Bug#489975: marked as done (cups: Serial backend not recognised by default)
Your message dated Thu, 8 Jul 2010 11:35:23 +0200
with message-id <AANLkTilyz6n3DtF713xC5yReK-4PfjJF-DqF-WEmiDMO@mail.gmail.com>
and subject line Re: Bug#489975: [Debian QA] please review your old bug reports against CUPS
has caused the Debian Bug report #489975,
regarding cups: Serial backend not recognised by default
to be marked as done.
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--- Begin Message ---
I have just tried connecting a serial printer to ttyS0, with default
crw-rw---- 1 root dialout 4, 64 2008-07-09 00:35 /dev/ttyS0
When I tried to add it via the web UI, the serial port wasn't listed in
the Device drop-down. I turned on debug logging, and sure enough:
I [09/Jul/2008:00:51:39 +0100] Started
D [09/Jul/2008:00:51:39 +0100] [cups-deviced] Added device "http"...
D [09/Jul/2008:00:51:39 +0100] [cups-deviced] Added device "lpd"...
D [09/Jul/2008:00:51:39 +0100] [cups-deviced] Added device "ipp"...
D [09/Jul/2008:00:51:40 +0100] [cups-deviced] Added device
D [09/Jul/2008:00:51:41 +0100] [cups-deviced] Added device "scsi"...
D [09/Jul/2008:00:51:41 +0100] [cups-deviced] Added device "socket"...
D [09/Jul/2008:00:51:41 +0100] [cups-deviced] Added device "beh"...
D [09/Jul/2008:00:51:41 +0100] PID 4656
(/usr/lib/cups/daemon/cups-deviced) exited with no errors.
I noted that cups runs as root, and sure enough, running cups-deviced as
root (with arbitrary parameters) gave the same result:
# /usr/lib/cups/daemon/cups-deviced 1 10 1 all
DEBUG: [cups-deviced] Added device "http"...
DEBUG: [cups-deviced] Added device "lpd"...
DEBUG: [cups-deviced] Added device "ipp"...
DEBUG: [cups-deviced] Added device "scsi"...
DEBUG: [cups-deviced] Added device "socket"...
DEBUG: [cups-deviced] Added device "beh"...
However, the same command as a normal user worked for the serial backend
if not others (snip follows):
DEBUG: [cups-deviced] Added device "serial:/dev/ttyS0?baud=115200"...
I then tried configuring a suitable printer directly in printers.conf.
When I tried printing to it, I got:
E [09/Jul/2008:01:03:02 +0100] PID 4767 (/usr/lib/cups/backend/serial)
stopped with status 1!
E [09/Jul/2008:01:03:02 +0100] [Job 217] Unable to open device file
"/dev/ttyS0": Permission denied
This led me to suspect permissions, and sure enough, changing /dev/ttyS0
to 0666 worked. I didn't really understand this, as root had rw
permissions anyway. I had a glance at scheduler/cups-deviced.c, and
there is certainly some magic there relating to the user that it runs
the backend as. Unfortunately, I don't have time to delve deeper, but
see comments around line 204.
I don't want /dev/ttyS0 to be world readable/writeable, but I don't see
what else to do for now. NB lp is already in my dialout group, that
Anyway, this wasn't documented anywhere that I can see. I'm not sure
what you think is the best way forward, but at the very least, I think
this should be documented in README.Debian, with suitable guidance
given as to possible permission settings for the serial port. Better
still, it should just work.
 Another thing - could we please have a README.Debian in
/usr/share/doc/cups that at least points to
/usr/share/doc/libcups2/README.Debian.gz? I had to use apt-file and
grep to find it.
-- System Information:
Debian Release: lenny/sid
APT prefers unstable
APT policy: (500, 'unstable'), (500, 'testing')
Architecture: amd64 (x86_64)
Kernel: Linux 2.6.24-1-amd64 (SMP w/1 CPU core)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash
Versions of packages cups depends on:
ii adduser 3.108 add and remove users and groups
ii cups-common 1.3.7-7 Common UNIX Printing System(tm) -
ii debconf [debconf-2.0] 1.5.22 Debian configuration management sy
ii ghostscript 8.62.dfsg.1-2.1 The GPL Ghostscript PostScript/PDF
ii libavahi-compat-libdn 0.6.22-3 Avahi Apple Bonjour compatibility
ii libc6 2.7-10 GNU C Library: Shared libraries
ii libcups2 1.3.7-7 Common UNIX Printing System(tm) -
ii libcupsimage2 1.3.7-7 Common UNIX Printing System(tm) -
ii libdbus-1-3 1.2.1-2 simple interprocess messaging syst
ii libgnutls26 2.4.0-2 the GNU TLS library - runtime libr
ii libkrb53 1.6.dfsg.4~beta1-3 MIT Kerberos runtime libraries
ii libldap-2.4-2 2.4.9-1 OpenLDAP libraries
ii libpam0g 0.99.7.1-6 Pluggable Authentication Modules l
ii libpaper1 1.1.23+nmu1 library for handling paper charact
ii libslp1 1.2.1-7.3 OpenSLP libraries
ii lsb-base 3.2-12 Linux Standard Base 3.2 init scrip
ii perl-modules 5.10.0-11 Core Perl modules
ii procps 1:3.2.7-8 /proc file system utilities
ii ssl-cert 1.0.21 simple debconf wrapper for OpenSSL
ii xpdf-utils [poppler-u 3.02-1.3 Portable Document Format (PDF) sui
Versions of packages cups recommends:
ii avahi-utils 0.6.22-3 Avahi browsing, publishing and dis
ii cups-client 1.3.7-7 Common UNIX Printing System(tm) -
ii foomatic-filters 3.0.2-20080211-3.1 OpenPrinting printer support - fil
ii smbclient 2:3.0.30-3 a LanManager-like simple client fo
-- debconf information:
cupsys/backend: ipp, lpd, parallel, scsi, serial, socket, usb, snmp, dnssd
--- End Message ---
--- Begin Message ---
2010/7/8 Antony Gelberg <email@example.com>
That's great, feel free to close, thanks for your efforts. :)
On 08/07/10 12:30, Jean Felder wrote:
I did a little research and i think that this bug corrected in version
I've found it in the changelog from the cups package:
[ Martin Pitt ]
* debian/rules: Install the serial backend with 0744 permissions to make it
run as root, since /dev/ttyS* are root:dialout and thus not accessible as
user "lp". Thanks to Chanoch (Ken) Bloom. (part of #506181, LP: #154277)
2010/7/8 Antony Gelberg<firstname.lastname@example.org>
On 08/07/10 11:17, Jean Felder wrote:
I have no idea - I'm not using serial printers any more. Sorry I can't be
You reported a bug against the CUPS package in Debian a long time ago
and the maintainers are wondering if you can still reproduce the bug
using version 1.4.4-1 that is currently in Debian unstable?
of more help.
--- End Message ---