Overlay network driver

Table of contents

The overlay network driver creates a distributed network among multiple Docker daemon hosts. This network sits on top of (overlays) the host-specific networks, allowing containers connected to it to communicate securely when encryption is enabled. Docker transparently handles routing of each packet to and from the correct Docker daemon host and the correct destination container.

You can create user-defined overlay networks using docker network create, in the same way that you can create user-defined bridge networks. Services or containers can be connected to more than one network at a time. Services or containers can only communicate across networks they're each connected to.

Overlay networks are often used to create a connection between Swarm services, but you can also use it to connect standalone containers running on different hosts. When using standalone containers, it's still required that you use Swarm mode to establish a connection between the hosts.

This page describes overlay networks in general, and when used with standalone containers. For information about overlay for Swarm services, see Manage Swarm service networks.

Requirements

Docker hosts must be part of a swarm to use overlay networks, even when connecting standalone containers. The following ports must be open between participating hosts:

  • 2377/tcp: Swarm control plane (configurable)
  • 4789/udp: Overlay traffic (configurable)
  • 7946/tcp and 7946/udp: Node communication (not configurable)

Create an overlay network

The following table lists the ports that need to be open to each host participating in an overlay network:

PortsDescription
2377/tcpThe default Swarm control plane port, is configurable with docker swarm join --listen-addr
4789/udpThe default overlay traffic port, configurable with docker swarm init --data-path-addr
7946/tcp, 7946/udpUsed for communication among nodes, not configurable

To create an overlay network that containers on other Docker hosts can connect to, run the following command:

$ docker network create -d overlay --attachable my-attachable-overlay

The --attachable option enables both standalone containers and Swarm services to connect to the overlay network. Without --attachable, only Swarm services can connect to the network.

You can specify the IP address range, subnet, gateway, and other options. See docker network create --help for details.

Encrypt traffic on an overlay network

Use the --opt encrypted flag to encrypt the application data transmitted over the overlay network:

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

This enables IPsec encryption at the level of the Virtual Extensible LAN (VXLAN). This encryption imposes a non-negligible performance penalty, so you should test this option before using it in production.

Warning

Don't attach Windows containers to encrypted overlay networks.

Overlay network encryption isn't supported on Windows. Swarm doesn't report an error when a Windows host attempts to connect to an encrypted overlay network, but networking for the Windows containers is affected as follows:

  • Windows containers can't communicate with Linux containers on the network
  • Data traffic between Windows containers on the network isn't encrypted

Attach a container to an overlay network

Adding containers to an overlay network gives them the ability to communicate with other containers without having to set up routing on the individual Docker daemon hosts. A prerequisite for doing this is that the hosts have joined the same Swarm.

To join an overlay network named multi-host-network with a busybox container:

$ docker run --network multi-host-network busybox sh
Note

This only works if the overlay network is attachable (created with the --attachable flag).

Container discovery

Publishing ports of a container on an overlay network opens the ports to other containers on the same network. Containers are discoverable by doing a DNS lookup using the container name.

Flag valueDescription
-p 8080:80Map TCP port 80 in the container to port 8080 on the overlay network.
-p 8080:80/udpMap UDP port 80 in the container to port 8080 on the overlay network.
-p 8080:80/sctpMap SCTP port 80 in the container to port 8080 on the overlay network.
-p 8080:80/tcp -p 8080:80/udpMap TCP port 80 in the container to TCP port 8080 on the overlay network, and map UDP port 80 in the container to UDP port 8080 on the overlay network.

Connection limit for overlay networks

Due to limitations set by the Linux kernel, overlay networks become unstable and inter-container communications may break when 1000 containers are co-located on the same host.

For more information about this limitation, see moby/moby#44973.

Usage examples

This section provides hands-on examples for working with overlay networks. These examples cover swarm services and standalone containers on multiple Docker hosts.

Prerequisites

All examples require at least a single-node swarm. Initialize one by running docker swarm init on the host. You can run these examples on multi-node swarms as well.

Use the default overlay network

This example shows how the default overlay network works with swarm services. You'll create an nginx service and examine the network from the service containers' perspective.

Prerequisites for multi-node setup

This walkthrough requires three Docker hosts that can communicate with each other on the same network with no firewall blocking traffic between them:

  • manager: Functions as both manager and worker
  • worker-1: Functions as worker only
  • worker-2: Functions as worker only

If you don't have three hosts available, you can set up three virtual machines on a cloud provider with Docker installed.

Create the swarm

  1. On manager, initialize the swarm. If the host has one network interface, the --advertise-addr flag is optional:

    $ docker swarm init --advertise-addr=<IP-ADDRESS-OF-MANAGER>

    Save the join token displayed for use with workers.

  2. On worker-1, join the swarm:

    $ docker swarm join --token <TOKEN> \
  --advertise-addr <IP-ADDRESS-OF-WORKER-1> \
  <IP-ADDRESS-OF-MANAGER>:2377

  3. On worker-2, join the swarm:

    $ docker swarm join --token <TOKEN> \
  --advertise-addr <IP-ADDRESS-OF-WORKER-2> \
  <IP-ADDRESS-OF-MANAGER>:2377

  4. On manager, list all nodes:

    $ docker node ls

ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS
d68ace5iraw6whp7llvgjpu48 *   ip-172-31-34-146    Ready               Active              Leader
nvp5rwavvb8lhdggo8fcf7plg     ip-172-31-35-151    Ready               Active
ouvx2l7qfcxisoyms8mtkgahw     ip-172-31-36-89     Ready               Active

    Filter by role if needed:

    $ docker node ls --filter role=manager
$ docker node ls --filter role=worker

  5. List Docker networks on all hosts. Each now has an overlay network called ingress and a bridge network called docker_gwbridge:

    $ docker network ls

NETWORK ID          NAME                DRIVER              SCOPE
495c570066be        bridge              bridge              local
961c6cae9945        docker_gwbridge     bridge              local
ff35ceda3643        host                host                local
trtnl4tqnc3n        ingress             overlay             swarm
c8357deec9cb        none                null                local

The docker_gwbridge connects the ingress network to the Docker host's network interface. If you create services without specifying a network, they connect to ingress. It's recommended to use separate overlay networks for each application or group of related applications.

