Xdmx - Failure (again)

Xdmx ("Distributed Multi-head X server") allows any networked screen that can run X to act as a secondary monitor to any other computer on the network. That means you can use an old laptop as a secondary display for your primary computer. Cool, huh? Except ... I've tried about three times in the last decade (2009, 2011, most recently tonight) to get it running, and have failed every time. Here are the steps so far:

  • install 'xorg-x11-server-Xdmx' on Fedora systems, and 'xdmx' (and possibly 'xdmx-tools') on Debian systems
  • two machines, both must be running X
  • theoretically, you can open port 6000 on one and communicate through that ... but it's an unencrypted protocol so I don't want to use it and no one recommends it
  • let's call the two machines "master" and "slave", where "master" is the better machine and the one you want to use the keyboard and mouse on
  • on slave, ssh -X giles@master
  • in that terminal, run:
$ export grep DISPLAY
declare -x DISPLAY="localhost:11.0"

You're more likely to see "localhost:10.0", but make note of whichever it is. - on master I run that command and I get 'DISPLAY=":0"', so although most sources say you should use ":0.0" as your first display, I'm trying both - then run - in that same ssh session on slave that's connected to master - startx -- /usr/bin/Xdmx :1 +xinerama -display :0 -display localhost:11.0 -norender -noglxproxy

Here are my two attempts:

$ startx -- /usr/bin/Xdmx :1 +xinerama -display :0.0 -display localhost:11.0 -norender -noglxproxy
xauth:  file /home/giles/.serverauth.1608 does not exist

(II) dmx: Generation:         1
(II) dmx: DMX version:        1.2.20070424 (DMX Project)
(II) dmx: DMX Build OS:        4.9.3-200.fc25.x86_64 ()
(II) dmx: DMX Build Compiler: gcc 6.3.1
(II) dmx: DMX Execution OS:   Linux 4.12.11-200.fc25.x86_64 #1 SMP Fri Sep 8 11:44:51 UTC 2017
(II) dmx: DMX Execution Host: toshi7
(II) dmx: MAXSCREENS:         16
(II) dmx: Using configuration from command line
(II) dmx: Added :0.0 at 0 0
(II) dmx: Added localhost:11.0 right of :0.0
(II) dmx[o0/:0.0]: No Xdmx server running on backend
(II) dmx[o0/:0.0]: Name of display: :0.0
(II) dmx[o0/:0.0]: Version number:  11.0
(II) dmx[o0/:0.0]: Vendor string:   Fedora Project
(II) dmx[o0/:0.0]: Vendor release:  11903000
(II) dmx[o0/:0.0]: Dimensions:      2048x1920 pixels
(II) dmx[o0/:0.0]: 7 depths on screen 0:  24,1,4,8,15,16,32
(II) dmx[o0/:0.0]: Depth of root window:  24 planes (24)
(II) dmx[o0/:0.0]: Number of colormaps:   1 min, 1 max
(II) dmx[o0/:0.0]: Options: backing-store when mapped, save-unders no
(II) dmx[o0/:0.0]: Window Manager running: yes
(**) dmx[o0/:0.0]: Window manager running -- colormaps not supported
(II) dmx[o0/:0.0]: 2048x1920+0+0 on 2048x1920 at depth=24, bpp=32
(II) dmx[o0/:0.0]: 0x68 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff *
(II) dmx[o0/:0.0]: 0x69 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xf8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xf9 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xfa TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xfb TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xfc TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xfd TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xfe TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xff TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x100 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x101 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x102 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x103 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x104 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x105 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x106 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x107 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x108 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x109 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10a DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10b DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10c DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10d DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10e DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x10f DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x110 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x111 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x112 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x113 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x114 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x115 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x116 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x117 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x118 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x119 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x11a DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x11b DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0x11c DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: 0xb7 TrueColor   32b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: No Xdmx server running on backend
(II) dmx[o1/localhost:11.0]: Name of display: localhost:11.0
(II) dmx[o1/localhost:11.0]: Version number:  11.0
(II) dmx[o1/localhost:11.0]: Vendor string:   The X.Org Foundation
(II) dmx[o1/localhost:11.0]: Vendor release:  11902000
(II) dmx[o1/localhost:11.0]: Dimensions:      3046x1050 pixels
(II) dmx[o1/localhost:11.0]: 7 depths on screen 0:  24,1,4,8,15,16,32
(II) dmx[o1/localhost:11.0]: Depth of root window:  24 planes (24)
(II) dmx[o1/localhost:11.0]: Number of colormaps:   1 min, 1 max
(II) dmx[o1/localhost:11.0]: Options: backing-store when mapped, save-unders no
(II) dmx[o1/localhost:11.0]: Window Manager running: yes
(**) dmx[o1/localhost:11.0]: Window manager running -- colormaps not supported
(II) dmx[o1/localhost:11.0]: 3046x1050+0+0 on 3046x1050 at depth=24, bpp=32
(II) dmx[o1/localhost:11.0]: 0x21 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff *
(II) dmx[o1/localhost:11.0]: 0x22 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc7 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc9 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xca TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcb TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcc TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcd TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xce TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcf TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd0 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd1 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd2 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd3 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd4 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd5 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd6 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd7 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd9 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xda DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdb DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdc DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdd DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xde DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdf DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe0 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe1 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe2 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe3 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe4 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe5 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe6 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe7 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe8 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe9 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xea DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xeb DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0x86 TrueColor   32b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0.0]: DPMS 1.1 (on, enabled, 0 0 0)
(EE)
(EE) Backtrace:
(EE) 0: /usr/bin/Xdmx (OsLookupColor+0x139) [0x55b089]
(EE) 1: /lib64/libpthread.so.0 (funlockfile+0x50) [0x7f483f10061f]
(EE) 2: /usr/bin/Xdmx (TimerForce+0x10) [0x555450]
(EE) 3: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x5211) [0x4344b1]
(EE) 4: /usr/bin/Xdmx (_start+0x4513) [0x425ad3]
(EE) 5: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x41aa) [0x4323ca]
(EE) 6: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x460b) [0x432aab]
(EE) 7: /usr/bin/Xdmx (AddScreen+0xd7) [0x521107]
(EE) 8: /usr/bin/Xdmx (InitOutput+0x406) [0x429256]
(EE) 9: /usr/bin/Xdmx (InitFonts+0x216) [0x524ca6]
(EE) 10: /lib64/libc.so.6 (__libc_start_main+0xf1) [0x7f483ed49431]
(EE) 11: /usr/bin/Xdmx (_start+0x2a) [0x41d1ba]
(EE)
(EE) Segmentation fault at address 0x0
(EE)
Fatal server error:
(EE) Caught signal 11 (Segmentation fault). Server aborting
(EE)
xinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error
Couldn't get a file descriptor referring to the console

