Use overlay networks
Estimated reading time: 11 minutesThe 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 (including swarm service
containers) to communicate securely. Docker transparently handles routing of
each packet to and from the correct Docker daemon host and the correct
destination container.
When you initialize a swarm or join a Docker host to an existing swarm, two new networks are created on that Docker host:
- an overlay network called
ingress
, which handles control and data traffic related to swarm services. When you create a swarm service and do not connect it to a user-defined overlay network, it connects to theingress
network by default. - a bridge network called
docker_gwbridge
, which connects the individual Docker daemon to the other daemons participating in the swarm.
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 are each connected to.
Although you can connect both swarm services and standalone containers to an overlay network, the default behaviors and configuration concerns are different. For that reason, the rest of this topic is divided into operations that apply to all overlay networks, those that apply to swarm service networks, and those that apply to overlay networks used by standalone containers.
Operations for all overlay networks
Create an overlay network
Prerequisites:
Firewall rules for Docker daemons using overlay networks
You need the following ports open to traffic to and from each Docker host participating on an overlay network:
- TCP port 2377 for cluster management communications
- TCP and UDP port 7946 for communication among nodes
- UDP port 4789 for overlay network traffic
Before you can create an overlay network, you need to either initialize your Docker daemon as a swarm manager using
docker swarm init
or join it to an existing swarm usingdocker swarm join
. Either of these creates the defaultingress
overlay network which is used by swarm services by default. You need to do this even if you never plan to use swarm services. Afterward, you can create additional user-defined overlay networks.
To create an overlay network for use with swarm services, use a command like the following:
$ docker network create -d overlay my-overlay
To create an overlay network which can be used by swarm services or
standalone containers to communicate with other standalone containers running on
other Docker daemons, add the --attachable
flag:
$ docker network create -d overlay --attachable my-attachable-overlay
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
All swarm service management traffic is encrypted by default, using the AES algorithm in GCM mode. Manager nodes in the swarm rotate the key used to encrypt gossip data every 12 hours.
To encrypt application data as well, add --opt encrypted
when creating the
overlay network. This enables IPSEC encryption at the level of the vxlan. This
encryption imposes a non-negligible performance penalty, so you should test this
option before using it in production.
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.
Do not attach Windows nodes to encrypted overlay networks.
Overlay network encryption is not supported on Windows. If a Windows node attempts to connect to an encrypted overlay network, no error is detected but the node cannot communicate.
Swarm mode overlay networks and standalone containers
You can use the overlay network feature with both --opt encrypted --attachable
and attach unmanaged containers to that network:
$ docker network create --opt encrypted --driver overlay --attachable my-attachable-multi-host-network
Customize the default ingress network
Most users never need to configure the ingress
network, but Docker 17.05 and
higher allow you to do so. This can be useful if the automatically-chosen subnet
conflicts with one that already exists on your network, or you need to customize
other low-level network settings such as the MTU.
Customizing the ingress
network involves removing and recreating it. This is
usually done before you create any services in the swarm. If you have existing
services which publish ports, those services need to be removed before you can
remove the ingress
network.
During the time that no ingress
network exists, existing services which do not
publish ports continue to function but are not load-balanced. This affects
services which publish ports, such as a WordPress service which publishes port
80.
-
Inspect the
ingress
network usingdocker network inspect ingress
, and remove any services whose containers are connected to it. These are services that publish ports, such as a WordPress service which publishes port 80. If all such services are not stopped, the next step fails. -
Remove the existing
ingress
network:$ docker network rm ingress WARNING! Before removing the routing-mesh network, make sure all the nodes in your swarm run the same docker engine version. Otherwise, removal may not be effective and functionality of newly created ingress networks will be impaired. Are you sure you want to continue? [y/N]
-
Create a new overlay network using the
--ingress
flag, along with the custom options you want to set. This example sets the MTU to 1200, sets the subnet to10.11.0.0/16
, and sets the gateway to10.11.0.2
.$ docker network create \ --driver overlay \ --ingress \ --subnet=10.11.0.0/16 \ --gateway=10.11.0.2 \ --opt com.docker.network.driver.mtu=1200 \ my-ingress
Note: You can name your
ingress
network something other thaningress
, but you can only have one. An attempt to create a second one fails. -
Restart the services that you stopped in the first step.
