Tue 26 April 2005
By markmc misc

Session Migration With GDM

Last year Caolan
and I demoed hotdesking with GDM and VNC. Owen later pointed out that VNC
probably wasn't the way to go once the rendering
improvements they're working come on line.

So, last week I picked up a patch I'd hacked up before Christmas,
finished it off and committed it to GDM. The idea is to do the same
thing as the VNC patch, but this time using a X proxy (like Xnest)
server on the terminal server instead of a VNC server.

Specifically, though, the features added to GDM are:

  • You can now configure GDM to run XDMCP sessions on a local X
    proxy server. This may be useful on its own for performance reasons;
    in theory, at least, an X proxy server should be able to limit the
    number of roundtrips it makes to the remote X server since if all you
    want to do is query server state, then that state is local. I've no
    idea yet how well Xnest and others do on this in practice, though.
  • If the proxy server supports disconnecting from its parent
    display and re-connecting later, you can configure GDM such that you
    can disconnect from your session and reconnect later simply by logging
    back in. The only proxy server's that I know of which support this is
    the DMX X server and NoMachine NX's nxagent. Its
    certainly possible to do this with any proxy though; I had it half
    done for Xnest before realizing DMX had good enough support to get the
    GDM patch done.

I've played around a little today with NoMachine's proxy. You can try
it out with GDM HEAD up by:

  1. Install NoMachine's server package
  2. Set xdmcp/EnableProxy=true in gdm.conf
  3. Download these scripts (run-nxagent.sh
    and reconnect-nxagent.sh)
    and stick them in /tmp
  4. Set xdmcp/ProxyXServer to
    ProxyXServer=/tmp/run-nxagent.sh -audit 0 -name NX -geometry 768x576 and xdmcp/ProxyReconnect to
    /tmp/reconnect-nxagent.sh
  5. Re-start GDM
  6. From another machine run X -query $server and login
    through the login screen
  7. Run /tmp/reconnect-nxagent.sh --to :20 on the server to
    disconnect your remote X server from the session
  8. Run X -query $server again on the server, login and you
    should be immediately re-directed to your original session
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