Can't keep systemd sockets open on Ubuntu 18.04.2

Came across this systemd "bug" tonight, while trying to setup Knot Resolver 2.1.1 on Ubuntu 18.04.2. The Github page in question explicitly calls out a documentation conflict between systemd and kresd for socket clearing behavior.

using Sockets= in a drop-in for instantiated service doesn't clear the list of previously defined sockets

changed the description

kresd >= 4.0.0 doesn't use Sockets= anywhere. This systemd ticket was reported by "us".

Can you more precisely describe what problem you experience? ("can't keep sockets open")

@vcunat I should've caught onto that. Sorry about that. Basically, the socket will stay alive, and then quickly die on the first query. Can restart the service or the socket, but it'll die after 1st use by something. Think I had this trouble before on other Ubuntu instances, regardless of architecture; got 3.2.x running on BSD instances, no problem (no systemd sockets there).

I have no idea what could cause the systemd socket to die. I can only suggest to avoid using systemd sockets and use the old-style net.listen() in configuration:

1. systemctl mask kresd{,-tls,-doh}.socket
2. systemctl edit kresd@.service

   # /etc/systemd/system/kresd@.service.d/override.conf
   [Service]
   User=root

3. systemctl daemon-reload
4. modify /etc/knot-resolver/kresd.conf and add net.listen() and switch user afterwards (for network configuration, see ​https://knot-resolver.readthedocs.io/en/stable/daemon.html#c.net.listen )

   net.listen('127.0.0.1', 53)
   
   user('knot-resolver', 'knot-resolver')

5. systemctl restart system-kresd.slice
6. systemctl stop kresd{,-tls,-doh}.socket

This isn't the only issue we've seen with systemd's socket activation. We may drop it in the future in favor of more simple and reliable net.listen().

changed title from Can't keep sockets open on Ubuntu 18.04.2; may be related to systemd bug #12415 to Can't keep systemd sockets open on Ubuntu 18.04.2

To be clear, I don't think we would drop code/support for passing sockets from systemd; the question is about future defaults for DNS traffic.

I would drop it all together. Maintaining two different documentations and kresd.systemd is madness.

maybe it won't be worth maintaining. The user docs for this might almost be more complex than the code supporting passing of sockets. We'll see.

@tkrizek @vcunat I'd be willing to help do some further testing later on; if you feel that folks could be steered from using the systemd sockets on the Ubuntu systems. I already saw one comment on Reddit the other day about a guy switching to the BSDs from Linux because of systemd; this stuff only bolsters their argument.

mentioned in issue #485 (closed)

FYI the long-term plan for 5.0.0 is to drop systemd sockets. Please stay tuned.

Closing because there is no new information and sockets are going to be removed in %5.0.0 anyway.

closed

BTW issue for dropping systemd sockets is #485 (closed).