If the Ubuntu box is active directory domain joined for user authentication, the domain users cannot authenticate without changes to the sssd.conf file.
sudo -i
cd /etc/sssd
nano sssd.conf
add 3 lines
case_sensitive = False
ad_gpo_access_control = enforcing
ad_gpo_map_remote_interactive = +xrdp-sesman
restart sssd
systemctl restart sssd
For domain user name use the full domain in this format.
user@nameof.domain
The usual instructions to connect from Windows via RDP to Ubuntu failed on18.0.4. The problem is a dependency in a needed package has been broken in the installation of XRDP on Ubuntu. The fix is to install xorgxrdp manually but it is missing dependencies so you have to install xserver-xorg-core first. However, that also uninstalls some packages needed for mouse and cursor control in an RDP session so you add them back with xserver-xorg-input-all. After that, you can install xorgxrdp manually and the connection works.
The web page with write up on details is: https://c-nergy.be/blog/?p=13390
The broken dependencies in XRDP seem to have been corrected in the 20.04 install.
Still some flakey stuff about color scheme authorization when you connect but that’s more of a nuisance.
Update 1 Aug 2021
Found the web site which correctly address the color scheme authorization problem. I used it for Ubuntu 20.04 since I had the same problem with it as I did with 18.0.4.
Xrdp – Connect Ubuntu Linux Remote Desktop via RDP from Windows
One gottcha is you have to log out of Ubuntu before you attempt the remote desktop, otherwise all you get is a blank screen. Likewise logout of Ubuntu when exiting the RDP session, otherwise you get a blank screen when trying to login in to the physical box. Reboot is necessary to clear it. Might be a command line session you can do from putty but rebooting is quick and easy.
For Raspberry Pi, the process is much easier. Just install XRDP. The connection spawns a new window so you can have one window on the Raspberry Pi while working on a completely different window in the RPD session.
Update 2 Nov 2022
For Ubuntu 22.04 the problems have been addressed and installation is trivially easy.
Change to the root account for all commands to work.
sudo -i
install xrdp
apt install xrdp
Add xrdp to the certificate. Must be root to work.
usermod -a -G ssl-cert xrdp
Modify the firewall
ufw allow 3389/tcp
ufw allow 3389/udp
Logoff before trying RDP from windows. There will be 2 additional password authentications on connection to enable color pallet and color device management. This is for local users not AD users.
Active Directory authentication for RDP session
If the Ubuntu box is active directory domain joined for user authentication, the domain users cannot authenticate without some changes to the sssd.conf file.
sudo -i
cd /etc/sssd
nano sssd.conf
add 2 lines
ad_gpo_access_control = enforcing
ad_gpo_map_remote_interface = +xrdp-sesman
restart sssd
For domain user name use the full domain in the format
user@nameof.domain
Another option.
You can also go to settings, enable remote desktop, check enable legacy VNC protocol. This has slightly different login behavior with the authentication requests: one for color pallet and another for keyring unlock. However, the 2 procedures do not conflict and both can be active at the same time. You still get an xrdp login box under RDP.
I removed VNC and tried to connect with RDP and could not authenticate. RDP would not even connect to the remote machine unless I was logged in on the remote machine. This is xrdp behavior where you must be logged out to authenticate even though a connection can be made. From some posts, it appears this protocol is showing the same screen and the person logged in, not spawning a new one. After some more searching, this login failure appears to be a common problem with no definitive answer for now. Some people get it to work, others can’t.
Jerry's blog 