Manage swarm service networks (2024)

Table of contents

This page describes networking for swarm services.

A Docker swarm generates two different kinds of traffic:

Control and management plane traffic: This includes swarm managementmessages, such as requests to join or leave the swarm. This traffic isalways encrypted.
Application data plane traffic: This includes container traffic andtraffic to and from external clients.

Key network concepts

The following three network concepts are important to swarm services:

Overlay networks manage communications among the Docker daemonsparticipating in the swarm. You can create overlay networks, in the same wayas user-defined networks for standalone containers. You can attach a serviceto one or more existing overlay networks as well, to enable service-to-servicecommunication. Overlay networks are Docker networks that use the overlaynetwork driver.
The ingress network is a special overlay network that facilitatesload balancing among a service's nodes. When any swarm node receives arequest on a published port, it hands that request off to a module calledIPVS. IPVS keeps track of all the IP addresses participating in thatservice, selects one of them, and routes the request to it, over theingress network.
The ingress network is created automatically when you initialize or join aswarm. Most users do not need to customize its configuration, but Docker allowsyou to do so.
The docker_gwbridge is a bridge network that connects the overlaynetworks (including the ingress network) to an individual Docker daemon'sphysical network. By default, each container a service is running is connectedto its local Docker daemon host's docker_gwbridge network.
The docker_gwbridge network is created automatically when you initialize orjoin a swarm. Most users do not need to customize its configuration, butDocker allows you to do so.

Tip
See also Networking overview for more details about Swarm networking in general.

Docker daemons participating in a swarm need the ability to communicate witheach other over the following ports:

Port 7946 TCP/UDP for container network discovery.
Port 4789 UDP (configurable) for the overlay network (including ingress) data path.

When setting up networking in a Swarm, special care should be taken. Consultthe tutorialfor an overview.

Overlay networking

When you initialize a swarm or join a Docker host to an existing swarm, twonew networks are created on that Docker host:

An overlay network called ingress, which handles the control and data trafficrelated to swarm services. When you create a swarm service and do notconnect it to a user-defined overlay network, it connects to the ingressnetwork by default.
A bridge network called docker_gwbridge, which connects the individualDocker daemon to the other daemons participating in the swarm.

Create an overlay network

To create an overlay network, specify the overlay driver when using thedocker network create command:

$ docker network create \ --driver overlay \ my-network

The above command doesn't specify any custom options, so Docker assigns asubnet and uses default options. You can see information about the network usingdocker network inspect.

When no containers are connected to the overlay network, its configuration isnot very exciting:

$ docker network inspect my-network[ { "Name": "my-network", "Id": "fsf1dmx3i9q75an49z36jycxd", "Created": "0001-01-01T00:00:00Z", "Scope": "swarm", "Driver": "overlay", "EnableIPv6": false, "IPAM": { "Driver": "default", "Options": null, "Config": [] }, "Internal": false, "Attachable": false, "Ingress": false, "Containers": null, "Options": { "com.docker.network.driver.overlay.vxlanid_list": "4097" }, "Labels": null }]

Customize an overlay network

There may be situations where you don't want to use the default configurationfor an overlay network. For a full list of configurable options, run thecommand docker network create --help. The following are some of the mostcommon options to change.

Configure the subnet and gateway

By default, the network's subnet and gateway are configured automatically whenthe first service is connected to the network. You can configure these whencreating a network using the --subnet and --gateway flags. The followingexample extends the previous one by configuring the subnet and gateway.

$ docker network create \ --driver overlay \ --subnet 10.0.9.0/24 \ --gateway 10.0.9.99 \ my-network

Using custom default address pools

To customize subnet allocation for your Swarm networks, you canoptionally configure them during swarm init.

For example, the following command is used when initializing Swarm:

$ docker swarm init --default-addr-pool 10.20.0.0/16 --default-addr-pool-mask-length 26

Whenever a user creates a network, but does not use the --subnet command line option, the subnet for this network will be allocated sequentially from the next available subnet from the pool. If the specified network is already allocated, that network will not be used for Swarm.

Multiple pools can be configured if discontiguous address space is required. However, allocation from specific pools is not supported. Network subnets will be allocated sequentially from the IP pool space and subnets will be reused as they are deallocated from networks that are deleted.

The default mask length can be configured and is the same for all networks. It is set to /24 by default. To change the default subnet mask length, use the --default-addr-pool-mask-length command line option.

Note
Default address pools can only be configured on swarm init and cannot be altered after cluster creation.

Overlay network size limitations

Docker recommends creating overlay networks with /24 blocks. The /24 overlay network blocks limit the network to 256 IP addresses.

This recommendation addresseslimitations with swarm mode.If you need more than 256 IP addresses, do not increase the IP block size. You can either use dnsrrendpoint mode with an external load balancer, or use multiple smaller overlay networks. SeeConfigure service discovery for more information about different endpoint modes.

Configure encryption of application data

Management and control plane data related to a swarm is always encrypted.For more details about the encryption mechanisms, see theDocker swarm mode overlay network security model.