The second attempt:

$ startx -- /usr/bin/Xdmx :1 +xinerama -display :0 -display localhost:11.0 -norender -noglxproxy
xauth:  file /home/giles/.serverauth.1665 does not exist

(II) dmx: Generation:         1
(II) dmx: DMX version:        1.2.20070424 (DMX Project)
(II) dmx: DMX Build OS:        4.9.3-200.fc25.x86_64 ()
(II) dmx: DMX Build Compiler: gcc 6.3.1
(II) dmx: DMX Execution OS:   Linux 4.12.11-200.fc25.x86_64 #1 SMP Fri Sep 8 11:44:51 UTC 2017
(II) dmx: DMX Execution Host: toshi7
(II) dmx: MAXSCREENS:         16
(II) dmx: Using configuration from command line
(II) dmx: Added :0 at 0 0
(II) dmx: Added localhost:11.0 right of :0
(II) dmx[o0/:0]: No Xdmx server running on backend
(II) dmx[o0/:0]: Name of display: :0
(II) dmx[o0/:0]: Version number:  11.0
(II) dmx[o0/:0]: Vendor string:   Fedora Project
(II) dmx[o0/:0]: Vendor release:  11903000
(II) dmx[o0/:0]: Dimensions:      2048x1920 pixels
(II) dmx[o0/:0]: 7 depths on screen 0:  24,1,4,8,15,16,32
(II) dmx[o0/:0]: Depth of root window:  24 planes (24)
(II) dmx[o0/:0]: Number of colormaps:   1 min, 1 max
(II) dmx[o0/:0]: Options: backing-store when mapped, save-unders no
(II) dmx[o0/:0]: Window Manager running: yes
(**) dmx[o0/:0]: Window manager running -- colormaps not supported
(II) dmx[o0/:0]: 2048x1920+0+0 on 2048x1920 at depth=24, bpp=32
(II) dmx[o0/:0]: 0x68 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff *
(II) dmx[o0/:0]: 0x69 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xf8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xf9 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xfa TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xfb TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xfc TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xfd TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xfe TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xff TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x100 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x101 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x102 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x103 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x104 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x105 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x106 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x107 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x108 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x109 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10a DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10b DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10c DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10d DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10e DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x10f DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x110 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x111 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x112 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x113 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x114 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x115 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x116 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x117 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x118 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x119 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x11a DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x11b DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0x11c DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: 0xb7 TrueColor   32b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: No Xdmx server running on backend
(II) dmx[o1/localhost:11.0]: Name of display: localhost:11.0
(II) dmx[o1/localhost:11.0]: Version number:  11.0
(II) dmx[o1/localhost:11.0]: Vendor string:   The X.Org Foundation
(II) dmx[o1/localhost:11.0]: Vendor release:  11902000
(II) dmx[o1/localhost:11.0]: Dimensions:      3046x1050 pixels
(II) dmx[o1/localhost:11.0]: 7 depths on screen 0:  24,1,4,8,15,16,32
(II) dmx[o1/localhost:11.0]: Depth of root window:  24 planes (24)
(II) dmx[o1/localhost:11.0]: Number of colormaps:   1 min, 1 max
(II) dmx[o1/localhost:11.0]: Options: backing-store when mapped, save-unders no
(II) dmx[o1/localhost:11.0]: Window Manager running: yes
(**) dmx[o1/localhost:11.0]: Window manager running -- colormaps not supported
(II) dmx[o1/localhost:11.0]: 3046x1050+0+0 on 3046x1050 at depth=24, bpp=32
(II) dmx[o1/localhost:11.0]: 0x21 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff *
