What is Docker Networking?

For Docker containers to communicate with each other and the outside world via the host machine, there has to be a layer of networking involved. Docker supports different types of networks, each fit for certain use cases.

For example, building an application that runs on a single Docker container will have a different network setup as compared to a web application with a cluster with a database, application, and load balancers that span multiple containers that need to communicate with each other. Additionally, clients from the outside world will need to access the web application container.

Docker Default Networking (docker0)

When Docker is installed, a default bridge network named docker0 is created. Each new Docker container is automatically attached to this network unless a custom network is specified.

Besides docker0, two other networks get created automatically by Docker: host (no isolation between host and containers on this network, to the outside world they are on the same network) and none (attached containers run on container-specific network stack).

Docker Network Types

Docker comes with network drivers geared toward different use cases. The most common network types are

1. Bridge
2. Overlay
3. Macvlan.

Bridge Networks

Bridge networking is the most common network type. It is limited to containers within a single host running the Docker engine. Bridge networks are easy to create, manage and troubleshoot.

For the containers on the bridge network to communicate or be reachable from the outside world, port mapping needs to be configured. As an example, consider you can have a Docker container running a web service on port 80. Because this container is attached to the bridge network on a private subnet, a port on the host system like 8000 needs to be mapped to port 80 on the container for outside traffic to reach the web service.

To create a bridge network named my-bridge-net, pass the argument bridge to the -d (driver) parameter as shown below:

$ docker network create -d bridge my-bridge-net

Overlay Networks

An overlay network uses software virtualization to create additional layers of network abstraction running on top of a physical network. In Docker, an overlay network driver is used for multi-host network communication. This driver utilizes Virtual Extensible LAN (VXLAN) technology which provide portability between cloud, on-premise and virtual environments. VXLAN solves common portability limitations by extending layer 2 subnets across layer 3 network boundaries, hence containers can run on foreign IP subnets.

To create an overlay network named my-overlay-net, you’ll also need the --subnet parameter to specify the network block that Docker will use to assign IP addresses to the containers:

$ docker network create -d overlay --subnet=192.168.10.0/24 my-overlay-net

Macvlan Networks

The macvlan driver is used to connect Docker containers directly to the host network interfaces through layer 2 segmentation. No use of port mapping or network address translation (NAT) is needed and containers can be assigned a public IP address which is accessible from the outside world. Latency in macvlan networks is low since packets are routed directly from Docker host network interface controller (NIC) to the containers.

Note that macvlan has to be configured per host, and has support for physical NIC, sub-interface, network bonded interfaces and even teamed interfaces. Traffic is explicitly filtered by the host kernel modules for isolation and security. To create a macvlan network named macvlan-net, you’ll need to provide a --gateway parameter to specify the IP address of the gateway for the subnet, and a -o parameter to set driver specific options. In this example, the parent interface is set to eth0 interface on the host:

$ docker network create -d macvlan \
  --subnet=192.168.40.0/24 \
  --gateway=192.168.40.1 \
  -o parent=eth0 my-macvlan-net

How Containers Communicate with Each Other

Different networks provide different communication patterns (for example by IP address only, or by container name) between containers depending on the network type and whether it’s a Docker default or a user-defined network.

Container discovery on docker0 network (DNS resolution) Docker will assign a name and hostname to each container created on the default docker0 network unless a different name/hostname is specified by the user. Docker then keeps a mapping of each name/hostname against the container’s IP address. This mapping allows pinging each container by name as opposed to IP address.

Directly linking containers

It is possible to directly link one container to another using the --link option when starting a container. This allows containers to discover each other and securely transfer information about one container to another container. However, Docker has deprecated this feature and recommends creating user-defined networks instead.

As an example, imagine you have a mydb container running database service. We can then create an application container named myweb and directly link it to mydb:

$ docker run --name myweb --link mydb:mydb -d -P myapp python app.py

How Containers Communicate with the Outside World

There are different ways in which Docker containers can communicate with the outside world, as detailed below.

Exposing Ports and Forwarding Traffic

In most cases, Docker networks use subnets without access from the outside world. To allow requests from the Internet to reach the container, you’ll need to map container ports to ports on the container’s host. For example, a request to hostname:8000 will be forwarded to whatever service is running inside the container on port 80, if a mapping from host port 8000 to container port 80 to was previously defined.

