What is Docker Networking?

Docker Networking is a fundamental aspect of Docker that enables communication between Docker containers and external networks and external networks. It allows you to connect containers together as well as connect containers to the host machine and other external resources.

A container has not information about what kind of network it's attached to, or whether their peers are also Docker workloads or not.

A container only sees a network interface with an IP address, a gateway, a routing table, DNS Services, and other networking details.

By default, when you create or run a container using docker create or docker run, the container doesn't expose any of its parts to the outside world.

Use the --publish or -p flag to make a port available to services outside of docker. This creates a firewall rule in the host, mapping a container port to a port on the docker host to the outside world.

Let's talk about some Network Drivers

Docker's networking is pluggable using drivers. Several drivers exist by default and provide core networking functionality.

In this context, "pluggable" means that Docker's networking system is modular and flexible. So, you can swap, add or customize networking components(drivers) without changing Docker's core itself.

Think of it like USB Devices, you can play in a mouse, keyboard, or external driver and each works differently, but the computer doesn't need to be redesigned.

Docker uses network drivers (like build-in or third party-ones) to handle how containers communicate Because it's pluggable.

Network Drivers

Bridge: The default bridge network is good for running containers that don't require special networking capabilities. User-defined bridge networks enable containers on the same docker host to communicate with each other. A user-defined network typically defines an isolated network for multiple containers belonging for a common project or component.

Host: Host network shares the hosts network with the container. When you use this driver, the containers network isn't isolated from the host.

Overlay: Overlay networks are best when you need containers running on different Docker hosts to communicate, or when multiple applications work together using swarm services.

Macvlan: Macvlan networks are best when you are migrating from a VM setup or need your containers to look like physical hosts on your network, each with unique MAC address.

IPvlan: The IPvlan driver gives users total control over both IPV4 and IPV6 addressing. IPvlan is new twist on the tried and true network virtualization technique. The linux implementation are extremely lightweight because rather than using the traditional linux bridge for isolation, they are associated to a Linux Ethernet interface or sub-interface to enforce separation between networks and connectivity to the physical network.

Bridge Network Drivers:

In terms of Docker, a bridge network uses a software bridge which allows containers connected to the same bridge network to communicate, while providing isolation from containers which are not connected to that bridge network.

The docker bridge driver automatically installs rules in the host machine so, that containers on different bridge networks cannot communicate directly with each other.

When you start, a default bridge network (also called a bridge) is created automatically, and newly started containers connect to it unless otherwise specified.

Containers can communicate to each other.
Containers and Host can communicate via bridge network.
Containers can only communicate by IP address not by container name.

User-defined Bridge Network

Possible to achieve isolation of container.
container 1 and container 2 can communicate to each other and to host via bridge
container 3 can't communicate to container 1 and container 2 but can't communicate to Host via bridge net1.
Containers can communicate by IP address and also resolve a container name to an IP address. This capability called automatic discovery.

Docker Networking Commands

docker network ls: It is used to list all networks. bridge is the default network.

docker network inspect bridge: It is used to inspect bridge network to see what containers are connected to it.

docker network create --driver bridge_network_name: It is used to create user defined bridge network.

docker run --name container_name -itd --network_name alpine: It is used to connect a container to a specified network e.g. host, user-defined bridge.

docker network connect container_name: It is used to connect a running container to an existing user-defined bridge.

docker network disconnect container_name: It is used to discover a running container to an existing defined bridge.

docker network rm network_name: It is used to delete specified network.

Let's look out some examples:

I created the container c1 from alpine image and after creating, if we inspect bridge network than you can see a container c1 is connected to bridge by default.

Let's create driver named net2

Now create a new container c3 and connect it to net2 network driver