SecureNAT VPN for LAN gaming - Connection issues

[deb_user]

Hi everyone!

Bit of a back story:
I'm running SoftEther VPN Server on a Debian 12 VM (Proxmox) for LAN gaming since about two years without any larger problems. All LAN games worked perfectly fine so far, except of one specific game: CoD - WaW Zombies / Coop.

The weird part:
I first tried this game back in Mai of 2024 and it worked perfectly fine. Now over a year later my friend and I wanted to play it again and... it does not work anymore. To be more precise the player who want's to join is able to see the Host in the server browser but as soon as he wants to join the message "This game session is no longer available" appears.

What changed:
Not that much honestly. Back then we've used SoftEther VPN Server v4.43. Both clients used SoftEther VPN Client v4.43. Since then I've updated the server and also client to v4.44. Other than that I've always installed / updated the packages on the server (Debian 12) and we both also always applied the various Windows updates. Back then my friend was on Windows 11, I was on Windows 10. Now we both are on the most recent versions of Windows 11.

Server configuration:
- Enabled SecureNAT with Virtual DHCP (No Default Gaetway or DNS is set)
- Disabled Virtual NAT
- No Local Bridge configured
- No specific Security Policy set for both users (already tried setting unlimitted broadcasts with no success)
- All other settings are basically the default / disabled

What I've already tried:
- As mentioned above I've set unlimitted broadcasts on both users via Security Policy
- Checked that the Virtual VPN adapter metric is set to "1" on both client systems
- Disabled SSDP on both Windows Clients
- Set up another fresh VM and tried with v4.43 of the SoftEther VPN Server, also installed v4.43 of the SoftEther CPN Client Manager for both clients
- Just to be sure: I've tried this game / mode on two pcs on my local LAN which worked perfectly fine

I've tried really hard and searched the internet for days now but I can't find a solution. Oh btw: The settings on the SoftEther Client Manager are all on default as far as I know (haven't changed anything special in there).

I'm happy to provide all important settings / screenshots of the server and / or client configs if needed. Would appreciate any help :)

[solo]

Hi, this is an interesting case.

- Just to be sure: I've tried this game / mode on two pcs on my local LAN which worked perfectly fine

Disconnect one of them from the LAN and connect via a mobile/cell hotspot, while the other connects with SE LAN address. If it still works for you, we'll focus on your friend's PC next.

[deb_user]

Hi, this is an interesting case.

Disconnect one of them from the LAN and connect via a mobile/cell hotspot, while the other connects with SE LAN address. If it still works for you, we'll focus on your friend's PC next.

Thank you for your response. I've already tried that with no luck. The weird thing is that if I connect both LAN pc's with SE LAN address it's the same behavior, I can see the Host (no matter who creates it) but can't connect. This tells me that it has to be something on SE / the connection itself that is blocking, either Server or Client if I'm not completely wrong.

[solo]

You have not mentioned anything about the firewalls - disable both for a test.

[deb_user]

You have not mentioned anything about the firewalls - disable both for a test.

Oh yes sorry I totally forgot to mention that. I already did, it was one of the first things I did on both client pc's. I'm also able to ping each other.

[solo]

Would you be able to test it out on a Windows SE server? If you you don't have a spare PC/VM, simply install it on one of the SE client PCs and from it connect to localhost. The server's settings need to be identical to your Debian's. More info https://www.vpnusers.com/viewtopic.php?f=7&t=68129 and https://sites.google.com/view/softether-vpn-gaming

[deb_user]

Would you be able to test it out on a Windows SE server? If you you don't have a spare PC/VM, simply install it on one of the SE client PCs and from it connect to localhost. The server's settings need to be identical to your Debian's. More info https://www.vpnusers.com/viewtopic.php?f=7&t=68129 and https://sites.google.com/view/softether-vpn-gaming

Yes I was able to test according to your advice. I've set up Windows SE Server on a spare PC and imported my config from my Debian SE Server. The result sadly is exactly the same. No matter who is hosting the game, it's visible to all other VPN clients but no one is able to connect. I even tried adding a third VPN client on the Windows SE Server pc, but it's the same.
I also followed the specific steps on your provided links and changed some of the settings on my server according to them but also with no luck.