Not having "ListenStream=631" in the systemd cups.socket file is intended. See the answer from Mike Sweet, author of CUPS:
---------- First, the reason why we don't include "ListenStream=631" is because that exposes cupsd to remote attack. And we don't listen on localhost (which has a similar security issue) because it causes problems when binding to the "any" address on Linux. Second, once cupsd starts it will create the localhost (or "any" address) listener for you, and stay running indefinitely. And cupsd uses a KeepAlive file to tell systemd to start it up on bootup once you enable the web interface, so it should be running right away unless you've added something to manually start/stop cupsd. ---------- In Yakkety (16.10), CUPS behaves as intended upstream, not automatically shutting down when it shares at least one printer or the web interface is active (turned on via "WebInterface Yes" in cupsd.conf). So there this bug should be fixed. In earlier Ubuntu versions, like Xenial (16.04 LTS), there is a patch which allows for CUPS auto shutdown due to another problem, whgich is fixed in later CUPS versions. This fix needs to get backported so that auto shutdown can be suppressed with active web interface, to match the upstream standard. ** Changed in: cups (Ubuntu) Status: Confirmed => Fix Released ** Also affects: cups (Ubuntu Xenial) Importance: Undecided Status: New ** Changed in: cups (Ubuntu Xenial) Status: New => Confirmed ** Changed in: cups (Ubuntu Xenial) Milestone: None => xenial-updates -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to cups in Ubuntu. https://bugs.launchpad.net/bugs/1598300 Title: cups hang after a while Status in cups package in Ubuntu: Fix Released Status in cups source package in Xenial: Confirmed Bug description: after 6 minutes or so, cups is not responding. it do not produce error on the log, just stop working, worse, it exit with 0 ⌌—————————————————————————————————————————————————————————————————————————————————————⌍ |root@cupsmachine :~# systemctl status cups | |● cups.service - CUPS Scheduler | | Loaded: loaded (/lib/systemd/system/cups.service; enabled; vendor preset: enabled)| | Active: inactive (dead) since ven. 2016-07-01 10:31:32 TAHT; 2min 16s ago | | Docs: man:cupsd(8) | | Process: 28686 ExecStart=/usr/sbin/cupsd -l (code=exited, status=0/SUCCESS) | | Main PID: 28686 (code=exited, status=0/SUCCESS) | | | |juil. 01 10:30:01 appli-client systemd[1]: Started CUPS Scheduler. | ⌎—————————————————————————————————————————————————————————————————————————————————————⌏ I got to launch it again, so I have finish with a cron job like */10 * * * * systemctl status cups.service|grep -q 'inactive (dead)' && systemctl start cups but it is a dirty solution. I have no idea of what make it stop. NB: I have seen problems related to apparmor, this machine has no apparmor package. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/cups/+bug/1598300/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp