How to make MacOS Catalina to receive incoming X connections (without ssh tunnels)

Solution 1:

You need to make sure that XQuartz is started before attempting to run xcalc.

When XQuartz is started, you can run this command in the xterm that appears:

echo $DISPLAY

It should let you know which display number you're using. Make sure to use the same display number after the : on the Linux side.

In addition you'll want to ensure that you haven't firewalled the incoming ports (6000, 6001, etc. depending on your display number) on the Mac computer. For example if you're using a tool like Little Snitch or similar, you should ensure that you haven't setup anything there to block this traffic.