add IPv6 NDP Proxying documentation

Signed-off-by: Malte Janduda <mail@janduda.net>

docs/sources/articles/networking.md

@@ -505,6 +505,67 @@ at `2001:db8::1`. The subnet `2001:db8:0:0:0:1::/80` with an address range from
 `2001:db8::1:0:0:0` to `2001:db8::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: <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:
+
+    docker -d --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.
+
+![](/article-img/ipv6_ndp_proxying.svg)
+
+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 noone 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 solication 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.
+
 ### Docker IPv6 Cluster
 
 #### Switched Network Environment