First off I want to say, that Systemd is quite new to me and I always used the old and classic init systems like OpenRC (which is still available on Gentoo—if someone cares about that more than I do).
Now let’s start right away, flushing the DNS cache on a linux box using Systemd is not that complicated, but the correct commands should be known.
And this post is all about that:
sudo systemd-resolve --flush-caches
You could also use
sudo resolvectl flush-caches if you like or prefer.
Now verify the empty cache:
I usually do not need to flush my computers DNS caches, but fiddling with my firewall at home made me use this very often last weeks.