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Adding and setting a custom resolution using xrandr

This guide details how to add and set a new custom screen resolution with xrandr. The inspiration for this post was the fact that this is a recurring question on help forums and have written this explanation up as comments multiple times, but have also gotten this question from multiple coworkers as they were setting up VMs for frontend testing.

The problem

Basically, when you want to go full screen in a VM, you might want to set such a resolution that isn’t one of the currently available options. E.g. 1920x1080 is often not available by default on a freshly installed debian in a VM. We will be using this scenario going forth, but aside the note specific to gnome the tips are applicable in general.

A note about wayland and Gnome

As established above, if you have installed debian with default settings, you probably have gnome + wayland installed. In this case, make sure you switch to X because there is no simple way to add a custom resolution with Gnome and Wayland at the time of writing this guide.

Anyway, to make the switch to X, become root and open the following file:

/etc/gdm3/daemon.conf

Find the following line:

#WaylandEnable=false

Uncomment it, save and exit, and then reboot.

Only after that, can you use the rest of the guide below.

Before you start, you need to make sure that you are running X. If it is not running, you will get “Can’t open display output” errors.

Finding and testing the right settings

To begin, we need to identify the primary connected monitor. To do that, execute:

xrandr --listmonitors

And look for the entry with the * symbol. This is the monitor/display we want to apply new settings to. In my case, this is Virtual1.

Next we want to get the modeline string for the resolution we desire.

For that, execute the following command:

gtf 1920 1080 60

It’s beyond the scope of this tutorial to explain in detaila about gtf, but the gist of it is that the string it produces is something that is compatible to be added into the xorg.conf file, as well as compatible for use with the xrandr command. So the output of the above command should give you some result like…

Modeline "1920x1080_60.00" 172.80 1920 2040 2248 2576 1080 1081 1084 1118 -HSync +Vsync

Copy out this string with the exception of the word Modeline. We will use xrandr to get the system to recognize these display parameters.

So now execute:

xrandr --newmode "1920x1080_60.00"  172.80  1920 2040 2248 2576  1080 1081 1084 1118  -HSync +Vsync

Actually the part enclosed in the double quotes is just the alias/name of the display setting, so you can call it anything you want, but it’s wise to keep it something understandable.

Now we can apply this mode to our display. In my case xrandr --listmonitors returned Virtual1 as my primary connected monitor. If you have are not in a VM, it will likely be something like VGA1 or HDMI-0.

If you are in a VM, it is probably Virtual1, so in that case execute (but change it to whatever makes sense to your situation):

xrandr --addmode Virtual1 "1920x1080_60.00"

Now at this point, 1920x1080 should be a valid display resolution, so let’s apply it by executing:

xrandr -s 1920x1080

And now you should be able to go full screen on a 1920x1080 resolution in your VM…

Setting the resolution this way only lasts until the end of the current session.

How to permanently add a resolution?

You might decide that you want to stick to a resolution permanently, and you do not want to switch with xrandr each time you log in etc. In this case, you will need to modify the xorg config (xorg.conf) file.

The xorg.conf does not exist by default any more, so the first step is that we create it. (But depending on your situation, it might already exist, so if it does, then you can skip ahead here)

However, you need a new display socket in order to run the necessary command. If you are using :0 on your current X session, you probably need to run the command on socket :1

To check what display sockets are in use (e.g. you might have multiple in use if you use something like VNC), check the contents of the following folder:

ls /tmp/.X11-unix/

Once you have found what ports are in use, become root, and execute (replace :1 with the next number depending on the results of running ls /tmp/.X11-unix:

X :1 -configure

This will generate a file called xorg.conf.new, which we want to move to a more appropriate place:

mv ~/xorg.conf.new /etc/X11/xorg.conf

Now reboot and we are ready to add our custom configuration. Become root, and create a new folder into /etc/X11:

mkdir /etc/X11/xorg.conf.d

and now create a custom configuration file into it, e.g.

vim /etc/X11/xorg.conf.d/10-monitor.conf

where you want to add a new Monitor section:

Section "Monitor"
    Identifier "Virtual1"
    Modeline "1920x1080_60.00"  172.80  1920 2040 2248 2576  1080 1081 1084 1118  -HSync +Vsync
    Option "PreferredMode" "1920x1080_60.00"
EndSection

Make sure that you get the syntax in this file absolutely right because a misconfigured xorg.conf file will render X unusable, and upon login, you will be stuck on a shell session with no way to start X. If this happens, delete/move your custom configuration file, reboot and X will work again.