I wrote this to use Teamviewer on a remote machine without a graphics-card but do want to use high resolutions. I wanted to use full-screen on my 21:9 shinyness which does 3440x1440. However I couldn’t get past 1324x768 with normal configs. Hard pressed to get it working I decided to sacrifice an evening to it. It now works, and it’s gorgeous :)

On Debian Jessie this file was found at /usr/share/X11/xorg.conf.d/xorg.conf

Other packages needed were: xserver-xorg-video-dummy and Teamviewer.

Sample of Teamviewer showing the new available resolutions: Teamviewer preview

# Xorg.conf config for dummy video driver
# For usage with for example TeamViewer on a machine without a monitor attached
# and you wanted more then just 1024x768 ;)
#
# Use at own risk, loosly based on info scattered around but these links really helped
# http://arachnoid.com/modelines/ for the modelines (lot of trial and error to figure out which worked over Teamviewer and Xorg)
# https://www.xpra.org/xorg.conf sample config from xpra who seems to use the dummy driver a lot (thanks guys!)

Section "Device"
	Identifier "dummy_videocard"
	Option "NoDDC" "true"
	Option "IgnoreEDID" "true"

	# Debian: apt-get install xserver-xorg-video-dummy
	Driver "dummy"

	# You could lower this to suit your needs, however you will need
	# a quite high amount to run such crazy resolutions
	VideoRam 524288
EndSection

Section "Monitor"
	Identifier "dummy_monitor"

	# 16:9 4k
	Modeline "3840x2160_20.00" 218.15 3840 4016 4416 4992 2160 2161 2164 2185

	# 21:9 "4k"
	Modeline "3440x1440_20.00" 124.95 3440 3520 3864 4288 1440 1441 1444 1457

	# Usual 27" suspect before 4k era
	Modeline "2560x1440" 42.12 2560 2592 2752 2784 1440 1475 1478 1513

	# Oddball 1920 resolution
	Modeline "1920x1440" 69.47 1920 1960 2152 2384 1440 1441 1444 1457

	# 16:10 "Full-HD"
	Modeline "1920x1200" 26.28 1920 1952 2048 2080 1200 1229 1231 1261

	# 16:9 Full HD
	Modeline "1920x1080" 23.53 1920 1952 2040 2072 1080 1106 1108 1135

	# These are just crazy high to ensure stuff works
	HorizSync   5.0 - 1000.0
	VertRefresh 5.0 - 1000.0
EndSection

Section "Screen"
	Identifier "dummy_screen"
	Device "dummy_videocard"
	Monitor "dummy_monitor"
	DefaultDepth 24
	SubSection "Display"
		Depth 24
		Modes "3840x2160_20.00" "3440x1440_20.00" "2650x1440" "1920x1440" "1920x1200" "1920x1080"

		# Not sure why, but 3440x1440 won't work when the Virtual is set to "3840 2160"
		# However it will complain in the Xorg.log when you didn't comment out the 3840x2160 resolution at the top
		#
		# Uncomment this for 21:9 4k
		Virtual 3440 1440

		# Uncomment this for 16:9 4k
		#Virtual 3840 2160
	EndSubSection
EndSection