The printer itself is a Lexmark E120n.

It is reachable over the LAN, at drucker.realraum.at, and you can print to it using JetDirect.

You can use directly LPRng to print from your computer, no CUPS needed!

The packages you will need (on Debian Stretch) are:

• printer-driver-hpijs
• ghostscript
• foomatic-filters and foomatic-db
• lprng (obviously!)

sudo checkpc -f # Fix various directory permissions
sudo systemctl enable lprng
sudo systemctl start  lprng

Autostarting the LPRng daemon is relatively safe: by default, it only listens on a UNIX socket.

The printer driver needs a PPD file, that you can create out of the foomatic database as follows:

foomatic-ppdfile -p Lexmark-E120n | sudo tee /etc/lprng/Lexmark-E120n.ppd > /dev/null

LPRng comes with an example /etc/printcap file, that you may or may not use. In any case, here is the configuration you actually need:

.common:
	:sd=/var/spool/lpd/%P
	:sh:mx=0:mc=0
	:force_localhost

realraum:tc=.common:lp=drucker.realraum.at%9100
	:if=/usr/bin/foomatic-rip: 
	:filter_options= --lprng $Z /etc/lprng/Lexmark-E120n.ppd: 

Write gtk-print-backends = “file,lpr” in ~/.gtkrc-2.0.

Unfortunately, AFAIK GTK 3 has no support for LPR.