Unable to start docker after configuring hosts in daemon.json

I found this on the Docker docs and it worked on Docker 18.09.1 and Centos 8:

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

Then reload

systemctl daemon-reload

The reason is:

Docker listens on a socket by default. On Debian and Ubuntu systems using systemd, this means that a host flag -H 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.

Here is the link: https://docs.docker.com/config/daemon/#troubleshoot-conflicts-between-the-daemonjson-and-startup-scripts

For systemd, my preferred method is to deploy a simple override file (you may need to first create the directory):

$ cat /etc/systemd/system/docker.service.d/override.conf
# Disable flags to dockerd, all settings are done in /etc/docker/daemon.json
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd

This removes the -H ... default flag from dockerd along with any other options and lets you manage docker from the daemon.json file. This also allows docker to make changes to their startup scripts as long as they don't modify the ExecStart and you'll continue to receive those changes without maintaining your own copy of the docker.service.

After creating this file, run systemctl daemon-reload; systemctl restart docker.

It looks like this is an issue merging configuration from both the command line and configuration file. The default systemd unit file is specifying -H fd:// and it conflicts with your tcp://0.0.0.0:1234 and unix:///var/run/docker.sock.

There are a number of GitHub issues on the subject:

• https://github.com/moby/moby/issues/22339
• https://github.com/moby/moby/issues/21559
• https://github.com/moby/moby/issues/25471
• https://github.com/moby/moby/pull/27473

They don't seem to consider this a bug. But it is definitely an annoyance. A workaround is to copy the default unit file and remove the -H fd:// from it:

$ sudo cp /lib/systemd/system/docker.service /etc/systemd/system/
$ sudo sed -i 's/\ -H\ fd:\/\///g' /etc/systemd/system/docker.service
$ sudo systemctl daemon-reload
$ sudo service docker restart