How it works

Reads and control go over IPP on the unix socket rather than through lpstat. The output of the command line tools is localised and free-form, while IPP returns typed attributes — printer-state is an integer and printer-state-reasons is machine-readable. Over the unix socket CUPS authenticates the local user; the same request over TCP to localhost:631 answers 401.

Submitting jobs and setting quotas go through lp and lpadmin instead. The Go IPP library available today sends copies twice, cannot encode page-ranges as a rangeOfInteger — it sends text, which the filter may ignore without saying so — and attributes the job to root instead of the real user. It also cannot encode the multi-valued access list a quota needs. Both were checked against CUPS 2.4.19 by comparing the attributes each route leaves stored on the job.

The IPP transport is a single connection with one shared http.Transport. The library builds a fresh transport per request and never closes it, which leaks a socket per query until the collector gets to it; refreshing every few seconds that exhausts the MaxClients of cupsd and stalls printing for every program on the machine, not just this one.