CentOS 6: Adjusting the remote desktop resolution without a monitor being connected

If you remote desktop share into a CentOS 6 system without a monitor being connected (i.e. a headless server) the screen resolution defaults to a tiny 800×600. Unfortunately the upstream provider (Redhat) has declined to provide system-config-display for EL6 (see this bug report), making it impossible to set the screen resolution in CentOS6 to something more useable as you would if you had a monitor attached.

The workaround is to use xrandr in a script to change the screen size. Here are the steps involved.

1. Open a terminal window and type xrandr. This will list your various screen devices.


You should get something like this

Screen 0: minimum 320 x 200, current 800 x 600, maximum 8192 x 8192
VGA-0 disconnected  (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)

2. Type cvt with the screen dimensions you want (e.g. 1920 x 1200).

cvt 1920 1200

This should output something like this. Copy everything after “Modline”.

# 1920x1200 59.88 Hz (CVT 2.30MA) hsync: 74.56 kHz; pclk: 193.25 MHz
Modeline "1920x1200_60.00"  193.25  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync

3. Create a new shell script to call xrandr. This script will use xrandr to create a new mode, add the new mode to the screen device, and finally sets the screen output to the device using this mode.

nano setscreen.sh

Paste the copied cvt output information to after the  xrandr newmode line. Adjust the xrandr addmode line to be the name of your device (e.g. VGA-0) and the name of the new mode (e.g. 1920x1200_60.00) . Modify the xrandr output line to be this new mode. Your script should look something like this.

xrandr --newmode "1920x1200_60.00" 193.25 1920 2056 2256 2592 1200 1203 1209 1245 -hsync +vsync
xrandr --addmode VGA-0 1920x1200_60.00
xrandr --output VGA-0 --mode 1920x1200_60.00

Save the script (control-x).

4. Change the script’s ownership and permissions to root and make it executable.

sudo chown root setscreen.sh
sudo chgrp root setscreen.sh
sudo chmod 755 setscreen.sh

5. Run the script to see if it works correctly.

sudo ./setscreen.sh

This should change the remote screen resolution to your desired size (e.g. 1920×1200). It can take a second or two to adjust.

6. To avoid having to run this script manually every time you log into the server, copy it into the /etc/X11/xinit/xinitrc.d/ directory.

sudo cp setscreen.sh /etc/X11/xinit/xinitrc.d/

Now that was so easy Redhat.

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