Create the services

  1. On manager, create a new overlay network:

    $ docker network create -d overlay nginx-net

    The overlay network is automatically created on worker nodes when they run service tasks that need it.

  2. On manager, create a 5-replica Nginx service connected to nginx-net:

    Note

    Services can only be created on a manager.

    $ docker service create \
  --name my-nginx \
  --publish target=80,published=80 \
  --replicas=5 \
  --network nginx-net \
  nginx

    The default ingress publish mode means you can browse to port 80 on any node and connect to one of the 5 service tasks, even if no tasks run on that node.

  3. Monitor service creation progress:

    $ docker service ls

  4. Inspect the nginx-net network on all hosts. The Containers section lists all service tasks connected to the overlay network from that host.

  5. From manager, inspect the service:

    $ docker service inspect my-nginx

    Notice the information about ports and endpoints.

  6. Create a second network and update the service to use it:

    $ docker network create -d overlay nginx-net-2
$ docker service update \
  --network-add nginx-net-2 \
  --network-rm nginx-net \
  my-nginx

  7. Verify the update completed:

    $ docker service ls

    Inspect both networks to verify containers moved from nginx-net to nginx-net-2.

    Note

    Overlay networks are automatically created on swarm worker nodes as needed, but aren't automatically removed.

  8. Clean up:

    $ docker service rm my-nginx
$ docker network rm nginx-net nginx-net-2

Use a user-defined overlay network

This example shows the recommended approach for production services using custom overlay networks.

Prerequisites

This assumes the swarm is already set up and you're on a manager node.

Steps

  1. Create a user-defined overlay network:

    $ docker network create -d overlay my-overlay

  2. Start a service using the overlay network, publishing port 80 to port 8080:

    $ docker service create \
  --name my-nginx \
  --network my-overlay \
  --replicas 1 \
  --publish published=8080,target=80 \
  nginx:latest

  3. Verify the service task is connected to the network:

    $ docker network inspect my-overlay

    Check the Containers section for the my-nginx service task.

  4. Clean up:

    $ docker service rm my-nginx
$ docker network rm my-overlay

Use an overlay network for standalone containers

This example demonstrates DNS container discovery between standalone containers on different Docker hosts using an overlay network.

Prerequisites

You need two Docker hosts that can communicate with each other with the following ports open between them:

  • TCP port 2377
  • TCP and UDP port 7946
  • UDP port 4789

This example refers to the hosts as host1 and host2.

Steps

  1. Set up the swarm:

    On host1, initialize a swarm:

    $ docker swarm init
Swarm initialized: current node (vz1mm9am11qcmo979tlrlox42) is now a manager.

To add a worker to this swarm, run the following command:

    docker swarm join --token SWMTKN-1-5g90q48weqrtqryq4kj6ow0e8xm9wmv9o6vgqc5j320ymybd5c-8ex8j0bc40s6hgvy5ui5gl4gy 172.31.47.252:2377

    On host2, join the swarm using the token from the previous output:

    $ docker swarm join --token <your_token> <your_ip_address>:2377
This node joined a swarm as a worker.

    If the join fails, run docker swarm leave --force on host2, verify network and firewall settings, and try again.

  2. On host1, create an attachable overlay network:

    $ docker network create --driver=overlay --attachable test-net
uqsof8phj3ak0rq9k86zta6ht

    Note the returned network ID.

  3. On host1, start an interactive container that connects to test-net:

    $ docker run -it --name alpine1 --network test-net alpine
/ #

  4. On host2, list available networks. Notice that test-net doesn't exist yet:

    $ docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
ec299350b504        bridge              bridge              local
66e77d0d0e9a        docker_gwbridge     bridge              local
9f6ae26ccb82        host                host                local
omvdxqrda80z        ingress             overlay             swarm
b65c952a4b2b        none                null                local

  5. On host2, start a detached, interactive container that connects to test-net:

    $ docker run -dit --name alpine2 --network test-net alpine
fb635f5ece59563e7b8b99556f816d24e6949a5f6a5b1fbd92ca244db17a4342
    Note

    Automatic DNS container discovery only works with unique container names.

  6. On host2, verify that test-net was created with the same network ID as on host1:

    $ docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
...
uqsof8phj3ak        test-net            overlay             swarm

  7. On host1, ping alpine2 from within alpine1:

    / # ping -c 2 alpine2
PING alpine2 (10.0.0.5): 56 data bytes
64 bytes from 10.0.0.5: seq=0 ttl=64 time=0.600 ms
64 bytes from 10.0.0.5: seq=1 ttl=64 time=0.555 ms

--- alpine2 ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 0.555/0.577/0.600 ms

    The two containers communicate over the overlay network connecting the two hosts. You can also run another container on host2 and ping alpine1:

    $ docker run -it --rm --name alpine3 --network test-net alpine
/ # ping -c 2 alpine1
/ # exit

  8. On host1, close the alpine1 session (which stops the container):

    / # exit

  9. Clean up. You must stop and remove containers on each host independently:

    On host2:

    $ docker container stop alpine2
$ docker network ls
$ docker container rm alpine2

    When you stop alpine2, test-net disappears from host2.

    On host1:

    $ docker container rm alpine1
$ docker network rm test-net

Next steps