Docker Swarm Services with IPv6

IPv6 does not work as expected with docker service deploy.

Using this trivial docker-compose.yml as an example.

version: "3.9"

services:

  caddy:
    image: caddy
    ports:
      - "80:80"
    volumes:
      - /etc/caddy:/etc/caddy:ro

  nginx:
    image: nginx

/etc/caddy/Caddyfile contains a reverse proxy definition like this:

:80 {
        reverse_proxy nginx:80
}

Basic connectivity can be checked using using docker compose up and using curl http://<ipv4>:80 and curl http://<ipv6>:80, which both return the nginx default website.

After docker swarm init, the stack can be deployed using docker stack deploy --compose-file ./docker-compose.yml test. Connectivity tests on IPv6 now fail. IPv4 works as expected.

Updating /etc/docker/daemon.json by adding ipv6 and fixed-cidr-v6 as described here does not help.

The situation can be worked around by deploying the caddy image with host ports like this. Now, IPv4 and IPv6 connectivity work.

version: "3.9"

services:

  caddy:
    image: caddy
    deploy:
      mode: global
    ports:
      - target: 80
        published: 80
        protocol: tcp
        mode: host
    volumes:
      - /etc/caddy:/etc/caddy:ro

  nginx:
    image: nginx