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Docker for Mac Configuration Persistence

07 May 2017

I use Docker for Mac (DFM) every single day. Up until now all of the out of the box configuration settings just worked for me. However, I ran into an issue with DNS and an issue with subnet collisions that required me to change the configuration. I also needed to script those changes so others could conveniently use them and the changes needed to persist between Docker for Mac restarts.

The versions used in this post are:

Temporary Configuration

If you want to make temporary configuration changes to your Docker for Mac environment, you can change the running xhyve VM directly. The changes won’t persist but it’s a good way to experiment. It’s also a good way to check your work after you’ve attempted to make something persistent.

To attach to the running xhyve VM, you can use screen. You might have to hit Enter again after running screen to get a prompt.

$ screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/tty

/ # cat /etc/hosts
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Use Ctrl+a Ctrl+d to detach from the session.

Persistent Configuration

Initially, the placeholder for the configuration is stored in a git bare repo. We need to do a git reset on it to get a working copy.

cd ~/Library/Containers/com.docker.docker/Data/database
git reset --hard

DNS

DFM wasn’t able to resolve the IP of my Docker registry. To workaround that issue, I had to change the /etc/hosts file with the proper IP address and the FQDN of the registry. Here’s how to customize the /etc/hosts file and commit the changes.

DFM_HOSTS_FILE="com.docker.driver.amd64-linux/etc/hosts"

if ! grep -q registry.example.com ${DFM_HOSTS_FILE}; then
  echo "adding docker registry ip to DFM /etc/hosts"

  echo -e "xxx.xxx.xxx.xxx\tregistry.example.com" >> ${DFM_HOSTS_FILE}

  git add ${DFM_HOSTS_FILE}
  git commit -m "add docker registry to /etc/hosts for dns lookup"
fi

Entries added to ${DFM_HOSTS_FILE} are appended the /etc/hosts file in the xhyve VM.

Subnet

The default subnet used by DFM is 172.17.0.0/16. Unfortunately for me this conflicted with a subnet in use by VirtualBox on my laptop. To change it you need to specify the network bridge IP using the bip flag from the dockerd options in the Docker daemon.json file.

DFM_DAEMON_FILE="com.docker.driver.amd64-linux/etc/docker/daemon.json"

if ! grep -q bip ${DFM_DAEMON_FILE}; then
  echo "changed DFM subnet to avoid conflict with VirtualBox subnet"

  DFM_DAEMON_JSON=$(cat ${DFM_DAEMON_FILE})
  echo ${DFM_DAEMON_JSON} | jq -r '.+ {"bip": "172.20.0.1/16"}' > ${DFM_DAEMON_FILE}

  git add ${DFM_DAEMON_FILE}
  git commit -m "changed subnet to avoid conflict with VirtualBox subnet"
fi

I used jq to merge the bip flag change with any preexisting changes that might already be there. For these change to take effect, you must restart Docker for Mac. To test the changes, inspect the network bridge, start a container, and check its IP address.

$ docker network inspect bridge | jq -r '.[0].IPAM.Config'
[
  {
    "Subnet": "172.20.0.1/16",
    "Gateway": "172.20.0.1"
  }
]

$ docker container run --detach --rm --name nginx --publish 80:80 nginx
34944a4a360364240636ae1436d2911909ad6c2f401c037daaeaeeb1502f701f

$ docker container inspect nginx | jq -r '.[0].NetworkSettings.Networks.bridge.IPAddress'
172.20.0.3

Coda

There’s much more you can do to configure Docker for Mac. These are just two examples based on issues I had to solve. You can experiment in a screen session and then make your configuration changes persistent.