nixos/services.printing: GTK applications freeze when opening print dialog on NixOS 25.11 (CUPS) - CLI printing works

[Borgvall]

Nixpkgs version

• Stable (25.11)

Describe the bug

I am experiencing an issue on NixOS 25.11 where GTK-based applications (Firefox, Evince/Papers, etc.) freeze indefinitely when attempting to open the print dialog.

The underlying printing system seems fully functional, as printing via the command line (lpr) works perfectly with the exact same CUPS queue. This suggests a problem specifically in the interaction between GTK (or libcups used by GTK) and the CUPS daemon/network backend in the current stable channel.

Steps to reproduce

1. Setup a network printer (HP OfficeJet 6950 in my case) on NixOS 25.11.

2. Configure the printer using socket://:9100 to rule out IPP/TLS handshake issues.

3. Verify printing works via terminal: lpr -P test.pdf (Successful).

4. Open a GTK application (Firefox, Evince or Gnome Papers).

5. Attempt to print.

6. Result: The application hangs/freezes immediately. The print dialog either does not appear (Papers) or appears but is unresponsive (Evince), or appears, but hangs, when the print-button is pressed.

Expected behaviour

The print dialog should open, query the printer capabilities, and submit the job, just like lpr does.

Screenshots

No response

Relevant log output

Additional context

No response

System metadata

have performed extensive troubleshooting to isolate the issue:

• Network/Hardware: The printer works fine from other distros (Fedora 43) and from NixOS via lpr.

• Protocols tried: tried both ipp:// and socket://. The freeze happens with both in the GUI.

• Drivers tried:

  • hpcups (HPLIP 3.25.2)

  • Generic PCL 6

  • Result: lpr works with both, GUI freezes with both.

• Firewall: Tested with networking.firewall.enable = false; -> Issue persists.

• AppArmor: Tested with security.apparmor.enable = false; -> Issue persists.

• DNS: Added printer IP to /etc/hosts to rule out DNS timeouts -> Issue persists.

• SNMP: Disabled SNMP on the printer queue (lpadmin -p ... -o snmp-enabled=false) -> Issue persists.

Notify maintainers

@matthewbauer

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[Borgvall]

It seems the release notes of CUPS v2.4.16 are specifically describing the issue.

[diogotcorreia]

See #473491 and #468820 (already merged to 25.11 staging)

[TobTobXX]

I tried setting services.printing.package = pkgs-unstable.cups, and http://localhost:631 shows CUPS v2.4.16, but the issue still persists. does printing.package not work in this case?

[Borgvall]

@TobTobXX the problem doesn't seem to be introduced by the cups service, but the libcups used by the GTK Apps. So the GTK Apps have to be rebuilt against the newer libcups to fix the issue.

[inthesquarehole]

A regression test for this would be most welcome.