design for xdm controlling a backend server in visualization cluster

[Brook Milligan]

It is becoming increasingly feasible to construct a visualization
cluster using one or more backend X servers.  For example, a small
cluster might involve one multiheaded X server, whereas a larger one
might involve a group of backend servers aggregated behind a single
Xdmx server.  In either case, it seems that xdm should be running on
the backend servers to manage the displays, create the window manager
processes, etc.

Currently, however, xdm cannot be used for this purpose because of its
login procedure, which requires a keyboard for login.  In a
visualization cluster, the backend servers will not have keyboards and
even if they did one does not want users to physically log into each
one individually.

It would seem that a solution to this would be to modify xdm to
optionally select a different greeter process.  This greeter process
would do the following (instead of displaying the login window and
waiting for the appropriate X events on the console):

- display a configurable image or set of images
- listen for connections from (possibly preconfigured) clients
- authenticate the connection
- optionally allow for transfer of a cookie for server authentication
  (i.e., provide another channel for inserting/removing cookies in
  $HOME/.Xauthority, so that xauth need not be run separately)
- provide a mechanism for verifying that the connection persists so
  that the X session can be closed (and optionally the cookie removed)
  upon disconnection

This greeter would interact with a client running on the frontend so
that users could initiate authenticated connections to the set of
backend servers in the visualization cluster.

Questions:

- Does this seem like an appropriate design for accomplishing the task
  of providing connections to a backend server in a visualization
  cluster?

- Are there alternative designs or prior art that should be
  considered?

- How can this design be improved and completed?

- Would X.org be interested in modifications to xdm that achieved this
  functionality?

- Are there any developers who would be interested in implementing or
  guiding the implementation of this functionality?

I look forward to any thoughts, suggestions, and assistance that you
might offer.

Cheers,
Brook