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Docker swarm mode overlay network security model

Overlay networking for Docker Engine swarm mode comes secure out of the box. The swarm nodes exchange overlay network information using a gossip protocol. By default the nodes encrypt and authenticate information they exchange via gossip using the AES algorithm in GCM mode. Manager nodes in the swarm rotate the key used to encrypt gossip data every 12 hours.

You can also encrypt data exchanged between containers on different nodes on the overlay network. To enable encryption, when you create an overlay network pass the --opt encrypted flag:

$ docker network create --opt encrypted --driver overlay my-multi-host-network

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When you enable overlay encryption, Docker creates IPSEC tunnels between all the nodes where tasks are scheduled for services attached to the overlay network. These tunnels also use the AES algorithm in GCM mode and manager nodes automatically rotate the keys every 12 hours.

Swarm mode overlay networks and unmanaged containers

Because the overlay networks for swarm mode use encryption keys from the manager nodes to encrypt the gossip communications, only containers running as tasks in the swarm have access to the keys. Consequently, containers started outside of swarm mode using docker run (unmanaged containers) cannot attach to the overlay network.

For example:

$ docker run --network my-multi-host-network nginx

docker: Error response from daemon: swarm-scoped network
(my-multi-host-network) is not compatible with `docker create` or `docker
run`. This network can only be used by a docker service.

To work around this situation, migrate the unmanaged containers to managed services. For instance:

$ docker service create --network my-multi-host-network my-image

Because swarm mode is an optional feature, the Docker Engine preserves backward compatibility. You can continue to rely on a third-party key-value store to support overlay networking if you wish. However, switching to swarm-mode is strongly encouraged. In addition to the security benefits described in this article, swarm mode enables you to leverage the substantially greater scalability provided by the new services API.

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https://docs.docker.com/engine/userguide/networking/overlay-security-model/