IPv6 with Docker

Estimated reading time: 10 minutes

The information in this section explains IPv6 with the Docker default bridge. This is a bridge network named bridge created automatically when you install Docker.

As we are running out of IPv4 addresses the IETF has standardized an IPv4 successor, Internet Protocol Version 6 , in RFC 2460. Both protocols, IPv4 and IPv6, reside on layer 3 of the OSI model.

How IPv6 works on Docker

By default, the Docker daemon configures the container network for IPv4 only. You can enable IPv4/IPv6 dualstack support by running the Docker daemon with the --ipv6 flag. Docker will set up the bridge docker0 with the IPv6 link-local address fe80::1.

By default, containers that are created will only get a link-local IPv6 address. To assign globally routable IPv6 addresses to your containers you have to specify an IPv6 subnet to pick the addresses from. Set the IPv6 subnet via the --fixed-cidr-v6 parameter when starting Docker daemon:

You can run dockerd with these flags directly, but it is recommended that you set them in the daemon.json configuration file instead. The following example daemon.json enables IPv6 and sets the IPv6 subnet to 2001:db8:1::/64.

{
  "ipv6": true,
  "fixed-cidr-v6": "2001:db8:1::/64"
}

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

By default, --fixed-cidr-v6 parameter causes Docker to add a new route to the routing table, by basically running the three commands below on your behalf. To prevent the automatic routing, set ip-forward to false in the daemon.json file or start the Docker daemon with the --ip-forward=false flag. Then, to get the same routing table that Docker would create automatically for you, issue the following commands:

$ ip -6 route add 2001:db8:1::/64 dev docker0

$ sysctl net.ipv6.conf.default.forwarding=1

$ sysctl net.ipv6.conf.all.forwarding=1

All traffic to the subnet 2001:db8:1::/64 will now be routed via the docker0 interface.

Note: IPv6 forwarding may interfere with your existing IPv6 configuration: If you are using Router Advertisements to get IPv6 settings for your host’s interfaces, set accept_ra to 2 using the following command. Otherwise IPv6 enabled forwarding will result in rejecting Router Advertisements.

$ sysctl net.ipv6.conf.eth0.accept_ra=2

Every new container will get an IPv6 address from the defined subnet, and a default route will be added on eth0 in the container via the address specified by the daemon option --default-gateway-v6 (or default-gateway-v6 in daemon.json) if present. The default gateway defaults to fe80::1.

This example provides a way to examine the IPv6 network settings within a running container.

docker run -it alpine ash -c "ip -6 addr show dev eth0; ip -6 route show"

15: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500
   inet6 2001:db8:1:0:0:242:ac11:3/64 scope global
      valid_lft forever preferred_lft forever
   inet6 fe80::42:acff:fe11:3/64 scope link
      valid_lft forever preferred_lft forever

2001:db8:1::/64 dev eth0  proto kernel  metric 256
fe80::/64 dev eth0  proto kernel  metric 256
default via fe80::1 dev eth0  metric 1024

In this example, the container is assigned a link-local address with the subnet /64 (fe80::42:acff:fe11:3/64) and a globally routable IPv6 address (2001:db8:1:0:0:242:ac11:3/64). The container will create connections to addresses outside of the 2001:db8:1::/64 network via the link-local gateway at fe80::1 on eth0.

Often servers or virtual machines get a /64 IPv6 subnet assigned (e.g. 2001:db8:23:42::/64). In this case you can split it up further and provide Docker a /80 subnet while using a separate /80 subnet for other applications on the host:

In this setup the subnet 2001:db8:23:42::/64 with a range from 2001:db8:23:42:0:0:0:0 to 2001:db8:23:42:ffff:ffff:ffff:ffff is attached to eth0, with the host listening at 2001:db8:23:42::1. The subnet 2001:db8:23:42:1::/80 with an address range from 2001:db8:23:42:1:0:0:0 to 2001:db8:23:42:1:ffff:ffff:ffff is attached to docker0 and will be used by containers.

Using NDP proxying

If your Docker host is the only part of an IPv6 subnet but does not have an IPv6 subnet assigned, you can use NDP proxying to connect your containers to the internet via IPv6. If the host with IPv6 address 2001:db8::c001 is part of the subnet 2001:db8::/64 and your IaaS provider allows you to configure the IPv6 addresses 2001:db8::c000 to 2001:db8::c00f, your network configuration may look like the following:

$ ip -6 addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:db8::c001/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::601:3fff:fea1:9c01/64 scope link
       valid_lft forever preferred_lft forever

To slit up the configurable address range into two subnets 2001:db8::c000/125 and 2001:db8::c008/125, use the following daemon.json settings. The first subnet will be used by non-Docker processes on the host, and the second will be used by Docker.

