We spent a morning trying to figure out why containers in a new installation of Swarm just couldn’t talk to each other. Overlay network looked fine. Firewall looked fine. You could get from the host to the container, just not from the container to a container on the other server. So … here’s a bug where your swarm (i.e. the thing you do when you want docker stuff to run across more than one server) cannot actually, ya know, talk to the other servers. Sigh!