the following directives are specified both as a flag and in the configuration
file: hosts: (from flag: [unix:///var/run/docker.sock], from file: [tcp://127.0.0.1:2376])
```

If you see an error similar to this one and you are starting the daemon manually
with flags, you may need to adjust your flags or the `daemon.json` to remove the
conflict.

> [!NOTE]
>
> If you see this specific error message about `hosts`, continue to the
> [next section](#configure-the-daemon-host-with-systemd)
> for a workaround.

If you are starting Docker using your operating system's init scripts, you may
need to override the defaults in these scripts in ways that are specific to the
operating system.

#### Configure the daemon host with systemd

One notable example of a configuration conflict that's 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 host flag `-H` is always used when starting
`dockerd`. If you specify a `hosts` entry in the `daemon.json`, this causes a
configuration conflict and results in the Docker daemon failing 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's used when starting the daemon by default.

```systemd
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd
```

There are other times when you might need to configure `systemd` with Docker,
such as [configuring a HTTP or HTTPS proxy](./proxy.md).

> [!NOTE]
>
> If you override this option without specifying a `hosts` entry in the
> `daemon.json` or a `-H` flag when starting Docker manually, Docker fails to
> start.

Run `sudo systemctl daemon-reload` before attempting to start Docker. If Docker
starts successfully, it's now listening on the IP address specified in the
`hosts` key of the `daemon.json` instead of a socket.

> [!IMPORTANT]
>
> Setting `hosts` in the `daemon.json` isn't supported on Docker
> Desktop for Windows or Docker Desktop for Mac.

### Out of memory issues

If your containers attempt to use more memory than the system has available, you
may experience an Out of Memory (OOM) exception, and a container, or the Docker
daemon, might be stopped by the kernel OOM killer. To prevent this from
happening, ensure that your application runs on hosts with adequate memory and
see
[Understand the risks of running out of memory](../containers/resource_constraints.md#understand-the-risks-of-running-out-of-memory).

### Kernel compatibility

Docker can't run correctly if your kernel is older than version 3.10, or if it's
missing kernel modules. To check kernel compatibility, you can download and run
the
[`check-config.sh`](https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh)
script.

```console
$ curl https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh > check-config.sh

$ bash ./check-config.sh
```

The script only works on Linux.

### Kernel cgroup swap limit capabilities

On Ubuntu or Debian hosts, you may see messages similar to the following when
working with an image.

```text
WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.
```

If you don't need these capabilities, you can ignore the warning.

You can turn on these capabilities on Ubuntu or Debian by following these
instructions. Memory and swap accounting incur an overhead of about 1% of the
total available memory and a 10% overall performance degradation, even when
Docker isn't running.

1. Log into the Ubuntu or Debian host as a user with `sudo` privileges.

2. Edit the `/etc/default/grub` file. Add or edit the `GRUB_CMDLINE_LINUX` line
   to add the following two key-value pairs:

   ```text
   GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"
   ```

   Save and close the file.

3. Update the GRUB boot loader.

   ```console
   $ sudo update-grub
   ```