuming you've manually enabled it in the first place.

If port forwards are really necessary for Xboxes to work, then IPv6 brings another advantage: you can run multiple Xboxes behind the same IPv4 address. That IPv4 address can be your home connection, or it can be a thousand people behind CGNAT. In countries where CGNAT is the norm (India comes to mind) you can't possibly expect UPNP to be a requirement for Xbox to work!

[martinald]

UPNP is just fine assuming a secure implementation (some of the early ones were awful, but that doesn't make the concept bad). It doesn't reduce security anywhere near like what is made out. If you already have a device on your network that is compromised and is able to do UPNP requests, then you have a much bigger problem (in a home setting).

But IPv6 doesn't solve the problem at all if all incoming connections are blocked on the firewall - which they have to be for security! You need some protocol for the xbox to be able to tell the firewall to open ports, which doesn't exist. Or users have to manually set the firewall rules, which is the same as port forwarding on NAT from a users perspective (ie impossibly complicated)

You can run multiple xboxes behind NAT right now, they get different ports.

Again, for a non technical user there is no major difference between being behind CGNAT IPv4 (impossible to get incoming connections), and firewalled IPv6 (possible to get incoming connections, but the firewall is too complicated to use so it may as well be impossible). If there was a protocol which would allow devices to open ports on IPv6 firewalls programmatically similar to UPNP it would be entirely different, but there isn't.

[jeroenhd]

libupnp/miniupnp seem to support IPv6 pinholes just fine through WANIPv6FirewallControl. I don't sell routers so I don't know how up to date the libraries on your average router firmware are (most likely "ancient") but this isn't a protocol problem. If you run firmware like OpenWRT you've had support for it for at least five years now.

Most consumer routers I've seen these days come with UPNP disabled by default, though.

For most P2P traffic (which includes a third party handshake server) you can probably skip port forwarding entirely; just send UDP packets both ways and the firewall will figure it out. For stateful protocols like TCP and friends (SCTP etc.) that's harder to accomplish this, that's where you need pinholing.

It's possible that your router simply doesn't support IPv6 pinholing but I think the more likely scenario for breakages is that client software doesn't bother implementing it.