$ 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
   ```

   An error occurs if your GRUB configuration file has incorrect syntax. In this
   case, repeat steps 2 and 3.

   The changes take effect when you reboot the system.

## Networking

### IP forwarding problems

If you manually configure your network using `systemd-network` with systemd
version 219 or later, Docker containers may not be able to access your network.
Beginning with systemd version 220, the forwarding setting for a given network
(`net.ipv4.conf.<interface>.forwarding`) defaults to off. This setting prevents
IP forwarding. It also conflicts with Docker's behavior of enabling the
`net.ipv4.conf.all.forwarding` setting within containers.

To work around this on RHEL, CentOS, or Fedora, edit the `<interface>.network`
file in `/usr/lib/systemd/network/` on your Docker host, for example,
`/usr/lib/systemd/network/80-container-host0.network`.

Add the following block within the `[Network]` section.

```systemd
[Network]
...
IPForward=kernel
# OR
IPForward=true
```

This configuration allows IP forwarding from the container as expected.

### DNS resolver issues

```console
DNS resolver found in resolv.conf and containers can't use it
```

Linux desktop environments often have a network manager program running, that
uses `dnsmasq` to cache DNS requests by adding them to `/etc/resolv.conf`. The
`dnsmasq` instance runs on a loopback address such as `127.0.0.1` or
`127.0.1.1`. It speeds up DNS look-ups and provides DHCP services. Such a
configuration doesn't work within a Docker container. The Docker container uses
its own network namespace, and resolves loopback addresses such as `127.0.0.1`
to itself, and it's unlikely to be running a DNS server on its own loopback
address.

If Docker detects that no DNS server referenced in `/etc/resolv.conf` is a fully
functional DNS server, the following warning occurs:

```text
WARNING: Local (127.0.0.1) DNS resolver found in resolv.conf and containers
can't use it. Using default external servers : [8.8.8.8 8.8.4.4]
```

If you see this warning, first check to see if you use `dnsmasq`:

```console
$ ps aux | grep dnsmasq
```

If your container needs to resolve hosts which are internal to your network, the
public nameservers aren't adequate. You have two choices:

- Specify DNS servers for Docker to use.
- Turn off `dnsmasq`.

  Turning off `dnsmasq` adds the IP addresses of actual DNS nameservers to
  `/etc/resolv.conf`, and you lose the benefits of `dnsmasq`.

You only need to use one of these methods.

### Specify DNS servers for Docker

The default location of the configuration file is `/etc/docker/daemon.json`. You
can change the location of the configuration file using the `--config-file`
daemon flag. The following instruction assumes that the location of the
configuration file is `/etc/docker/daemon.json`.

1. Create or edit the Docker daemon configuration file, which defaults to
   `/etc/docker/daemon.json` file, which controls the Docker daemon
   configuration.

   ```console
   $ sudo nano /etc/docker/daemon.json