Where's docker's daemon.json? (missing)

From docs:

The default location of the configuration file on Linux is /etc/docker/daemon.json

But I don't have it on my fresh docker installation:

# docker --version
Docker version 17.03.1-ce, build c6d412e
# ls -la /etc/docker/
total 12
drwx------  2 root root 4096 Apr 28 17:58 .
drwxr-xr-x 96 root root 4096 Apr 28 17:58 ..
-rw-------  1 root root  244 Apr 28 17:58 key.json
# lsb_release -cs
trusty

The default config file path on Linux is /etc/docker/daemon.json like you said, but it doesn't exist by default. You can write one yourself and put additional docker daemon configuration stuff in there instead of passing in those configuration options into the command line. You don't even have to do dockerd --config-file /etc/docker/daemon.json since that's the default path, but it can be useful to make it explicit for others who are inspecting the system.

Also ensure that any configuration you set in /etc/docker/daemon.json doesn't conflict with options passed into the command line evocation of dockerd. For reference:

The options set in the configuration file must not conflict with options set via flags. The docker daemon fails to start if an option is duplicated between the file and the flags, regardless their value.

If you have installed Docker as part of installation of Ubuntu, then Docker is installed as a snap.

The config can be found in /var/snap/docker/current/config/daemon.json.

Refer to https://github.com/docker-archive/docker-snap/issues/22#issuecomment-423361607

Summary:

anonymouse64 commented on 21 Sep 2018

Modifying the daemon.json file is now supported in the version of the snap
I have published in the edge channel. The daemon is now hard-coded to read
the config file for it's settings, so you can now edit the daemon.json
located in $SNAP_DATA/config/daemon.json (on Ubuntu for example $SNAP_DATA
is /var/snap/docker/current, it may be different on your distribution) and
then restart docker for the changes to take effect with:

sudo snap restart docker

You may switch the snap to the edge channel to test this by running:

sudo snap refresh docker --edge

The changes in the edge channel should show up in stable in a short while
if you don't wish to use edge.

This now seems to be present in 'stable'. I am using Ubunu 20.04 and I found daemon.json in /var/snap/docker/current/config/daemon.json.

I changed the 'log-driver' to 'local' and this was picked up by docker after restarting it:

docker info --format '{{.LoggingDriver}}'
json-file
nano /var/snap/docker/current/config/daemon.json
    # added line:  "log-driver":"local",
snap restart docker
docker info --format '{{.LoggingDriver}}'
local