How to set up a headless VNC on Slackware
 

On slackware one can use runlevel 3 for a multi-user console based UI, or runlevel 4 for a multi-user X11 based GUI.

One can set up (fairly easy, with distro available packages) a VNC server inside a logged in X11 session.

Also, X11 forwarding over the network is not useful because it still requires a local X11 login and programs displayed remotely close when the client exists.

My question, and problem, is as follows:

• a system running most often without a monitor (ie server)
• the physical monitor shows console interface only
• GUI with login manager available via VNC

The purpose is to have the server administration on console level and no interference from unneeded GUI (specifically no X11 on physical monitor),
and
have the ability to remotely connect to the server into X11 in such a way that it does not require X11 on the physical monitor and programs running in the GUI do not terminate when client disconnects (like happens with remote display by X11_forwarding).

Thank you

Just use some vnc server. tigervnc is in extra.
Start vncserver from console or ssh session and connect with vncviewer.
I use this on my server which doesn't have any GPU.

Can it be done to work unattended (so that vncserver runs at startup, instead of from the ssh session, so no need for ssh session)?

Yes, just put something like this in /etc/rc.d/rc.local:
/bin/su -l USERNAME -c "vncserver"

But first run it manually to make sure it works and set password.

But how about running an X11 with display via VNC, unattended, such that it does not run as a specific user, but rather it displays the logni prompt like XDM ?

Not true. x11 clients on the remote computer can use the display on your computer; no need for a display at their end. But if you don't want to use SSH you need to find some other way of setting the display explicitly at the remote PC:
or let SSH handle that for you automatically
SSH is the most convenient option for x11 and VNC. I use TightVNC through a SSH tunnel and there's no need for X running on the remote computer; TightVNC will set up a virtual framebuffer for you when you connect. I think it should be possible as well to start the vncserver in tmux so it continues to run even when you close the SSH session.

I could be wrong but I think you would need X running on the console to do that, and Karl Runge's x11vnc sever, which attaches to a real display.

It should be possbile if you put something like that in inetd.conf:

I didn't test this, but accrding to man page it should work.

The above solution does not allow client to disconnect without closing the remotely running programs, which is not good.

I will look into the suggested x11vncserver.