Docker - Enable Remote HTTP API with SystemD and "daemon.json" - json

Question

Disclaimer:
On a old machine with Ubuntu 14.04 with Upstart as init system I have enabled the HTTP API by defining DOCKER_OPTS on /etc/default/docker. It works.
$ docker version
Client:
Version: 1.11.2
(...)
Server:
Version: 1.11.2
(...)
Problem:
This does solution does not work on a recent machine with Ubuntu 16.04 with SystemD.
As stated on the top of the recent file installed /etc/default/docker:
# Docker Upstart and SysVinit configuration file
#
# THIS FILE DOES NOT APPLY TO SYSTEMD
#
# Please see the documentation for "systemd drop-ins":
# https://docs.docker.com/engine/articles/systemd/
#
(...)
As I checked this information on the Docker documentation page for SystemD I need to fill a daemon.json file but as stated on the reference there are some properties self-explanatory but others could be under-explained.
That being said, I'm looking for help to convert this:
DOCKER_OPTS="-H tcp://0.0.0.0:2375 -H unix:///var/run/docker.sock -G myuser --debug"
to the daemon.jsonobject?
Notes
PS1: I'm aware that the daemon.json have a debug: true as default.
PS2: Probably the group: "myuser" it will work like this or with an array of strings.
PS3: My main concern is to use SOCK and HTTP simultaneous.
EDIT (8/08/2017)
After reading the accepted answer, check the #white_gecko answer for more input on the matter.

Answer 1

With a lot of fragmented documentation it was difficult to solve this.
My first solution was to create the daemon.json with
{
"hosts": [
"unix:///var/run/docker.sock",
"tcp://127.0.0.1:2376"
]
}
This does not worked this error docker[5586]: unable to configure the Docker daemon with file /etc/docker/daemon.json after tried to restart the daemon with service docker restart.
Note: There was more on the error that I failed to copy.
But what this error meant it at the start the daemon it a conflict with a flag and configurations on daemon.json.
When I looked into it with service docker status this it was the parent process: ExecStart=/usr/bin/docker daemon -H fd://.
What it was strange because is different with configurations on /etc/init.d/docker which I thought that were the service configurations.
The strange part it was that the file on init.d does contain any reference to daemon argument neither -H fd://.
After some research and a lot of searches of the system directories, I find out these directory (with help on the discussion on this issue docker github issue #22339).
Solution
Edited the ExecStart from /lib/systemd/system/docker.service with this new value:
/usr/bin/docker daemon
And created the /etc/docker/daemon.json with
{
"hosts": [
"fd://",
"tcp://127.0.0.1:2376"
]
}
Finally restarted the service with service docker start and now I get the "green light" on service docker status.
Tested the new configurations with:
$ docker run hello-world
Hello from Docker!
(...)
And,
$ curl http://127.0.0.1:2376/v1.23/info
[JSON]
I hope that this will help someone with a similar problem as mine! :)

Answer 2

I had the same problem and actually in my eyes the easiest solution which should doesn't touch any existing files, which are managed by the system update process is, to use a systemd drop-in:
Just create a file /etc/systemd/system/docker.service which overwrites the specific part of the service in /lib/systemd/system/docker.service.
In this case the content of /etc/systemd/system/docker.service would be:
[Service]
ExecStart=/usr/bin/dockerd --tlsverify --tlscacert=/etc/docker/ca.pem --tlscert=/etc/docker/server-cert.pem --tlskey=/etc/docker/server-key.pem -H=tcp://127.0.0.1:2375 -H=fd://
(You could even create a directory docker.service.d which contains multiple files to overwrite different parameters.)
After adding the file you just run:
$ sudo systemctl daemon-reload
$ sudo systemctl restart docker

Answer 3

The solution described at https://docs.docker.com/engine/admin/#troubleshoot-conflicts-between-the-daemonjson-and-startup-scripts works for me:
One notable example of a configuration conflict that is difficult to
troubleshoot is when you want to specify a different daemon address
from the default. Docker listens on a socket by default. On Debian and
Ubuntu systems using systemd), this means that a -H flag is always
used when starting dockerd. If you specify a hosts entry in the
daemon.json, this causes a configuration conflict (as in the above
message) and Docker fails to start.
To work around this problem, create a new file
/etc/systemd/system/docker.service.d/docker.conf with the following
contents, to remove the -H argument that is used when starting the
daemon by default.
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd
Note that the line with ExecStart= is actually required, otherwise it'll fail with the error:
docker.service: Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.
After creating the file you must run:
sudo systemctl daemon-reload
sudo systemctl restart docker

Answer 4

For me worked on Ubuntu 18.04.1 LTS and Docker 18.06.0-ce create
/etc/systemd/system/docker.service.d/remote-api.conf
with following content:
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:2376 -H unix:///var/run/docker.sock
then run sudo systemctl daemon-reload and sudo systemctl restart docker
See result calling:
curl http://localhost:2376/info
You might need to configure proxy, if your docker is behind a proxy.
To achiev this paste in /etc/default/docker file following:
http_proxy="http://85.22.53.71:8080/"
https_proxy="http://85.22.53.71:8080/"
HTTP_PROXY="http://85.22.53.71:8080/"
HTTPS_PROXY="http://85.22.53.71:8080/"
# below you can list some *.enterprise_domain.com as well
NO_PROXY="localhost,127.0.0.1,::1"
Or Create
/etc/systemd/system/docker.service.d/remote-api.conf with following content:
[Service]
Environment="HTTP_PROXY=http://<you_proxy_ip>:<port>"
Environment="HTTPS_PROXY=https://<you_proxy_ip>:<port>/"
Environment="NO_PROXY=localhost,127.0.0.1,::1"
I hope it helps someone...