# IPv6 with Docker 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](http://en.wikipedia.org/wiki/IPv4_address_exhaustion) the IETF has standardized an IPv4 successor, [Internet Protocol Version 6](http://en.wikipedia.org/wiki/IPv6) , in [RFC 2460](https://www.ietf.org/rfc/rfc2460.txt). Both protocols, IPv4 and IPv6, reside on layer 3 of the [OSI model](http://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/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: 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::/80` with a range from `2001:db8:23:42:0:0:0:0` to `2001:db8:23:42:0: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 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 has 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: mtu 65536 inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: 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](https://github.com/DanielAdolfsson/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 `/78` 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.