Arch Linux

Wild Penguin

Member

Registered: 2015-03-19

Posts: 320

USB printer device nodes are created with wrong permissions [SOLVED?]

Hi,

I got some problems getting CUPS to recognize my USB printer.

In the end I figured CUPS can not read and write the device node for some reason. The device node is created with cups:lp ownership and 664 permissions:

EDIT: I'm actually not certain it was cups:lp previously, only thought so. Only now noticed after copy+paste it is actully root:lp! However, other points still stand, as the process should be cups:lp and the group still has permissions!

crw-rw-r--+ 1 root lp   189, 135  6. 6. 11:54 008

According to the wiki (see here), this should work, and also I see no reason why it should not:

$ groups cups
lp cups

However, CUPS does not see the printer in the web interface. But changing the permissions all problems go away and I can now use the printer:

$ sudo chmod 666 008 

This is just FYI if someone is in the same boat :-)

But, can anyone tell why is this a case? A bug in a newish CUPS?

That change will not persis over reboots. It is trivial to make an udev rule which will make the permissions 0666, and I don't mind the permissions being that relaxed on this box. However, I believe a user should not need to make that change, and also, the group rw permission should be enough, but for some reason is not. Any ideas how to make a proper fix? Or, why isn't CUPS able to read/write? Is the process dropping the group permission at some point for some reason?

Cheers!

p.s. search came up empty, if this issue is already known, I used wrong search words. Apologies in that case!
p.s.s. some perhaps interesting background: I have no idea if this is a regression or not, since I haven't been using my printer locally (as USB) on this Arch installation. My printer was previously next to my router. But I needed to move my printer (physically) because of interior changes in my apartment, so local use is now more sensible!

Last edited by Wild Penguin (2019-06-12 16:59:04)

Wild Penguin

Member

Registered: 2015-03-19

Posts: 320

Re: USB printer device nodes are created with wrong permissions [SOLVED?]

Printed something again today and magically the USB printer now works with 664 permissions (root:lp). Not sure what changed between! There has been at least one reboot, but I'm certain I already tried that before making the OP.

So, marking as [SOLVED], although I have no idea why it now works as it is supposed to!

If someone has the same situation and 666 permissions are reguired (persistently and over reboots), then .... I guess there is some kind of bug which only manifest itself in some weird situations.

Last edited by Wild Penguin (2019-06-12 17:06:27)