Attach services to an overlay network

Estimated reading time: 8 minutes

Docker Engine swarm mode natively supports overlay networks, so you can enable container-to-container networks. When you use swarm mode, you don’t need an external key-value store. Features of swarm mode overlay networks include the following:

You can attach multiple services to the same network.
By default, service discovery assigns a virtual IP address (VIP) and DNS entry to each service in the swarm, making it available by its service name to containers on the same network.
You can configure the service to use DNS round-robin instead of a VIP.

In order to use overlay networks in the swarm, you need to have the following ports open between the swarm nodes before you enable swarm mode:

Port 7946 TCP/UDP for container network discovery.
Port 4789 UDP for the container overlay network.

Create an overlay network in a swarm

When you run Docker Engine in swarm mode, you can run docker network create from a manager node to create an overlay network. For instance, to create a network named my-network:

$ docker network create \
  --driver overlay \
  --subnet 10.0.9.0/24 \
  --opt encrypted \
  my-network

273d53261bcdfda5f198587974dae3827e947ccd7e74a41bf1f482ad17fa0d33

By default nodes in the swarm encrypt traffic between themselves and other nodes. The optional --opt encrypted flag enables an additional layer of encryption in the overlay driver for vxlan traffic between containers on different nodes. For more information, refer to Docker swarm mode overlay network security model.

The --subnet flag specifies the subnet for use with the overlay network. When you don’t specify a subnet, the swarm manager automatically chooses a subnet and assigns it to the network. On some older kernels, including kernel 3.10, automatically assigned adresses may overlap with another subnet in your infrastructure. Such overlaps can cause connectivity issues or failures with containers connected to the network.

Before you attach a service to the network, the network only extends to manager nodes. You can run docker network ls to view the network:

$ docker network ls

NETWORK ID          NAME        DRIVER   SCOPE
f9145f09b38b        bridge      bridge   local
..snip..
273d53261bcd        my-network  overlay  swarm

The swarm scope indicates that the network is available for use with services deployed to the swarm. After you create a service attached to the network, the swarm only extends the network to worker nodes where the scheduler places tasks for the service. On workers without tasks running for a service attached to the network, network ls does not display the network.

Attach a service to an overlay network

To attach a service to an overlay network, pass the --network flag when you create a service. For example to create an nginx service attached to a network called my-network:

$ docker service create \
  --replicas 3 \
  --name my-web \
  --network my-network \
  nginx

Note: You have to create the network before you can attach a service to it.

The containers for the tasks in the service can connect to one another on the overlay network. The swarm extends the network to all the nodes with Running tasks for the service.

From a manager node, run docker service ps <SERVICE> to view the nodes where tasks are running for the service:

$ docker service ps my-web

NAME                                IMAGE  NODE   DESIRED STATE  CURRENT STATE               ERROR
my-web.1.63s86gf6a0ms34mvboniev7bs  nginx  node1  Running        Running 58 seconds ago
my-web.2.6b3q2qbjveo4zauc6xig7au10  nginx  node2  Running        Running 58 seconds ago
my-web.3.66u2hcrz0miqpc8h0y0f3v7aw  nginx  node3  Running        Running about a minute ago

service vip image

You can inspect the network from any node with a Running task for a service attached to the network:

$ docker network inspect <NETWORK>

The network information includes a list of the containers on the node that are attached to the network. For instance:

$ docker network inspect my-network
[
    {
        "Name": "my-network",
        "Id": "273d53261bcdfda5f198587974dae3827e947ccd7e74a41bf1f482ad17fa0d33",
        "Scope": "swarm",
        "Driver": "overlay",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "10.0.9.0/24",
                    "Gateway": "10.0.9.1"
                }
            ]
        },
        "Internal": false,
        "Containers": {
            "404d1dec939a021678132a35259c3604b9657649437e59060621a17edae7a819": {
                "Name": "my-web.1.63s86gf6a0ms34mvboniev7bs",
                "EndpointID": "3c9588d04db9bc2bf8749cb079689a3072c44c68e544944cbea8e4bc20eb7de7",
                "MacAddress": "02:42:0a:00:09:03",
                "IPv4Address": "10.0.9.3/24",
                "IPv6Address": ""
            }
        },
        "Options": {
            "com.docker.network.driver.overlay.vxlanid_list": "257"
        },
        "Labels": {}
    }
]

In the example above, the container my-web.1.63s86gf6a0ms34mvboniev7bs for the my-web service is attached to the my-network network on node1.

