I used to play Stardew Valley with friends on openvpn. My Friend is host and I am farmhands.
But I've found that when the host turns on SMAPI, I can't connect the way I did before (whether I turn on SMAPI or not) .
When the host shuts down SMAPI, I can connect to it (whether I turn on SMAPI or not) .
Then, after trying, I found that by sending the byte 0x880000000 to a host without SMAPI turned on via openvpn, I was able to receive the string"StardewValley".
Conversely, if the host turns on SMAPI, I receive nothing.

This is the instruction I use to send the data，10.8.0.10 is the host。
echo -ne '\x88\x00\x00\x00\x00' | nc -unv 10.8.0.10 24642

Hi! See Can't connect to another player in multiplayer in the troubleshooting guide for some common fixes. I'll close this since I only use GitHub issues to track specific development tasks; if the suggested fixes don't work for you, come ask in one of the recommended places, and we'll help get it working.