Bridge network driver
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 lets containers connected to the same bridge network communicate, while providing isolation from containers that aren't connected to that bridge network. The Docker bridge driver automatically installs rules in the host machine so that containers on different bridge networks can't 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
anddb
, the web container can connect to the db container atdb
, 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 isn't 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.
Options
The following table describes the driver-specific options that you can pass to
--option
when creating a custom network using the bridge
driver.
Option | Default | Description |
---|---|---|
com.docker.network.bridge.name | Interface name to use when creating the Linux bridge. | |
com.docker.network.bridge.enable_ip_masquerade | true | Enable IP masquerading. |
com.docker.network.bridge.gateway_mode_ipv4 com.docker.network.bridge.gateway_mode_ipv6 | nat | Enable NAT and masquerading (nat ), or only allow direct routing to the container (routed ). |
com.docker.network.bridge.enable_icc | true | Enable or Disable inter-container connectivity. |
com.docker.network.bridge.host_binding_ipv4 | all IPv4 and IPv6 addresses | Default IP when binding container ports. |
com.docker.network.driver.mtu | 0 (no limit) | Set the containers network Maximum Transmission Unit (MTU). |
com.docker.network.container_iface_prefix | eth | Set a custom prefix for container interfaces. |
com.docker.network.bridge.inhibit_ipv4 | false | Prevent Docker from assigning an IP address to the network. |
Some of these options are also available as flags to the dockerd
CLI, and you
can use them to configure the default docker0
bridge when starting the Docker
daemon. The following table shows which options have equivalent flags in the
dockerd
CLI.
Option | Flag |
---|---|
com.docker.network.bridge.name | - |
com.docker.network.bridge.enable_ip_masquerade | --ip-masq |
com.docker.network.bridge.enable_icc | --icc |
com.docker.network.bridge.host_binding_ipv4 | --ip |
com.docker.network.driver.mtu | --mtu |
com.docker.network.container_iface_prefix | - |
The Docker daemon supports a --bridge
flag, which you can use to define
your own docker0
bridge. Use this option if you want to run multiple daemon
instances on the same host. For details, see
Run multiple daemons.
Default host binding address
When no host address is given in port publishing options like -p 80
or -p 8080:80
, the default is to make the container's port 80 available on all
host addresses, IPv4 and IPv6.
The bridge network driver option com.docker.network.bridge.host_binding_ipv4
can be used to modify the default address for published ports.
Despite the option's name, it is possible to specify an IPv6 address.
When the default binding address is an address assigned to a specific interface, the container's port will only be accessible via that address.
Setting the default binding address to ::
means published ports will only be
available on the host's IPv6 addresses. However, setting it to 0.0.0.0
means it
will be available on the host's IPv4 and IPv6 addresses.
To restrict a published port to IPv4 only, the address must be included in the
container's publishing options. For example, -p 0.0.0.0:8080:80
.
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 an 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 in a user-defined bridge network
When you create your network, you can specify the --ipv6
flag to enable IPv6.
$ docker network create --ipv6 --subnet 2001:db8:1234::/64 my-net
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're 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",
"mtu": 1500,
"default-gateway": "192.168.1.254",
"dns": ["10.20.1.2","10.20.1.3"]
}
Restart Docker for the changes to take effect.
Use IPv6 with the default bridge network
IPv6 can be enabled for the default bridge using the following options in
daemon.json
, or their command line equivalents.
These three options only affect the default bridge, they are not used by user-defined networks. The addresses in below are examples from the IPv6 documentation range.
- Option
ipv6
is required - Option
fixed-cidr-v6
is required, it specifies the network prefix to be used.- The prefix should normally be
/64
or shorter. - For experimentation on a local network, it is better to use a Unique Local
prefix (matching
fd00::/8
) than a Link Local prefix (matchingfe80::/10
).
- The prefix should normally be
- Option
default-gateway-v6
is optional. If unspecified, the default is the first address in thefixed-cidr-v6
subnet.
{
"ipv6": true,
"fixed-cidr-v6": "2001:db8::/64",
"default-gateway-v6": "2001:db8:abcd::89"
}
Connection limit for bridge networks
Due to limitations set by the Linux kernel, bridge networks become unstable and inter-container communications may break when 1000 containers or more connect to a single network.
For more information about this limitation, see moby/moby#44973.
Skip IP address configuration
The com.docker.network.bridge.inhibit_ipv4
option lets you create a network
that uses an existing bridge and have Docker skip configuring the IPv4 address
on the bridge. This is useful if you want to configure the IP address for the
bridge manually. For instance if you add a physical interface to your bridge,
and need to move its IP address to the bridge interface.
To use this option, you should first configure the Docker daemon to use a
self-managed bridge, using the bridge
option in the daemon.json
or the
dockerd --bridge
flag.
With this configuration, north-south traffic won't work unless you've manually configured the IP address for the 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