LAN play: why, how it works, and what is left to test (#144)

[EdouardSence]

Mario Kart World has a hidden LAN Play mode, like Mario Kart 8 Deluxe: on console, hold L+R and click the left stick on Wireless Play. Wireless Play itself uses nn::ldn, the Switch's own radio, which a PC cannot provide. LAN Play goes through ordinary UDP sockets on a local network, which a PC can. That makes it the realistic way to play this port together on several PCs, without Nintendo's servers.

What works now (#144, behind MKW_LAN=1, off by default)

• nn::socket runs on the host's sockets (runtime/sdk/ov/ov_socket.c). It translates the console's FreeBSD sockaddr, options and flags, and gives the game Linux errno numbers.
• nifm's local network mode is connected and reports the PC's address (MKW_LAN_IP to choose one).
• The title menu opens with LAN Play chosen. The game keeps the L+R+left stick choice in one byte of the title page (+0x77c). That byte picks the entry's label and the menu state that leads to the LAN lobby and the LAN session (network mode 3). The patch set stores 1 there when the page is set up.
• On the way: the error dialog overload that crashed Wireless Play for everyone (Wireless Play crashes in nn::applet::CreateLibraryApplet (error dialog) #147), and the random bytes Pia asks the csrng service for.

Tested on one PC (CachyOS, RX 9070 XT): the menu reads "Mode LAN", the lobby says "no group available", and creating a group reaches the character select.

Update 12/11: a LAN race works between two instances on one PC (tools/run/lan2.sh, each in its own network namespace, 10.77.0.1 and 10.77.0.2). A group created on one is listed and joined from the other, the host's rules reach the guest, both vote, and the race starts on both together, each kart visible on the other screen; a second race followed. Screenshot in #144.

What does not work yet

• No game between two real PCs yet (two instances on one PC work, see above), and nothing on Windows. If you have two machines on one network: build feat(net): LAN play over the host's sockets (MKW_LAN=1) #144, start both with MKW_LAN=1, pick LAN Play, create a group on one and join from the other; the host starts the race from its menu (Plus, Tab on the keyboard). Your firewall must let UDP in (Pia uses ports 49152 and 35000). Please report what you see.
• The real L+R+left stick combo does not switch the menu entry here. I have not found its handler yet. The patch set does the same thing directly; with the handler found, the patch could go.
• Building the LAN session in place of the ldn one, which I tried first, does not work: creating a group crashes.

Also

#145 is unrelated to the game: on a machine without the maintainers' job runner, tools/run/job.sh started test runs without the memory cap they assume. One of them stalled my whole desktop. Runs now go in a capped systemd user unit, and ctl.sh stop can stop them.

Written with AI assistance (Claude Code); I am responsible for it.

[adriadam10]

Thanks for writing this up, it's a very clear explanation of why LAN Play is the way to play between PCs.

Status: the ShowError(ErrorResultVariant) fix went into main on its own and ships in v0.1.9 (#147). #144 is rebased on main (by us, 12/11) and builds; the review points in the PR still stand (the socket slot race, eventfd and WSAECONNRESET on Windows, and nn::socket with MKW_LAN off). We have a Linux and a Windows PC on one network, so we'll also try a game between the two once those are in, and post what we see in #144.

[EdouardSence]

Update (12/11):

• The review points of feat(net): LAN play over the host's sockets (MKW_LAN=1) #144 are answered in 4a00ccb. The socket table is now under a lock. On Windows, ICMP resets are off, and eventfd is only reached by gRPC. With MKW_LAN off, nn::socket answers exactly as main's stub did; a run through the online menu made zero nn::socket calls.
• First LAN race played, with two instances on one PC: tools/run/lan2.sh (fe5e314) puts each instance in its own network namespace (no root). A group created on one instance was listed and joined from the other, and the host's rules reached the guest. Both voted, the race started on both at the same moment, and a second race followed after the pause, with no disconnection. A screenshot is in feat(net): LAN play over the host's sockets (MKW_LAN=1) #144.
• Your Linux + Windows test would be the first between two real machines, and the first on Windows. UDP ports 49152 and 35000 need to be open. The host starts the race from its menu (Plus, which is Tab on the keyboard).

[adriadam10]

Thanks for the update and for 4a00ccb, it answers the review points.

Where #144 stands now: we rebased it on main and added one change of our own (dca2a18, explained in #144): LAN play reaches only local network addresses. Bind and SendTo accept loopback, the private ranges, shared address space (Tailscale-style virtual LANs), link-local, multicast and broadcast, and refuse anything else with ENETUNREACH before it reaches a host socket. A LAN mode must never be able to talk to a server on the internet, whatever the game asks for. The sockets probe passes with it.

#144 goes into 0.1.9, which comes out today, with LAN Play still behind MKW_LAN=1 (off by default). Then we'll run the Linux + Windows game between our two PCs (UDP 49152 and 35000 open) and post what we see in #144.

[adriadam10]

#144 is in main and goes into 0.1.9 (out today) as Settings > Game > LAN play, off by default. Turn it on and restart, and the title menu's Wireless Play becomes LAN Play. It only reaches local network addresses, never the internet; UDP ports 49152 and 35000 must be open. So far it has been played between two instances on one Linux PC. If you have two PCs on the same network (Linux, Windows or one of each), please try a group and a race and tell us here what you see, with logs/mkw.log from both if something fails. (Correction to my earlier reply: our two PCs are on different networks, so we can't run that test ourselves yet.)

[adriadam10]

LAN play is in v0.1.9: Settings > Game > LAN play (off by default; turn it on and restart, and the title menu's Wireless Play becomes LAN Play). It only reaches local network addresses, never the internet; UDP ports 49152 and 35000 must be open. So far it has been played between two instances on one Linux PC. If you have two PCs on one network (Linux, Windows or one of each), please update both to v0.1.9, create a group on one and join from the other, and tell us what you see here, with logs/mkw.log from both if something fails.

Release: https://github.com/adriadam10/mkw-recomp/releases/tag/v0.1.9