At first I want to say that I have thoroughly searched for a solution to my problems both with search engines and here on this platform but to no avail.
I have a headless Debian server (Headless in the sense that I just installed the standard Debian Jessie package, uninstalled gnome and installed xdm), on a Dell XPS 420, on which I am running xdm as the "minimal x setup", according to the hashcat webpage's instructions on configuring an ubuntu server. The card I am trying to use is a Radeon HD 6570.
It does work at first and I get about 14000 H/s with WPA/WPA2 which is fine, but then after a while Hashcat is calculating at 0 H/s and when I try to quit the application it freezes and stops responding to my commands. I guess this is a problem with the OpenCL video drivers, which currently is fglrx from the Debian Experimental repos.
Everything I have installed is directly from the Debian repos because the manual way to build a Debian package with the official AMD installer doesn't work for some reason.
Now to the question: What is the proper way to install Radeon HD video card drivers and use them with a headless server as if I would with a monitor, but without outputting the video data anywhere?
Edit: I'm on Jessie right now because Stretch was also very much a pain to configure. In case it matters. xserver-xorg-video-radeon is the driver I'm using.
When I follow the instructions of this site regarding how to check for drivers,
lspci -n, my graphics card shows up and says it is working. It even identifies the correct model series and tells me im using the Radeon driver. It does say the video driver is working and that it's called Radeon.