Intro

With a floating IPV4 address, the traffic comes into the droplet via the anchor ip address. This is a private IP address in the 10.0.0.0 network. Docker options setup IPV4 and IPV6 neatly, then the default route needs adjusting, otherwise traffic is routed out via the droplets IP address

Docker configuration

Depending on your linux variant, you’ll need to set some DOCKER_OPTS options.
1. Determine the anchor IP address 2. Determine the IPV6 subnet 3. Add DOCKER_OPTS

Determine the anchor IP address and Gateway
1
2

curl -s http://169.254.169.254/metadata/v1/interfaces/public/0/anchor_ipv4/address
curl -s http://169.254.169.254/metadata/v1/interfaces/public/0/anchor_ipv4/gateway
Determine the IPV6 subnet
1

ip -6 addr

Find the line for eth0 starting with inet6. Don’t use the line fe80:: - that’s the link local. Then I use this subnet calculator to get the network. Put that together for your linux distribution - you’ll need the following three options

1
2
3

--ip=<anchor address>
--ipv6
--fixed-cidr-v6=<ipv6 subnet>/64

This is a systemd (works with CoreOs) /etc/systemd/system/docker.service.d/10-ipv6.conf

1
2

[Service]
Environment=DOCKER_OPTS="--ip=10.1.0.72 --ipv6 --fixed-cidr-v6=20a3:b1d9:3:d1::/64"

Routing

Final part - outbound traffic needs to look like it comes from the floating ip. That means sending back to the anchor ip address. Something like this should run at boot:

1
2

#!/bin/bash
route del default && route add default gw 10.1.0.72 eth0