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Estimated reading time: 8 minutes
In terms of networking, a bridge network is a Link Layer devicewhich forwards traffic between network segments. A bridge can be a hardwaredevice or a software device running within a host machine’s kernel.
In terms of Docker, a bridge network uses a software bridge which allowscontainers connected to the same bridge network to communicate, while providingisolation from containers which are not connected to that bridge network. TheDocker bridge driver automatically installs rules in the host machine so thatcontainers on different bridge networks cannot communicate directly with eachother.
Bridge networks apply to containers running on the same Docker daemon host.For communication among containers running on different Docker daemon hosts, youcan either manage routing at the OS level, or you can use anoverlay network.
When you start Docker, a default bridge network (alsocalled bridge
) is created automatically, and newly-started containers connectto it unless otherwise specified. You can also create user-defined custom bridgenetworks. User-defined bridge networks are superior to the default bridge
network.
Differences between user-defined bridges and the default bridge
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User-defined bridges provide automatic DNS resolution between containers.
Containers on the default bridge network can only access each other by IPaddresses, unless you use the
--link
option, which isconsidered legacy. On a user-defined bridge network, containers can resolveeach other by name or alias.Imagine an application with a web front-end and a database back-end. If you callyour containers
web
anddb
, the web container can connect to the db containeratdb
, no matter which Docker host the application stack is running on.If you run the same application stack on the default bridge network, you needto manually create links between the containers (using the legacy
--link
flag). These links need to be created in both directions, so you can see thisgets 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 fromuser-defined networks on the fly. To remove a container from the defaultbridge network, you need to stop the container and recreate it with differentnetwork options.
Each user-defined network creates a configurable bridge.
If your containers use the default bridge network, you can configure it, butall the containers use the same settings, such as MTU and
iptables
rules.In addition, configuring the default bridge network happens outside of Dockeritself, and requires a restart of Docker.User-defined bridge networks are created and configured using
docker network create
. If different groups of applications have differentnetwork 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 containerswas to link them using the
--link
flag. This type ofvariable sharing is not possible with user-defined networks. However, thereare superior ways to share environment variables. A few ideas:Multiple containers can mount a file or directory containing the sharedinformation, using a Docker volume.
Multiple containers can be started together using
docker-compose
and thecompose file can define the shared variables.You can use swarm services instead of standalone containers, and takeadvantage of shared secrets andconfigs.
Containers connected to the same user-defined bridge network effectively expose all portsto each other. For a port to be accessible to containers or non-Docker hosts ondifferent 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 bridgenetwork.
You can specify the subnet, the IP address range, the gateway, and otheroptions. See thedocker network createreference or the output of docker network create --help
for details.
Use the docker network rm
command to remove a user-defined bridgenetwork. If containers are currently connected to the network,disconnect themfirst.
What’s really happening?
When you create or remove a user-defined bridge or connect or disconnect acontainer from a user-defined bridge, Docker uses tools specific to theoperating system to manage the underlying network infrastructure (such as addingor removing bridge devices or configuring iptables
rules on Linux). Thesedetails should be considered implementation details. Let Docker manage youruser-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 alsopublishes port 80 in the container to port 8080 on the Docker host, so externalclients 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.
To connect a running container to an existing user-defined bridge, use thedocker network connect
command. The following command connects an already-runningmy-nginx
container to an already-existing my-net
network:
Disconnect a container from a user-defined bridge
To disconnect a running container from a user-defined bridge, use the dockernetwork disconnect
command. The following command disconnects the my-nginx
container from the my-net
network.
Use IPv6
If you need IPv6 support for Docker containers, you need toenable the option on the Docker daemon and reload itsconfiguration, before creating any IPv6 networks or assigning containers IPv6addresses.
When you create your network, you can specify the --ipv6
flag to enableIPv6. 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 isnot forwarded to the outside world. To enable forwarding, you need to changetwo settings. These are not Docker commands and they affect the Docker host’skernel.
Configure the Linux kernel to allow IP forwarding.
Change the policy for the
iptables
FORWARD
policy fromDROP
toACCEPT
.
These settings do not persist across a reboot, so you may need to add them to astart-up script.
Use the default bridge network
The default bridge
network is considered a legacy detail of Docker and is notrecommended for production use. Configuring it is a manual operation, and it hastechnical shortcomings.
Connect a container to the default bridge network
If you do not specify a network using the --network
flag, and you do specify anetwork driver, your container is connected to the default bridge
network bydefault. Containers connected to the default bridge
network can communicate,but only by IP address, unless they are linked using thelegacy --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 specifythe settings you need to customize.
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), thedefault bridge network is also configured for IPv6 automatically. Unlikeuser-defined bridges, you can’t selectively disable IPv6 on the default bridge.
Next steps
- Go through the standalone networking tutorial
- Learn about networking from the container’s point of view
- Learn about overlay networks
- Learn about Macvlan networks