How to find which screen (and thus port) the VNC UI for a kvm guest has? Or how to assign it

Is there a straightforward way to find the VNC screen (i.e. port number minus 5900) onto which a KVM guest is bound?

My guests are all configured to run with VNC enabled, but the order in which they occupy the ports is random.

Alternately, is there a way to assign them in the configuration (of guest or host), so that each respective guest will occupy a predefined port?!

Since you're using libvirt, you can just ask it!

root@onhost1:~# virsh list
 Id Name                 State
----------------------------------
  1 one-34               running
  2 one-36               running
  3 one-38               running

root@onhost1:~# virsh vncdisplay one-34
:34

root@onhost1:~# virsh vncdisplay 1
:34

(my particular correlation of name to VNC display port is due to the use of Open Nebula)

EDIT: Nowadays, you can use domdisplay to get the URI of the display whether it's VNC or Spice:

○ → virsh domdisplay win-michael
vnc://127.0.0.1:0

○ → for dom in $(virsh list --name); do echo -n "$dom: "; virsh domdisplay $dom; done
win-michael-m: vnc://127.0.0.1:0

for i in $(virsh -q list|awk '{print $2}'|sort); do
  echo -e "\033[01;31m$i\033[00m -> $(virsh vncdisplay $i)"
done

Also made it into a function that sorts output by port number:

function vnc-list
{
  for i in $(virsh -q list|awk '{print $2}'|sort); do
    PORTNUM=$(virsh vncdisplay $i|cut -f 2 -d ':')
    printf "% 2d: \033[01;32m%.20s\033[00m\n" "$PORTNUM" "$i";
  done | sort -n
}

I would run :

ps aux | grep "VM name/config"

Note the process ID and then

netstat -apn | grep "process ID"

This should show you are port open by that process.