Match Codes

Using Match Codes, Flycast Dojo can start a P2P game session behind firewalls, so that you can play against others without the need of Radmin or Fightcade. No need to sign up for any accounts or download additional software than Flycast Dojo itself. You should be able to play any games supported by Flycast, so long as you and your opponent have the same ROM.
The methods used by this feature should work with most consumer routers, but you may run into trouble over some cloud gaming services or corporate networks. (It is confirmed to work over Paperspace though, where I did much of my testing).

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As Host

You can set yourself as a host by making sure HOST is selected in the drop-down box on the left, then click on the game to begin.
This will contact the default matchmaking relay and give you a code to send to your opponent. To make code transfer easier, press the Copy Match Code button to add the code to your clipboard to paste to your opponent.
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Set Delay
When a guest joins a session, the host can set the delay according to packet round trip time by pressing "Detect Delay". Use the slider to adjust the game to your liking, and press "Start Game" to begin your session.
Depending on the connection between you and your opponent and the tendency for network spikes, you may have to bump delay up to make your game smoother. The best course of action is to start low, and go higher until both you and your opponent have a smooth framerate.

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As Guest

To join a game, click on the dropdown on the left and select JOIN then select the game of your choice. It will prompt you for the match code. If you have copied the Match Code from your opponent, you can press the Paste button, otherwise, you can enter in the match code manually.
Once you are done, you can press Start Session.

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Match Code Troubleshooting

If you notice disconnects at some regular interval (e.g. every 6 minutes, 10, minutes, 15 minutes), it may be possible that your router is closing the UDP route that was established by the matchmaking service by having a low UDP Timeout. This is similar to what can happen with VoIP calls on restrictive routers.
To fix this, find your router's UDP Timeout settings and set it to 90 seconds or higher.
If you are unable to find this setting, you can forward your router's port and connect manually, or use the LAN Lobby over a Virtual LAN service like ZeroTier or Radmin VPN.
For more information, check out 10 VoIP Problems: How to Fix Them Forever.