Containers Connected to Multiple Networks

Fine-grained network policies for connectivity and isolation can be achieved by joining containers to multiple networks. By default each container will be attached to a single network. More networks can be attached to a container by creating it first with docker create (instead of docker run) and then running the command docker network connect. For example:

$ docker network create net1 # creates bridge network name net1
$ docker network create net2 # creates bridge network name net2
$ docker create -it --net net1 --name cont1 busybox sh # creates container named cont1 attached to network net1
$ docker network connect net2 cont1 # further attaches container cont1 to network net2

The container is now connected to two distinct networks simultaneously.

Common Operations

Some common operations with Docker networking include:

Inspect a network: To see a specific network’s configuration details like subnet information, network name, IPAM driver, network ID, network driver, or connected containers, use the docker network inspect command.

 docker network inspect [OPTIONS] NETWORK [NETWORK...]

List all networks: Run docker network ls to display all networks (along with their type and scope) present on the current host.

 docker network ls

Create a new network: To create a new network, use the docker network create command and specify if it’s of type bridge (default), overlay, or macvlan.

 docker network create [OPTIONS] NETWORK

Run or connect a container to a specific network: Note, first of all, that the network must exist already on the host. Either specify the network at container creation/startup time (docker create or docker run) with the --net option; or attach an existing container by using the docker network connect command.

docker network connect my-network my-container

Disconnect a container from a network: The container must be running to disconnect it from the network using the docker network disconnect command.

docker network disconnect [OPTIONS] NETWORK CONTAINER

Remove an existing network: A network can only be removed using the command docker network rm if there are no containers attached to it. When a network is removed, the associated bridge will be removed as well.

 docker network rm NETWORK [NETWORK...]

Docker Networking with Multiple Hosts

When working with multi-host, there is a need to use higher-level Docker orchestration tools to ease the management of networking between a cluster of machines. Popular orchestration tools today include Docker Swarm, Kubernetes, and Apache Mesos.

Docker Swarm

Docker Swarm is a Docker Inc. native tool used to orchestrate Docker containers. It enables you to manage a cluster of hosts as a single resource pool.

Docker Swarm makes use of overlay networks for inter-host communication. The swarm manager service is responsible for automatically assigning IP addresses to the containers.

For service discovery, each service in the swarm gets assigned a unique DNS name. Additionally, Docker Swarm has an embedded DNS server. You can query every container running in the swarm through this embedded DNS server.

Kubernetes

Kubernetes Guide is a system used for automating deployment, scaling, and management of containerized applications, either on a single host or across a cluster of hosts.

Kubernetes approaches networking in a different way as compared to Docker, using native concepts like services and pods. Each pod has an IP address and no linking of pods is required, neither do you need to explicitly map container ports to host ports. There are DNS-based service discovery plugins that can be used for service discovery. Apache Mesos Apache Mesos is an open-source project used to manage a cluster of containers, providing efficient resource sharing and isolation across distributed applications.

Mesos uses an IP address management (IPAM) server and client to manage container networking. The role of the IPAM server is to assign IP addresses on demand while the IPAM client acts as a bridge between a network isolator module and the IPAM server. A network isolator module is a lightweight module that’s loaded into the Mesos agent. It looks at scheduler task requirements and uses IPAM and network isolator services to provide IP addresses to the containers.

Mesos-DNS is a DNS-based service discovery for Mesos. It allows applications and services running on Mesos to find each other through the DNS service.

Creating a New Network Driver Plugin

Docker plugins are out-of-process extensions that add capabilities to the Docker Engine. The Docker engine network plugins API allows for the support of a wide range of networking technologies to be realized. Once a networking plugin has been developed and installed, they behave just like the built-in bridge, overlay, and macvlan network drivers.

Summary

Docker offers a mature networking model. There are three common Docker network types – bridge networks, used within a single host, overlay networks, for multi-host communication, and macvlan networks which are used to connect Docker containers directly to host network interfaces.

In this page, we explained how Docker containers discover and communicate with each other and how they communicate with the outside world. We showed how to perform common operations such as inspecting a network, creating a new network, and disconnecting a container from a network. Finally, we briefly reviewed how docker networking works in the context of common orchestration platforms – Docker Swarm, Kubernetes Guide, and Apache Mesos.