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title = "Linking containers together"
description = "Learn how to connect Docker containers together."
keywords = ["Examples, Usage, user guide, links, linking, docker, documentation, examples, names, name, container naming, port, map, network port,  network"]
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parent = "smn_containers"
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# Linking containers together

In [the Using Docker section](usingdocker.md), you saw how you can
connect to a service running inside a Docker container via a network
port. But a port connection is only one way you can interact with services and
applications running inside Docker containers. In this section, we'll briefly revisit
connecting via a network port and then we'll introduce you to another method of access:
container linking.

## Connect using network port mapping

In [the Using Docker section](usingdocker.md), you created a
container that ran a Python Flask application:

    $ docker run -d -P training/webapp python app.py

> **Note:** 
> Containers have an internal network and an IP address
> (as we saw when we used the `docker inspect` command to show the container's
> IP address in the [Using Docker](usingdocker.md) section).
> Docker can have a variety of network configurations. You can see more
> information on Docker networking [here](../articles/networking.md).

When that container was created, the `-P` flag was used to automatically map
any network port inside it to a random high port within an *ephemeral port
range* on your Docker host. Next, when `docker ps` was run, you saw that port
5000 in the container was bound to port 49155 on the host.

    $ docker ps nostalgic_morse
    CONTAINER ID  IMAGE                   COMMAND       CREATED        STATUS        PORTS                    NAMES
    bc533791f3f5  training/webapp:latest  python app.py 5 seconds ago  Up 2 seconds  0.0.0.0:49155->5000/tcp  nostalgic_morse

You also saw how you can bind a container's ports to a specific port using
the `-p` flag. Here port 80 of the host is mapped to port 5000 of the 
container:

    $ docker run -d -p 80:5000 training/webapp python app.py

And you saw why this isn't such a great idea because it constrains you to
only one container on that specific port.

Instead, you may specify a range of host ports to bind a container port to
that is different than the default *ephemeral port range*:

    $ docker run -d -p 8000-9000:5000 training/webapp python app.py

This would bind port 5000 in the container to a randomly available port
between 8000 and 9000 on the host.

There are also a few other ways you can configure the `-p` flag. By
default the `-p` flag will bind the specified port to all interfaces on
the host machine. But you can also specify a binding to a specific
interface, for example only to the `localhost`.

    $ docker run -d -p 127.0.0.1:80:5000 training/webapp python app.py

This would bind port 5000 inside the container to port 80 on the
`localhost` or `127.0.0.1` interface on the host machine.

Or, to bind port 5000 of the container to a dynamic port but only on the
`localhost`, you could use:

    $ docker run -d -p 127.0.0.1::5000 training/webapp python app.py

You can also bind UDP ports by adding a trailing `/udp`. For example:

    $ docker run -d -p 127.0.0.1:80:5000/udp training/webapp python app.py

You also learned about the useful `docker port` shortcut which showed us the
current port bindings. This is also useful for showing you specific port
configurations. For example, if you've bound the container port to the
`localhost` on the host machine, then the `docker port` output will reflect that.

    $ docker port nostalgic_morse 5000
    127.0.0.1:49155

> **Note:** 
> The `-p` flag can be used multiple times to configure multiple ports.

## Connect with the linking system

Network port mappings are not the only way Docker containers can connect
to one another. Docker also has a linking system that allows you to link
multiple containers together and send connection information from one to another.
When containers are linked, information about a source container can be sent to a
recipient container. This allows the recipient to see selected data describing
aspects of the source container.

### The importance of naming

To establish links, Docker relies on the names of your containers.
You've already seen that each container you create has an automatically
created name; indeed you've become familiar with our old friend
`nostalgic_morse` during this guide. You can also name containers
yourself. This naming provides two useful functions:

1. It can be useful to name containers that do specific functions in a way
   that makes it easier for you to remember them, for example naming a
   container containing a web application `web`.

2. It provides Docker with a reference point that allows it to refer to other
   containers, for example, you can specify to link the container `web` to container `db`.

You can name your container by using the `--name` flag, for example:

    $ docker run -d -P --name web training/webapp python app.py

This launches a new container and uses the `--name` flag to
name the container `web`. You can see the container's name using the
`docker ps` command.

    $ docker ps -l
    CONTAINER ID  IMAGE                  COMMAND        CREATED       STATUS       PORTS                    NAMES
    aed84ee21bde  training/webapp:latest python app.py  12 hours ago  Up 2 seconds 0.0.0.0:49154->5000/tcp  web

You can also use `docker inspect` to return the container's name.