Use swarm mode service discovery

By default, when you create a service attached to a network, the swarm assigns the service a VIP. The VIP maps to a DNS alias based upon the service name. Containers on the network share DNS mappings for the service via gossip so any container on the network can access the service via its service name.

You don’t need to expose service-specific ports to make the service available to other services on the same overlay network. The swarm’s internal load balancer automatically distributes requests to the service VIP among the active tasks.

You can inspect the service to view the virtual IP. For example:

$ docker service inspect \
  --format='{{json .Endpoint.VirtualIPs}}' \
  my-web

[{"NetworkID":"7m2rjx0a97n88wzr4nu8772r3" "Addr":"10.0.0.2/24"}]

The following example shows how you can add a busybox service on the same network as the nginx service and the busybox service is able to access nginx using the DNS name my-web:

From a manager node, deploy a busybox service to the same network as my-web:

$ docker service create \
  --name my-busybox \
  --network my-network \
  busybox \
  sleep 3000

Lookup the node where my-busybox is running:

$ docker service ps my-busybox

NAME                                    IMAGE    NODE   DESIRED STATE  CURRENT STATE          ERROR
my-busybox.1.1dok2cmx2mln5hbqve8ilnair  busybox  node1  Running        Running 5 seconds ago

From the node where the busybox task is running, open an interactive shell to the busybox container:
```
$ docker exec -it my-busybox.1.1dok2cmx2mln5hbqve8ilnair /bin/sh
```
You can deduce the container name as <TASK-NAME>+<ID>. Alternatively, you can run docker ps on the node where the task is running.
From inside the busybox container, query the DNS to view the VIP for the my-web service:
```
$ nslookup my-web

Server:    127.0.0.11
Address 1: 127.0.0.11

Name:      my-web
Address 1: 10.0.9.2 ip-10-0-9-2.us-west-2.compute.internal
```
Note: the examples here use nslookup, but you can use dig or any available DNS query tool.

From inside the busybox container, query the DNS using a special query <tasks.SERVICE-NAME> to find the IP addresses of all the containers for the my-web service:

$ nslookup tasks.my-web

Server:    127.0.0.11
Address 1: 127.0.0.11

Name:      tasks.my-web
Address 1: 10.0.9.4 my-web.2.6b3q2qbjveo4zauc6xig7au10.my-network
Address 2: 10.0.9.3 my-web.1.63s86gf6a0ms34mvboniev7bs.my-network
Address 3: 10.0.9.5 my-web.3.66u2hcrz0miqpc8h0y0f3v7aw.my-network

From inside the busybox container, run wget to access the nginx web server running in the my-web service:
```
$ wget -O- my-web

Connecting to my-web (10.0.9.2:80)
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...snip...
```
The swarm load balancer automatically routes the HTTP request to the service’s VIP to an active task. It distributes subsequent requests to other tasks using round-robin selection.

Use DNS round-robin for a service

You can configure the service to use DNS round-robin directly without using a VIP, by setting the --endpoint-mode dnsrr when you create the service. DNS round-robin is useful in cases where you want to use your own load balancer.

The following example shows a service with dnsrr endpoint mode:

$ docker service create \
  --replicas 3 \
  --name my-dnsrr-service \
  --network my-network \
  --endpoint-mode dnsrr \
  nginx

When you query the DNS for the service name, the DNS service returns the IP addresses for all the task containers:

$ nslookup my-dnsrr-service
Server:    127.0.0.11
Address 1: 127.0.0.11

Name:      my-dnsrr
Address 1: 10.0.9.8 my-dnsrr-service.1.bd3a67p61by5dfdkyk7kog7pr.my-network
Address 2: 10.0.9.10 my-dnsrr-service.3.0sb1jxr99bywbvzac8xyw73b1.my-network
Address 3: 10.0.9.9 my-dnsrr-service.2.am6fx47p3bropyy2dy4f8hofb.my-network

Confirm VIP connectivity

In genaral we recommend you use dig, nslookup, or another DNS query tool to test access to the service name via DNS. Because a VIP is a logical IP, ping is not the right tool to confirm VIP connectivity.

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