Investigate why UDP connections fail with remote desktop using Coder Connect and our dogfood template

[spikecurtis]

Template: https://dev.coder.com/templates/coder/windows-with-good-rdp/docs

When we connect using Coder Connect, it seems to upgrade to UDP as the protocol, and then give a blank screen.

[Window Title]
Remote Desktop Connection

[Content]
Your connection round trip time is high. You may experience issues. UDP is enabled.

[^] Hide details  [OK]

[Expanded Information]
Timestamp (UTC): 05/05/25 10:36:11 AM
Activity ID: 6fa698e5-c6e2-4b9b-9bff-ad94f3ee0100

[Client details]
Client version: 10.0.26100.3775 (x64)
Local OS: Windows 10 Pro x64 (10.0, Build 26100)

[Network details]
Transport protocol: UDP
Round-trip time: 130 ms
Available bandwidth: 75.03 Mbps
Frame rate: 0 FPS

[Remote computer details]
Remote session type: Remote desktop
Gateway name: Not in use
Gateway logon method: Not in use
Remote computer: web-rdp.coder

Press Ctrl+C to copy.

[spikecurtis]

Workaround is to set a registry key on the client as described here: https://zomro.com/blog/faq/429-dissable-udp-for-rdp-on-windows

[spikecurtis]

We can disable this server side by setting

key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services
name: SelectTransport
type: REG_DWORD
value: 1

[spikecurtis]

UDP connections fail with Coder Connect because the Windows RDP Terminal Server sends packets larger than can be transmitted over the Coder tailnet tunnel. This is in spite of the fact that the MTU is supposedly capped at 1232 and negotiated over the protocol. c.f. https://winprotocoldoc.z19.web.core.windows.net/MS-RDPEUDP/%5bMS-RDPEUDP%5d.pdf for details

1232 makes a lot of sense, given that a UDP header is 8 bytes and an IPv6 header is 40 bytes, for a grand total of 1280, the minimum tx size for links that can run IPv6. (This is the MTU of the Coder Connect interface.) However, via Wireshark captures, we observe the Windows RDP terminal server sending packets with UDP payloads of 5-15 bytes larger than this

[spikecurtis]

It seems Microsoft has either deliberately removed or inadvertently broken RDP over UDP in Windows 11 24H2 and Server 2025.

https://www.reddit.com/r/sysadmin/comments/1g7gaxq/server_2025_rdp_only_using_tcp/

https://learn.microsoft.com/en-us/answers/questions/2190047/host-will-not-use-udp-for-rdp-on-24h2?forum=windowsclient-all&referrer=answers

I've confirmed with a Windows Server 2025 workspace. Out of the box, I can connect to it via RDP over Coder Connect. Capturing packets, I can see the client attempts to make a UDP connection but the server just never responds over UDP, only TCP.