One user with multiple sessions reconnecting to a specific session

[lcniel]

I have a weird edge case that I'd like to support in an HPC context, for purely administrative reasons.

1. User launches session 1 on Machine 1.
2. User launches session 2 on Machine 1.
3. User disconnects from session 1 and reconnects to it (not to session 2).
4. User disconnects from session 2 and reconnects to it (not to session 1).

Can I achieve this kind of sandboxing in any way or does systemd make it impossible? I am fine with running multiple instances of xrdp on a different port for each connection, using unique tokens to reconnect, etc.

[matt335672]

Have a look at systemd --user, and how it works.

Each user can only have at most one $DISPLAY variable set.

You can look at non systemd-based systems, but even there, xrdp doesn't have a lot of options for connecting to a particular session. Any ideas we had along those lines sort of disappeared with systemd.

[lcniel]

Thanks! I actually just tried using UBD Policy and connecting with two different initial screen sizes. And that seems to work with no issue - I can easily disconnect and reconnect with xfce4 and there are no obvious issues with running both of those concurrently. So probably this works with XFCE.

I think I can use this as a workaround for the time being (although it's a bit of a silly hack to use the size in this way), but I think the most natural way for me to manage this going forward would be to have a policy that keeps track of the port of the xrdp process so that one can handle this seamlessly by just starting up several xrdp processes with different ports. That's basically how you handle it with VNC as far as I am aware. So that xrdp-sesadmin -c=list would print something like

[leoniel@rdp-test ~]$ xrdp-sesadmin -c=list
Session ID: 547302
        Display: :10
        User: leoniel
        Session type: Xorg
        RDP port: 3389
        Screen size: 644x480, color depth 24
        Started: Wed May  7 14:59:58 2025
        Start IP address: ::ffff:10.51.1.185
Session ID: 548175
        Display: :11
        User: leoniel
        Session type: Xorg
        RDP port: 3390
        Screen size: 800x600, color depth 24
        Started: Wed May  7 15:00:19 2025
        Start IP address: ::ffff:10.51.1.185

[matt335672]

You could just use a single xrdp server listening on multiple ports using something like port=tcp://:3389 tcp://:3390 in the config (off the top of my head). The listening port could then maybe be a discriminator which could be passed to the Policy.

[lcniel]

You could just use a single xrdp server listening on multiple ports using something like port=tcp://:3389 tcp://:3390 in the config (off the top of my head). The listening port could then maybe be a discriminator which could be passed to the Policy.

In isolation one could do that, but I need to make this work with Slurm spinning up and shutting down "N" of these as user services (theoretically up to 64 per machine), this will not be a static server or anything, and it will run many types of other workloads. I'd then run a single session manager as a system service at all times, this seems to work when testing.

In either case whether xrdp would listen to multiple ports or multiple concurrent sessions would each use one port is secondary to the possibility of using the port as a discriminator, which would be really useful. I could see other ways of leveraging this as well (e.g. forwarding connections with iptables and Wireguard to specific session).

[matt335672]

We can work out a way to pass the server port and/or IP address through to sesman - it could then be used as a discriminator.

Are you able to build xrdp from source? If so we can prototype something between us. I'm off-net for the next 10 days or so BTW.

[lcniel]

We can work out a way to pass the server port and/or IP address through to sesman - it could then be used as a discriminator.

Nice!

Are you able to build xrdp from source? If so we can prototype something between us.

Sure, I have built RPM:s of it from source at least, so that shouldn't be a big problem. We mostly run Rocky 8 and 9, and I'm mostly interested in running on Rocky 9, but I can spin up VM:s if needed to test on other operating systems as well.

I'm off-net for the next 10 days or so BTW.

OK, no problem, I will be available and probably have plenty of time during the summer -- things will probably slow down here and my vacation is not until September.