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Use bridge networks

In terms of networking, a bridge network is a Link Layer device
which forwards traffic between network segments. A bridge can be a hardware
device or a software device running within a host machine’s kernel.

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.

Bridge networks apply to containers running on the same Docker daemon host.
For communication among containers running on different Docker daemon hosts, you
can either manage routing at the OS level, or you can use an
overlay network.

When you start Docker, a default bridge network (also
called bridge) is created automatically, and newly-started containers connect
to it unless otherwise specified. You can also create user-defined custom bridge
networks. User-defined bridge networks are superior to the default bridge
network.

Differences between user-defined bridges and the default bridge

User-defined bridges provide automatic DNS resolution between containers.

Containers on the default bridge network can only access each other by IP
addresses, unless you use the --link option, which is
considered legacy. On a user-defined bridge network, containers can resolve
each other by name or alias.

Imagine an application with a web front-end and a database back-end. If you call
your containers web and db, the web container can connect to the db container
at db, no matter which Docker host the application stack is running on.

If you run the same application stack on the default bridge network, you need
to manually create links between the containers (using the legacy --link
flag). These links need to be created in both directions, so you can see this
gets complex with more than two containers which need to communicate.
Alternatively, you can manipulate the /etc/hosts files within the containers,
but this creates problems that are difficult to debug.
User-defined bridges provide better isolation.

All containers without a --network specified, are attached to the default bridge network. This can be a risk, as unrelated stacks/services/containers are then able to communicate.

Using a user-defined network provides a scoped network in which only containers attached to that network are able to communicate.
Containers can be attached and detached from user-defined networks on the fly.

During a container’s lifetime, you can connect or disconnect it from
user-defined networks on the fly. To remove a container from the default
bridge network, you need to stop the container and recreate it with different
network options.
Each user-defined network creates a configurable bridge.

If your containers use the default bridge network, you can configure it, but
all the containers use the same settings, such as MTU and iptables rules.
In addition, configuring the default bridge network happens outside of Docker
itself, and requires a restart of Docker.

User-defined bridge networks are created and configured using
docker network create. If different groups of applications have different
network requirements, you can configure each user-defined bridge separately,
as you create it.
Linked containers on the default bridge network share environment variables.

Originally, the only way to share environment variables between two containers
was to link them using the --link flag. This type of
variable sharing is not possible with user-defined networks. However, there
are superior ways to share environment variables. A few ideas:
- Multiple containers can mount a file or directory containing the shared
  information, using a Docker volume.
- Multiple containers can be started together using docker-compose and the
  compose file can define the shared variables.
- You can use swarm services instead of standalone containers, and take
  advantage of shared secrets and
  configs.

Containers connected to the same user-defined bridge network effectively expose all ports
to each other. For a port to be accessible to containers or non-Docker hosts on
different networks, that port must be published using the -p or --publish
flag.

Manage a user-defined bridge

Use the docker network create command to create a user-defined bridge
network.

$
 docker network create my-net

You can specify the subnet, the IP address range, the gateway, and other
options. See the
docker network create
reference or the output of docker network create --help for details.

Use the docker network rm command to remove a user-defined bridge
network. If containers are currently connected to the network,
disconnect them
first.

$
 
docker network rm my-net

What’s really happening?

When you create or remove a user-defined bridge or connect or disconnect a
container from a user-defined bridge, Docker uses tools specific to the
operating system to manage the underlying network infrastructure (such as adding
or removing bridge devices or configuring iptables rules on Linux). These
details should be considered implementation details. Let Docker manage your
user-defined networks for you.

Connect a container to a user-defined bridge

When you create a new container, you can specify one or more --network flags.
This example connects a Nginx container to the my-net network. It also
publishes port 80 in the container to port 8080 on the Docker host, so external
clients can access that port. Any other container connected to the my-net
network has access to all ports on the my-nginx container, and vice versa.

$
 
docker create --name
 my-nginx \
  --network
 my-net \
  --publish
 8080:80 \
  nginx:latest

To connect a running container to an existing user-defined bridge, use the
docker network connect command. The following command connects an already-running
my-nginx container to an already-existing my-net network:

$
 docker network connect my-net my-nginx

Disconnect a container from a user-defined bridge

To disconnect a running container from a user-defined bridge, use the docker network disconnect command. The following command disconnects the my-nginx
container from the my-net network.

$
 docker network disconnect my-net my-nginx

Use IPv6

If you need IPv6 support for Docker containers, you need to
enable the option on the Docker daemon and reload its
configuration, before creating any IPv6 networks or assigning containers IPv6
addresses.

When you create your network, you can specify the --ipv6 flag to enable
IPv6. You can’t selectively disable IPv6 support on the default bridge network.

Enable forwarding from Docker containers to the outside world

By default, traffic from containers connected to the default bridge network is
not forwarded to the outside world. To enable forwarding, you need to change
two settings. These are not Docker commands and they affect the Docker host’s
kernel.

Configure the Linux kernel to allow IP forwarding.
```
$
 
sysctl net.ipv4.conf.all.forwarding=1
```
Change the policy for the iptables FORWARD policy from DROP to
ACCEPT.
```
$
 
sudo 
iptables -P FORWARD ACCEPT
```

These settings do not persist across a reboot, so you may need to add them to a
start-up script.

Use the default bridge network

The default bridge network is considered a legacy detail of Docker and is not
recommended for production use. Configuring it is a manual operation, and it has
technical shortcomings.

Connect a container to the default bridge network

If you do not specify a network using the --network flag, and you do specify a
network driver, your container is connected to the default bridge network by
default. Containers connected to the default bridge network can communicate,
but only by IP address, unless they are linked using the
legacy --link flag.

Configure the default bridge network

To configure the default bridge network, you specify options in daemon.json.
Here is an example daemon.json with several options specified. Only specify
the settings you need to customize.

{

  
"bip"
:
 
"192.168.1.1/24"
,

  
"fixed-cidr"
:
 
"192.168.1.0/25"
,

  
"fixed-cidr-v6"
:
 
"2001:db8::/64"
,

  
"mtu"
:
 
1500
,

  
"default-gateway"
:
 
"192.168.1.254"
,

  
"default-gateway-v6"
:
 
"2001:db8:abcd::89"
,

  
"dns"
:
 
[
"10.20.1.2"
,
"10.20.1.3"
]

}

Restart Docker for the changes to take effect.

Use IPv6 with the default bridge network

If you configure Docker for IPv6 support (see Use IPv6), the
default bridge network is also configured for IPv6 automatically. Unlike
user-defined bridges, you can’t selectively disable IPv6 on the default bridge.