(II) dmx[o1/localhost:11.0]: 0x22 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc7 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xc9 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xca TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcb TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcc TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcd TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xce TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xcf TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd0 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd1 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd2 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd3 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd4 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd5 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd6 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd7 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd8 TrueColor   24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xd9 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xda DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdb DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdc DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdd DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xde DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xdf DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe0 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe1 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe2 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe3 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe4 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe5 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe6 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe7 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe8 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xe9 DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xea DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0xeb DirectColor 24b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o1/localhost:11.0]: 0x86 TrueColor   32b 8b/rgb 256 0xff0000 0xff00 0x00ff
(II) dmx[o0/:0]: DPMS 1.1 (on, enabled, 0 0 0)
(EE)
(EE) Backtrace:
(EE) 0: /usr/bin/Xdmx (OsLookupColor+0x139) [0x55b089]
(EE) 1: /lib64/libpthread.so.0 (funlockfile+0x50) [0x7f7a722b061f]
(EE) 2: /usr/bin/Xdmx (TimerForce+0x10) [0x555450]
(EE) 3: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x5211) [0x4344b1]
(EE) 4: /usr/bin/Xdmx (_start+0x4513) [0x425ad3]
(EE) 5: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x41aa) [0x4323ca]
(EE) 6: /usr/bin/Xdmx (RemoveInputDeviceTraces+0x460b) [0x432aab]
(EE) 7: /usr/bin/Xdmx (AddScreen+0xd7) [0x521107]
(EE) 8: /usr/bin/Xdmx (InitOutput+0x406) [0x429256]
(EE) 9: /usr/bin/Xdmx (InitFonts+0x216) [0x524ca6]
(EE) 10: /lib64/libc.so.6 (__libc_start_main+0xf1) [0x7f7a71ef9431]
(EE) 11: /usr/bin/Xdmx (_start+0x2a) [0x41d1ba]
(EE)
(EE) Segmentation fault at address 0x0
(EE)
Fatal server error:
(EE) Caught signal 11 (Segmentation fault). Server aborting
(EE)
xinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error
Couldn't get a file descriptor referring to the console

I can think of a long list of possible reasons for the failure:

  • I'm running Fedora on one machine and Debian on the other
  • I'm running different versions of the libraries involved on the different machines (libpthread.so.0 and libc.so.6 are both visibly mentioned in the errors)
  • I'm running different versions of Xdmx itself
  • I'm running an odd setup on my primary/master, with two screens of different sizes
  • I'm running Openbox on one machine and Fluxbox on the other

But ... having searched repeatedly over the years online, my suspicion is that while the package is bundled with OSes, it's not actually maintained. Just because it successfully compiles doesn't mean it actually works when you run it. And it's apparently not running for anyone - I can't find documentation of a successful run past 2007, only bug reports very similar to what you see above.

I turned to ArchWiki's documentation. You know you're in trouble when you get to ArchWiki. Not that their documentation is bad - in fact it's fairly routinely the best there is. But if you end up there, you've exhausted all of the easy solutions and you're grinding: you're really digging in and it's going to take a while. Their page says "At present either Xdmx itself or the current state of window managers do not work well together for complex arrangements of multihead set-ups; the server tends to crash as soon as window drawing is required after the integrated desktop has been established."

I may have been driven to pessimism by my multiple failures. Let's try again with two laptops both with the same OS and window manager on them and no extra screens (of course part of the reason I want Xdmx is to have lots of screens, but you have to start somewhere). Huh - two laptops, both with Debian 9 and all recent updates, both running Fluxbox ... very similar failure. I saw a recommendation to use TWM - I installed it on both, same errors. Makes me feel a lot more certain about my diagnosis that this is abandonware. It shouldn't be shipped with distros anymore.