{
  "ipv6": true,
  "fixed-cidr-v6": "2001:db8::c008/125"
}

The Docker subnet is within the subnet managed by your router and connected to eth0. All containers with addresses assigned by Docker are expected to be found within the router subnet, and the router can communicate with these containers directly.

When the router wants to send an IPv6 packet to the first container, it transmits a neighbor solicitation request, asking “Who has 2001:db8::c009?” However, no host on the subnet has the address; the container with the address is hidden behind the Docker host. The Docker host therefore must listen for neighbor solicitation requests and respond that it is the device with the address. This functionality is called the NDP Proxy and is handled by the kernel on the host machine. To enable the NDP proxy, execute the following command:

$ sysctl net.ipv6.conf.eth0.proxy_ndp=1

Next, add the container’s IPv6 address to the NDP proxy table:

$ ip -6 neigh add proxy 2001:db8::c009 dev eth0

From now on, the kernel answers neighbor solicitation addresses for this address on the device eth0. All traffic to this IPv6 address is routed through the Docker host, which will forward it to the container’s network according to its routing table via the docker0 device:

$ ip -6 route show

2001:db8::c008/125 dev docker0  metric 1
2001:db8::/64 dev eth0  proto kernel  metric 256

You have to execute the ip -6 neigh add proxy ... command for every IPv6 address in your Docker subnet. Unfortunately there is no functionality for adding a whole subnet by executing one command. An alternative approach would be to use an NDP proxy daemon such as ndppd.

Docker IPv6 cluster

Switched network environment

Using routable IPv6 addresses allows you to realize communication between containers on different hosts. Let’s have a look at a simple Docker IPv6 cluster example:

The Docker hosts are in the 2001:db8:0::/64 subnet. Host1 is configured to provide addresses from the 2001:db8:1::/64 subnet to its containers. It has three routes configured:

Route all traffic to 2001:db8:0::/64 via eth0
Route all traffic to 2001:db8:1::/64 via docker0
Route all traffic to 2001:db8:2::/64 via Host2 with IP 2001:db8::2

Host1 also acts as a router on OSI layer 3. When one of the network clients tries to contact a target that is specified in Host1’s routing table Host1 will forward the traffic accordingly. It acts as a router for all networks it knows: 2001:db8::/64, 2001:db8:1::/64, and 2001:db8:2::/64.

On Host2 we have nearly the same configuration. Host2’s containers will get IPv6 addresses from 2001:db8:2::/64. Host2 has three routes configured:

Route all traffic to 2001:db8:0::/64 via eth0
Route all traffic to 2001:db8:2::/64 via docker0
Route all traffic to 2001:db8:1::/64 via Host1 with IP 2001:db8:0::1

The difference to Host1 is that the network 2001:db8:2::/64 is directly attached to Host2 via its docker0 interface whereas Host2 reaches 2001:db8:1::/64 via Host1’s IPv6 address 2001:db8::1.

This way every container is able to contact every other container. The containers Container1-* share the same subnet and contact each other directly. The traffic between Container1-* and Container2-* will be routed via Host1 and Host2 because those containers do not share the same subnet.

In a switched environment every host has to know all routes to every subnet. You always have to update the hosts’ routing tables once you add or remove a host to the cluster.

Every configuration in the diagram that is shown below the dashed line is handled by Docker: The docker0 bridge IP address configuration, the route to the Docker subnet on the host, the container IP addresses and the routes on the containers. The configuration above the line is up to the user and can be adapted to the individual environment.

Routed network environment

In a routed network environment you replace the layer 2 switch with a layer 3 router. Now the hosts just have to know their default gateway (the router) and the route to their own containers (managed by Docker). The router holds all routing information about the Docker subnets. When you add or remove a host to this environment you just have to update the routing table in the router - not on every host.

In this scenario containers of the same host can communicate directly with each other. The traffic between containers on different hosts will be routed via their hosts and the router. For example packet from Container1-1 to Container2-1 will be routed through Host1, Router, and Host2 until it arrives at Container2-1.

To keep the IPv6 addresses short in this example a /48 network is assigned to every host. The hosts use a /64 subnet of this for its own services and one for Docker. When adding a third host you would add a route for the subnet 2001:db8:3::/48 in the router and configure Docker on Host3 with --fixed-cidr-v6=2001:db8:3:1::/64.

Remember the subnet for Docker containers should at least have a size of /80. This way an IPv6 address can end with the container’s MAC address and you prevent NDP neighbor cache invalidation issues in the Docker layer. So if you have a /64 for your whole environment use /76 subnets for the hosts and /80 for the containers. This way you can use 4096 hosts with 16 /80 subnets each.

Every configuration in the diagram that is visualized below the dashed line is handled by Docker: The docker0 bridge IP address configuration, the route to the Docker subnet on the host, the container IP addresses and the routes on the containers. The configuration above the line is up to the user and can be adapted to the individual environment.

docker, network, IPv6