rdp connection fails if MTU is not adjusted

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nobody12
Posts: 139
Joined: Sat Feb 13, 2021 10:22 pm

rdp connection fails if MTU is not adjusted

Post by nobody12 » Mon Jan 10, 2022 6:15 pm

I have a strange problem (windows) when connecting to a rdp server (windows 10 rdp, not terminalserver):
If I am located in a foreign Wifi, then connect to the remote network using the SE client. Connection is ok, but I cannot access any host using rdp in the remote network.
If I use the mobile brodband adapter (built into the notebook) to connect to the VPN everythiing works.
The VPN is a L2 bridge VPN
I can fix this by lowering the MTU of the SE VPN adapter to 1290 (tested a ping with the DF flag and found that from 1300up the ping will not return).

What could be the reason behind this?

solo
Posts: 1228
Joined: Sun Feb 14, 2021 10:31 am

Re: rdp connection fails if MTU is not adjusted

Post by solo » Wed Jan 12, 2022 1:24 am

The RDP MTU issue can be fixed by disabling URCP but you also have MTU ping problems. Perhaps a global URCP-off can fix it too.

nobody12
Posts: 139
Joined: Sat Feb 13, 2021 10:22 pm

Re: rdp connection fails if MTU is not adjusted

Post by nobody12 » Wed Jan 12, 2022 7:42 pm

Thank you!
I never heard about this issue, but now found several posts in the internet about it.
What I do not understand: global URCP turn-off: Does this effect other services besides RDP, and, how can I turn this off global?

solo
Posts: 1228
Joined: Sun Feb 14, 2021 10:31 am

Re: rdp connection fails if MTU is not adjusted

Post by solo » Wed Jan 12, 2022 9:30 pm

URCP is enabled by default for all applications. Disabling it for RDP-only is documented. I suppose there may be a way to turn it off 'globally' but it is my speculation.

PH-IT
Posts: 26
Joined: Tue Jan 18, 2022 9:47 am
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Re: rdp connection fails if MTU is not adjusted

Post by PH-IT » Sun Jan 23, 2022 4:39 pm

Can anyone please discribe where exactly to modify the MTU setting?

Destination server, connecting client, SE SecureNAT settings, SE bridge interface? All?
Which seams suiteable or reasonable?

Please let us know.

solo
Posts: 1228
Joined: Sun Feb 14, 2021 10:31 am

Re: rdp connection fails if MTU is not adjusted

Post by solo » Sun Jan 23, 2022 9:36 pm

on RDP server PC:

Code: Select all

REG ADD "HKLM\SOFTWARE\Microsoft\Terminal Server Client" /v UseURCP /t REG_DWORD /d 0 /f

nobody12
Posts: 139
Joined: Sat Feb 13, 2021 10:22 pm

Re: rdp connection fails if MTU is not adjusted

Post by nobody12 » Mon Jan 24, 2022 8:10 am

in my case I had to change the MTU on the client (see first post).

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