NPF and Nintendo Switch multiplayer

[nia <nia%NetBSD.org@localhost>]

Hello, since there was a thread earlier in the year about
IPF and the Nintendo Switch, I thought I'd add my anecdote
for future readers to (hopefully) go through less pain.

Background:

The Nintendo Switch hates firewalls and does not use IPv6.
It also uses peer-to-peer multiplayer and is not good at
UPnP or hole punching.

Nintendo's official advice is that to use multiplayer, you
should disable the firewall or use a DMZ. Obviously, this
is non-preferrable.

If you go into the "Internet" settings on a Switch, it'll
give you a "NAT Rating" (letter graded). In many cases this
will be D (i.e. multiplayer will not work in most cases).

Solution:

In the end, I assigned the Nintendo Switches in the house
static addresses on the local network, and added "no-ports"
to my NPF firewall's NAT rules:

inet4($ext_if) dynamic no-ports $localnet -> inet4($ext_if)

This disables port translation and improved my Switch's
NAT grade from D to B. Everyone in the house is very happy
that they can now play Pokémon online.