couldnt connect to docker daemon. you might need to start docker for mac

Stefan Kleeschulte Feb 8, 2020 9 min read In this article I will describe the steps necessary to enable IPv6 connectivity for Docker containers on an Ubuntu 18.04 host.

This is based on my findings for a standalone Docker 19.03 installation on a virtual server.

It should be applicable to similar setups and covers the following topics: Enabling IPv6 on the Docker host Enabling IPv6 in the Docker daemon Accessing containers over IPv6 Enabling IPv6 on the Docker host Enabling IPv6 on the host is obviously only necessary if it is not already working.

An easy way to check this is to try to send IPv6 ping messages from and to the machine.

To identify the default ethernet interface, find the default line in the output of ip -6 route show.

Or, to do it all in one command: ip addr show (ip -6 route show awk default print 5) awk inet6.

Alternatively you can do it from your local computer if it has IPv6 connectivity.

If like me you have a server where IPv6 first needs to be manually configured, you can execute the following steps.

Manually configure IPv6 in Netplan By default, Ubuntu 18.04 uses Netplan for the network configuration.

To enable IPv6 connectivity in Netplan, add IPv6 address and gateway to the.yaml-file in the etcnetplan directory.

I have seen this file being named 01-netcfg.yaml or 50-cloud-init.yaml.

Here is an example of the altered file: etcnetplan01-netcfg.yaml Be careful when editing this file, it is in YAML format and uses 2 or 4 spaces for indentation no tabs Misformatting this file will probably break the machines network connection at the next reboot.

You should also ensure that the nameservers can resolve IPv6 queries.

Then apply the new configuration: netplan apply Thats it now test again if IPv6 connections are working on the host.

Enabling IPv6 in the Docker daemon The next step is to enable IPv6 in the Docker daemon.

Looking at the documentation for the current Docker version 19.03 is not very helpful: It only states to add ipv6: true to etcdockerdaemon.json and systemctl reload docker.

While reloading succeeds, it does not seem to actually enable IPv6 for Dockers default bridge network (if you want to try it, look at the output of docker network inspect bridge ).

On the other hand restarting Docker with systemctl restart docker now entirely fails, giving the following message in varlogsyslog: failed to start daemon: Error initializing network controller: Error creating default bridge network: could not find an available, non-overlapping IPv6 address pool among the defaults to assign to the network Useful information can be found in the issues mobymoby29443 and mobymoby36954 on GitHub: In addition to setting the ipv6 -flag in daemon.json, an IPv6 subnet must be defined in fixed-cidr-v6.

The subnet configured in fixed-cidr-v6 is used to assign IPv6 addresses to containers that are connected to Dockers default bridge network.

It should have a prefix length of at most 80 (i.e.

Docker 17.09: The subnet for Docker containers should at least have a size of 80, so that an IPv6 address can end with the containers MAC address and you prevent NDP neighbor cache invalidation issues in the Docker layer.

Note that this prefix length requirement is specific to the default bridge network, induced by its MAC address based IP address generation.

In manually added IPv6 enabled Docker bridge networks, IPv6 addresses are assigned sequentially so that smaller subnets are sufficient here (e.g.

Next arises the question which subnet to specify in fixed-cidr-v6.

This can either be a subnet of publicly routable addresses or a subnet defining a private address space.

Using publicly routable addresses has the advantage that no further configuration is necessary on the host to enable IPv6 internet connectivity in the containers.

In addition, it requires that you have a public subnet of at least size 80 available.

For this reason my recommendation is to always set fixed-cidr-v6 to a private subnet, e.g.

Configuring Docker To enable IPv6 in Docker, first create the file etcdockerdaemon.json with the following content (or if it already exists, add the values): etcdockerdaemon.json Then restart Docker: systemctl restart docker Configuring IPv6 NAT To enable IPv6 internet access from containers, enable NAT for the private Docker subnet on the host: ip6tables -t nat -A POSTROUTING -s fd00::80 -o docker0 -j MASQUERADE Note that this configuration does not survive a reboot of the host, so you might want to persist it once you confirmed it is working as expected.

Couldnt Connect To Docker Daemon. You Might Need To Start Docker For Mac