Customize the docker_gwbridge interface
The docker_gwbridge
is a virtual bridge that connects the overlay networks
(including the ingress
network) to an individual Docker daemon’s physical
network. Docker creates it automatically when you initialize a swarm or join a
Docker host to a swarm, but it is not a Docker device. It exists in the kernel
of the Docker host. If you need to customize its settings, you must do so before
joining the Docker host to the swarm, or after temporarily removing the host
from the swarm.
-
Stop Docker.
-
Delete the existing
docker_gwbridge
interface.$ sudo ip link set docker_gwbridge down $ sudo ip link del dev docker_gwbridge
-
Start Docker. Do not join or initialize the swarm.
-
Create or re-create the
docker_gwbridge
bridge manually with your custom settings, using thedocker network create
command. This example uses the subnet10.11.0.0/16
. For a full list of customizable options, see Bridge driver options.$ docker network create \ --subnet 10.11.0.0/16 \ --opt com.docker.network.bridge.name=docker_gwbridge \ --opt com.docker.network.bridge.enable_icc=false \ --opt com.docker.network.bridge.enable_ip_masquerade=true \ docker_gwbridge
-
Initialize or join the swarm. Since the bridge already exists, Docker does not create it with automatic settings.
Operations for swarm services
Publish ports on an overlay network
Swarm services connected to the same overlay network effectively expose all
ports to each other. For a port to be accessible outside of the service, that
port must be published using the -p
or --publish
flag on docker service
create
or docker service update
. Both the legacy colon-separated syntax and
the newer comma-separated value syntax are supported. The longer syntax is
preferred because it is somewhat self-documenting.
Flag value | Description |
---|---|
-p 8080:80 or -p published=8080,target=80 |
Map TCP port 80 on the service to port 8080 on the routing mesh. |
-p 8080:80/udp or -p published=8080,target=80,protocol=udp |
Map UDP port 80 on the service to port 8080 on the routing mesh. |
-p 8080:80/tcp -p 8080:80/udp or -p published=8080,target=80,protocol=tcp -p published=8080,target=80,protocol=udp |
Map TCP port 80 on the service to TCP port 8080 on the routing mesh, and map UDP port 80 on the service to UDP port 8080 on the routing mesh. |
Bypass the routing mesh for a swarm service
By default, swarm services which publish ports do so using the routing mesh.
When you connect to a published port on any swarm node (whether it is running a
given service or not), you are redirected to a worker which is running that
service, transparently. Effectively, Docker acts as a load balancer for your
swarm services. Services using the routing mesh are running in virtual IP (VIP)
mode. Even a service running on each node (by means of the --mode global
flag) uses the routing mesh. When using the routing mesh, there is no guarantee
about which Docker node services client requests.
To bypass the routing mesh, you can start a service using DNS Round Robin
(DNSRR) mode, by setting the --endpoint-mode
flag to dnsrr
. You must run
your own load balancer in front of the service. A DNS query for the service name
on the Docker host returns a list of IP addresses for the nodes running the
service. Configure your load balancer to consume this list and balance the
traffic across the nodes.
Separate control and data traffic
By default, control traffic relating to swarm management and traffic to and from
your applications runs over the same network, though the swarm control traffic
is encrypted. You can configure Docker to use separate network interfaces for
handling the two different types of traffic. When you initialize or join the
swarm, specify --advertise-addr
and --datapath-addr
separately. You must do
this for each node joining the swarm.
Operations for standalone containers on overlay networks
Attach a standalone container to an overlay network
The ingress
network is created without the --attachable
flag, which means
that only swarm services can use it, and not standalone containers. You can
connect standalone containers to user-defined overlay networks which are created
with the --attachable
flag. This gives standalone containers running on
different Docker daemons the ability to communicate without the need to set up
routing on the individual Docker daemon hosts.
Publish ports
Flag value | Description |
---|---|
-p 8080:80 |
Map TCP port 80 in the container to port 8080 on the overlay network. |
-p 8080:80/udp |
Map UDP port 80 in the container to port 8080 on the overlay network. |
-p 8080:80/sctp |
Map SCTP port 80 in the container to port 8080 on the overlay network. |
-p 8080:80/tcp -p 8080:80/udp |
Map 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. |
Container discovery
For most situations, you should connect to the service name, which is load-balanced and handled by all containers (“tasks”) backing the service. To get a list of all tasks backing the service, do a DNS lookup for tasks.<service-name>.
Next steps
- Go through the overlay networking tutorial
- Learn about networking from the container’s point of view
- Learn about standalone bridge networks
- Learn about Macvlan networks