IPv6 the technology solves it, IPv6 as it exists in the real world does not. Even if we got 100% IPv6 adoption overnight, you would still not have a universally adopted API for punching holes through router firewalls for p2p applications.
Here’s the attack:
- You want to get to A
- And you’re on a device B that has access to hole-punch-as-a-service but no direct access to A. Good network administrator / sysadmin!
- Attack NDP (ie ARP spoofing but IPv6 flavored) to trick the firewall just for a moment that you’re A [1] and request the port be opened to your evil server C on the public internet.
- Oops.
AFAIK it doesn't use third party servers and it seems to get through any router just fine...
This can have quite an effect on sync speed. I set it up for my dad and initially his PC and laptop were syncing incredibly slowly. It turned out they both thought his home network was a public one, so everything was going out to a relay server and back.
EDIT: I agree we definitely need something like UPnP, or maybe even UPnP itself, but it's another adoption problem currently.