(closed) Configuring and using Xdmcp with gdm3 on ubuntu 18.04

Question

On a fresh minimal ubuntu 18.04 installation, I tried to activate and use the gdm3 xdmcp server.

Here are steps I followed:

• Installation

I used the normal installation.

"Download updates during installation" was checked

"Install 3rd parties for Gfx/Wifi/.." was not checked.

• Xdmcp server

1 . Edit /etc/hosts.allow to set (cf. Gnome doc.).

ALL: ALL #Wide opened to be sure the problem isn't here

2 . Edit /etc/gdm3/custom.conf to set:

[security] DisallowTCP=false [xdmcp] Enable=true

3 . Restart gdm

sudo /etc/init.d/gdm3 restart

• Xdmcp client

sudo apt install xnest Xnest -query localhost -geometry 1024x768 :1

Unfortunately, I only have a black screen with regular white flash.

Where did I do something wrong ?

Thanks for your help.

Christophe C.

Solution 1

Sorry to post many months after my initial question but I prefered to confirm my results.

Here they are:

Working solutions or workarrounds

• Debian 10.3 xdmcp access with lightdm & LxQT

   /usr/share/lightdm/lightdm.conf.d/50-remote-session.conf
  
   [SeatDefaults]
   xserver-allow-tcp=true
   [XDMCPServer]
   enabled=true
  

• Ubuntu 20.4 x2go access with LxQT

Not working solutions

• Ubuntu 20.4 xdmcp access with gdm3 & LxQT

   I did many tries (including the Pathin tip about wayland) under
   18.4 and now... 20.4.
  

• Ubuntu 20.4 xdmcp access with lightdm & LxQT

   I unsuccessfully tried the "lightdm.conf.d/50-remote-session.conf"
   change validated under debian.
  

Due to problems I had with xdmcp, I am now using different alternatives to connect remotly.

I hope my tests will help someone.

Solution 2

In case soemone else finds this question and has problems getting XDMCP to work with gdm3, I managed to get it working by setting gdm3 to use xorg instead of wayland.

/etc/gdm3/custom.conf

[daemon]
# Uncoment the line below to force the login screen to use Xorg
WaylandEnable=false

Comments

[Csabi Vidó]

Caution: »XDMCP is inherently insecure. XDMCP is not a ciphered protocol. This may allow an attacker to capture keystrokes entered by a user. XDMCP is vulnerable to man-in-the-middle attacks. This may allow an attacker to steal the credentials of legitimate users by impersonating the XDMCP server.« cisecurity.org CIS Ubuntu Linux Benchmark 20.04

[chris_asn]

Hello LiveWireBT, Thanks a lot for your remark. The initial plan was to use it thru an ssh tunnel but you are right, I could have the problem you describe locally. Since my question, I am using in parallel x2go for a machine and vnc over ssh for another. Both match my needs.

[chris_asn]

Hello Pathin, Thanks a lot for those information. I didn't yet have the time to test it but I will comment when it will be done.