Configure service discovery

Service discovery is the mechanism Docker uses to route a request from yourservice's external clients to an individual swarm node, without the clientneeding to know how many nodes are participating in the service or theirIP addresses or ports. You don't need to publish ports which are used betweenservices on the same network. For instance, if you have aWordPress service that stores its data in a MySQL service,and they are connected to the same overlay network, you do not need to publishthe MySQL port to the client, only the WordPress HTTP port.

Service discovery can work in two different ways: internal connection-basedload-balancing at Layers 3 and 4 using the embedded DNS and a virtual IP (VIP),or external and customized request-based load-balancing at Layer 7 using DNSround robin (DNSRR). You can configure this per service.

By default, when you attach a service to a network and that service publishesone or more ports, Docker assigns the service a virtual IP (VIP), which is the"front end" for clients to reach the service. Docker keeps a list of allworker nodes in the service, and routes requests between the client and one ofthe nodes. Each request from the client might be routed to a different node.
If you configure a service to use DNS round-robin (DNSRR) service discovery,there is not a single virtual IP. Instead, Docker sets up DNS entries for theservice such that a DNS query for the service name returns a list of IPaddresses, and the client connects directly to one of these.
DNS round-robin is useful in cases where you want to use your own loadbalancer, such as HAProxy. To configure a service to use DNSRR, use the flag--endpoint-mode dnsrr when creating a new service or updating an existingone.

Customize the ingress network

Most users never need to configure the ingress network, but Docker allows youto do so. This can be useful if the automatically-chosen subnetconflicts with one that already exists on your network, or you need to customizeother low-level network settings such as the MTU, or if you want toenable encryption.

Customizing the ingress network involves removing and recreating it. This isusually done before you create any services in the swarm. If you have existingservices which publish ports, those services need to be removed before you canremove the ingress network.

During the time that no ingress network exists, existing services which do notpublish ports continue to function but are not load-balanced. This affectsservices which publish ports, such as a WordPress service which publishes port80.

Inspect the ingress network using docker network inspect ingress, andremove any services whose containers are connected to it. These are servicesthat publish ports, such as a WordPress service which publishes port 80. Ifall such services are not stopped, the next step fails.

Remove the existing ingress network:

$ docker network rm ingressWARNING! Before removing the routing-mesh network, make sure all the nodesin your swarm run the same docker engine version. Otherwise, removal may notbe effective and functionality of newly created ingress networks will beimpaired.Are you sure you want to continue? [y/N]

Create a new overlay network using the --ingress flag, along with thecustom options you want to set. This example sets the MTU to 1200, setsthe subnet to 10.11.0.0/16, and sets the gateway to 10.11.0.2.
$ 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 onefails.
Restart the services that you stopped in the first step.

The docker_gwbridge is a virtual bridge that connects the overlay networks(including the ingress network) to an individual Docker daemon's physicalnetwork. Docker creates it automatically when you initialize a swarm or join aDocker host to a swarm, but it is not a Docker device. It exists in the kernelof the Docker host. If you need to customize its settings, you must do so beforejoining the Docker host to the swarm, or after temporarily removing the hostfrom the swarm.

You need to have the brctl application installed on your operating system inorder to delete an existing bridge. The package name is bridge-utils.

Stop Docker.
Use the brctl show docker_gwbridge command to check whether a bridgedevice exists called docker_gwbridge. If so, remove it usingbrctl delbr docker_gwbridge.
Start Docker. Do not join or initialize the swarm.

Create or re-create the docker_gwbridge bridge with your custom settings.This example uses the subnet 10.11.0.0/16. For a full list of customizableoptions, seeBridge 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.

Use a separate interface for control and data traffic

By default, all swarm traffic is sent over the same interface, including controland management traffic for maintaining the swarm itself and data traffic to andfrom the service containers.

You can separate this traffic by passingthe --data-path-addr flag when initializing or joining the swarm. If there aremultiple interfaces, --advertise-addr must be specified explicitly, and--data-path-addr defaults to --advertise-addr if not specified. Traffic aboutjoining, leaving, and managing the swarm is sent over the--advertise-addr interface, and traffic among a service's containers is sentover the --data-path-addr interface. These flags can take an IP address ora network device name, such as eth0.

This example initializes a swarm with a separate --data-path-addr. It assumesthat your Docker host has two different network interfaces: 10.0.0.1 should beused for control and management traffic and 192.168.0.1 should be used fortraffic relating to services.

$ docker swarm init --advertise-addr 10.0.0.1 --data-path-addr 192.168.0.1

This example joins the swarm managed by host 192.168.99.100:2377 and sets the--advertise-addr flag to eth0 and the --data-path-addr flag to eth1.

$ docker swarm join \ --token SWMTKN-1-49nj1cmql0jkz5s954yi3oex3nedyz0fb0xx14ie39trti4wxv-8vxv8rssmk743ojnwacrr2d7c \ --advertise-addr eth0 \ --data-path-addr eth1 \ 192.168.99.100:2377

Swarm services connected to the same overlay network effectively expose allports to each other. For a port to be accessible outside of the service, thatport must be published using the -p or --publish flag on docker service create or docker service update. Both the legacy colon-separated syntax andthe newer comma-separated value syntax are supported. The longer syntax ispreferred 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.

Learn more

Deploy services to a swarm
Swarm administration guide
Swarm mode tutorial
Networking overview
Docker CLI reference