IPv6 with Docker

Estimated reading time: 9 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 server 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:

dockerd --ipv6 --fixed-cidr-v6="2001:db8:1::/64"

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.

With the --fixed-cidr-v6 parameter set Docker will add a new route to the routing table. Further IPv6 routing will be enabled (you may prevent this by starting dockerd with --ip-forward=false):

$ 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.

Be aware that IPv6 forwarding may interfere with your existing IPv6 configuration: If you are using Router Advertisements to get IPv6 settings for your host’s interfaces you should set accept_ra to 2. Otherwise IPv6 enabled forwarding will result in rejecting Router Advertisements. E.g., if you want to configure eth0 via Router Advertisements you should set:

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

Every new container will get an IPv6 address from the defined subnet. Further a default route will be added on eth0 in the container via the address specified by the daemon option --default-gateway-v6 if present, otherwise via fe80::1:

docker run -it ubuntu bash -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 Docker container is assigned a link-local address with the network suffix /64 (here: fe80::42:acff:fe11:3/64) and a globally routable IPv6 address (here: 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 has not got an IPv6 subnet assigned you can use NDP proxying to connect your containers via IPv6 to the internet. For example your host with the 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:

$ 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

Let’s split up the configurable address range into two subnets 2001:db8::c000/125 and 2001:db8::c008/125. The first one can be used by the host itself, the latter by Docker:

dockerd --ipv6 --fixed-cidr-v6 2001:db8::c008/125

You notice the Docker subnet is within the subnet managed by your router that is connected to eth0. This means all devices (containers) with the addresses from the Docker subnet are expected to be found within the router subnet. Therefore the router thinks it can talk to these containers directly.

As soon as the router wants to send an IPv6 packet to the first container it will transmit a neighbor solicitation request, asking, who has 2001:db8::c009? But it will get no answer because no one on this subnet has this address. The container with this address is hidden behind the Docker host. The Docker host has to listen to neighbor solicitation requests for the container address and send a response that itself is the device that is responsible for the address. This is done by a Kernel feature called NDP Proxy. You can enable it by executing

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

Now you can add the container’s IPv6 address to the NDP proxy table:

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

This command tells the Kernel to answer to incoming neighbor solicitation requests regarding the IPv6 address 2001:db8::c009 on the device eth0. As a consequence of this all traffic to this IPv6 address will go into the Docker host and it will forward it according to its routing table via the docker0 device to the container network:

$ 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 the host via its docker0 interface whereas it 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