weave/site/using-weave/dynamically-attach-containers.md

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Dynamically Attaching and Detaching Applications	default

When containers may not know the network to which they will be attached, Weave Net enables you to dynamically attach and detach containers to and from a given network, even when a container is already running.

To illustrate these scenarios, imagine a netcat service running in a container on $host1 and you need to attach it to another subnet. To attach the netcat service container to a given subnet run:

host1$ C=$(docker run -e WEAVE_CIDR=none -dti ubuntu)
host1$ weave attach $C
10.2.1.3

Where,

• C=$(Docker run -e WEAVE_CIDR=none -dti ubuntu) is a variable for the subnet on which to attach
• weave attach – the Weave Net command to attach to the specified subnet, which takes the variable for the subnet
• 10.2.1.3 - the allocated IP address output by weave attach and in this case, represents the default subnet

Note If you are using the Weave Docker API proxy, it will have modified DOCKER_HOST to point to the proxy and therefore you will have to pass -e WEAVE_CIDR=none to start a container that doesn't get automatically attached to the weave network for the purposes of this example.

###Dynamically Detaching Containers

A container can be detached from a subnet, by using the weave detach command:

host1$ weave detach $C
10.2.1.3

You can also detach a container from one network and then attach it to a different one:

host1$ weave detach net:default $C
10.2.1.3
host1$ weave attach net:10.2.2.0/24 $C
10.2.2.3

or, attach a container to multiple application networks, effectively sharing the same container between applications:

host1$ weave attach net:default
10.2.1.3
host1$ weave attach net:10.2.2.0/24
10.2.2.3

Finally, multiple addresses can be attached or detached using a single command:

host1$ weave attach net:default net:10.2.2.0/24 net:10.2.3.0/24 $C
10.2.1.3 10.2.2.3 10.2.3.1
host1$ weave detach net:default net:10.2.2.0/24 net:10.2.3.0/24 $C
10.2.1.3 10.2.2.3 10.2.3.1

Important! Any addresses that were dynamically attached will not be re-attached if the container restarts.

See Also

• Adding and Removing Hosts Dynamically