[Bug] ldn_mitm UDP discovery on linux

[superjcvd]

Prerequisites

• I am confident that my issue has not been reported already in open OR closed issues.
• I am able to reliably reproduce the issue on the latest stable or canary version.
• I am not using an unofficial build (Flatpak, Emudeck, etc.).
• I am a silly goose and didn't read these boxes.
• I am a silly goose and DID read these boxes.

Description of the issue

On Linux when using ldn_mitm to play on local network between Ryubing instances (or modded switches) the network discovery feature is broken. The lobbies stay forever empty.

There is a comment about this issue in the source code but the fix is not correctly implemented.

/ryujinx/src/Ryujinx.HLE/HOS/Services/Ldn/UserServiceCreator/LdnMitm/LanDiscovery.cs Line 320

We need to listen on the INADDR_BROADCAST address instead of the local address.
We need to replace LocalBroadcastAddr keyword with IPAddress.Broadcast

I have no C# knowledge and don't know how to complie the project to validate but the fix looks pretty logic to me.

            {
                if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
                {
                    _udp2 = new LdnProxyUdpServer(_protocol, LocalBroadcastAddr, DefaultPort);
                }
                _udp = new LdnProxyUdpServer(_protocol, LocalAddr, DefaultPort);
            }

It should be:

            try
            {
                if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
                {
                    _udp2 = new LdnProxyUdpServer(_protocol, IPAddress.Broadcast, DefaultPort);
                }

                _udp = new LdnProxyUdpServer(_protocol, LocalAddr, DefaultPort);
            }

Reproduction steps

• Install latest Ryubing version on Linux.
• Run it with ldn_mitm
• Create a lobby on a compatible game and try to join/refresh the lobby list from another computer

Log file

...

OS

Linux

Ryujinx version

1.3.3

Game version

No response

CPU

No response

GPU

No response

RAM

No response

List of applied mods

No response

Additional context?

No response

[PiplupHelper]

can confirm it happens on my end with mario kart 8, it hits a LDN timeout error unsure why yet but looking into it

[superjcvd]

/ryujinx/src/Ryujinx.HLE/HOS/Services/Ldn/UserServiceCreator/LdnMitm/LanDiscovery.cs Line 320

We must change LocalBroadcastAddr to IPAddress.Broadcast
It should resolve the issue

[sh0inx]

/ryujinx/src/Ryujinx.HLE/HOS/Services/Ldn/UserServiceCreator/LdnMitm/LanDiscovery.cs Line 320

We must change LocalBroadcastAddr to IPAddress.Broadcast It should resolve the issue

Did you test it?

[superjcvd]

Nope sorry I have no idea how to build the project...

But I am trying to find a workaround without the need to update the code.
In order to (at least) validate the issue...
I will post the result here as soon as possible.

Possible fix

On the 192.168.1.0/24 network

• Server = 192.168.1.1
• Client = 192.168.1.2

The client sends a UDP Discover on port 11451 on local network broadcast 192.168.1.255
The Linux host receives UDP discover packet packet but doesnt forward it to the server socket
On linux you can't easily bind your server to the local network broacast.
Instead you need to use the global Broadcast 255.255.255.255 to make the server receive the packet.

In order to validate and fix the issue we can use a tool like socat to forward the UDP discover to the server socket.
Something like socat UDP4-RECVFROM:11451,broadcast,fork UDP4-SENDTO:127.0.0.1:11445

I will test it this weekend and post the result here

[sh0inx]

All you have to do to test is clone the repo, change the code you're talking about, and then select "Run" at the top of the screen. I use JetBrains Rider for this, the IDE is available for free with the community edition.

[superjcvd]

Ok I can try it too :)
I will do that tomorrow and post my results here !!!

[sh0inx]

Keep us updated! If you want to submit an MR to our git repo, you can reach out to @officialryubing on Discord to request access. For your reference, our GitLab is here: https://git.ryujinx.app/ryubing/ryujinx

[capy-security]

@sh0inx I need to push the work further.
I successfuly compile the project and now ldn_mitm listen on 255.255.255.255 but still nothing happens when creating game lobbies. I will try to find the issue this week-end if I have time (and compare with Windows that seems to work fine)

[capy-security]

@sh0inx I just added a few comments in the ldn_mitmcode to have more insights about the issue and suddently it was working.

It looks like this is more a timing issue (with UDP) or something like that. I am working on code improvements (using cursor) in order to make the UDP discovery more stable.