[cups] MacOS clients using n-up printing to CUPS server doubles up the n-up option

[Douglas Kosovic]

Hi Jeff,

> Mostly Apple shop here (education) and we print via IPP Everywhere /
> AirPrint through CUPS servers that are advertised via DNS-SD records.
> I've recently received complaints about the number-up printer option
> when printing from MacOS…

I believe the issue is related to not advertising the _cups dns-sd subtype to let macOS clients know they are talking to a CUPS shared print queue instead of a printer.

CUPS shared print queues appear as "Bonjour Shared" on the macOS clients if the _cups dns-sd subtype is used.

But there are complications if you use the _cups dns-sd subtype as you won't be able to have macOS clients automatically add the queues as AirPrint queues, see:
https://github.com/OpenPrinting/cups/discussions/841

But this approach works fine if you use other mechanisms to add the queues as either AirPrint or IPP Everywhere queues.

> Basically after some investigation, when a user on MacOS chooses to
> print say 4 pages per physical piece of paper (number-up=4),
> MacOS automatically generates a PDF that is already set to 4 pages
> per page, but then it sends that to the CUPS server with the
> number-up=4 attribute, so CUPS takes the PDF that already has 4
> pages per page and manipulates that to be 4 pages of 4 pages per
> page, so 16 pages per page total.
>
> Is there some way to configure CUPS to ignore that option?
> Or other ideas to make this so it doesn’t stack these options
> and prints correctly?

I've been using the following patch to avoid applying options twice with macOS clients :
https://github.com/eait-cups-printing/cups/blob/main/cups-exclude-filter-options.patch

For the patch to work, you need to explicitly let the CUPS server know that the _cups dns-sd subtype is not being used by explicitly setting BrowseDNSSDSubTypes in cupsd.conf, e.g. :

BrowseDNSSDSubTypes _print,_universal


I haven't submitted a bug report yet. I also haven't tested the patch with iOS or iPadOS clients, nor have I tested if the "collate" option should also be excluded.





Cheers,
Doug