> **Note:**
> Container names have to be unique. That means you can only call
> one container `web`. If you want to re-use a container name you must delete
> the old container (with `docker rm`) before you can create a new
> container with the same name. As an alternative you can use the `--rm`
> flag with the `docker run` command. This will delete the container
> immediately after it is stopped.

## Communication across links

Links allow containers to discover each other and securely transfer information about one
container to another container. When you set up a link, you create a conduit between a
source container and a recipient container. The recipient can then access select data
about the source. To create a link, you use the `--link` flag. First, create a new
container, this time one containing a database.

    $ docker run -d --name db training/postgres

This creates a new container called `db` from the `training/postgres`
image, which contains a PostgreSQL database.

Now, you need to delete the `web` container you created previously so you can replace it
with a linked one:

    $ docker rm -f web

Now, create a new `web` container and link it with your `db` container.

    $ docker run -d -P --name web --link db:db training/webapp python app.py

This will link the new `web` container with the `db` container you created
earlier. The `--link` flag takes the form:

    --link <name or id>:alias

Where `name` is the name of the container we're linking to and `alias` is an
alias for the link name. You'll see how that alias gets used shortly.
The `--link` flag also takes the form:

	--link <name or id>

In which case the alias will match the name. You could have written the previous
example as:

    $ docker run -d -P --name web --link db training/webapp python app.py

Next, inspect your linked containers with `docker inspect`:

    $ docker inspect -f "{{ .HostConfig.Links }}" web
    [/db:/web/db]

You can see that the `web` container is now linked to the `db` container
`web/db`. Which allows it to access information about the `db` container.

So what does linking the containers actually do? You've learned that a link allows a
source container to provide information about itself to a recipient container. In
our example, the recipient, `web`, can access information about the source `db`. To do
this, Docker creates a secure tunnel between the containers that doesn't need to
expose any ports externally on the container; you'll note when we started the
`db` container we did not use either the `-P` or `-p` flags. That's a big benefit of
linking: we don't need to expose the source container, here the PostgreSQL database, to
the network.

Docker exposes connectivity information for the source container to the
recipient container in two ways:

* Environment variables,
* Updating the `/etc/hosts` file.

### Environment variables

Docker creates several environment variables when you link containers. Docker
automatically creates environment variables in the target container based on
the `--link` parameters.  It will also expose all environment variables 
originating from Docker from the source container. These include variables from:

* the `ENV` commands in the source container's Dockerfile
* the `-e`, `--env` and `--env-file` options on the `docker run`
command when the source container is started

These environment variables enable programmatic discovery from within the
target container of information related to the source container.

> **Warning**:
> It is important to understand that *all* environment variables originating
> from Docker within a container are made available to *any* container
> that links to it. This could have serious security implications if sensitive
> data is stored in them.

Docker sets an `<alias>_NAME` environment variable for each target container
listed in the `--link` parameter. For example, if a new container called
`web` is linked to a database container called `db` via `--link db:webdb`,
then Docker creates a `WEBDB_NAME=/web/webdb` variable in the `web` container.

Docker also defines a set of environment variables for each port exposed by the
source container.  Each variable has a unique prefix in the form:

`<name>_PORT_<port>_<protocol>`

The components in this prefix are:

* the alias `<name>` specified in the `--link` parameter (for example, `webdb`)
* the `<port>` number exposed
* a `<protocol>` which is either TCP or UDP

Docker uses this prefix format to define three distinct environment variables:

* The `prefix_ADDR` variable contains the IP Address from the URL, for
example `WEBDB_PORT_5432_TCP_ADDR=172.17.0.82`.
* The `prefix_PORT` variable contains just the port number from the URL for
example `WEBDB_PORT_5432_TCP_PORT=5432`.
* The `prefix_PROTO` variable contains just the protocol from the URL for
example `WEBDB_PORT_5432_TCP_PROTO=tcp`.

If the container exposes multiple ports, an environment variable set is
defined for each one. This means, for example, if a container exposes 4 ports
that Docker creates 12 environment variables, 3 for each port.

Additionally, Docker creates an environment variable called `<alias>_PORT`.
This variable contains the URL of the source container's first exposed port.
The  'first' port is defined as the exposed port with the lowest number.
For example, consider the `WEBDB_PORT=tcp://172.17.0.82:5432` variable.  If
that port is used for both tcp and udp, then the tcp one is specified.

Finally, Docker also exposes each Docker originated environment variable
from the source container as an environment variable in the target. For each
variable Docker creates an `<alias>_ENV_<name>` variable in the target 
container. The variable's value is set to the value Docker used when it 
started the source container.

Returning back to our database example, you can run the `env`
command to list the specified container's environment variables.

```
    $ docker run --rm --name web2 --link db:db training/webapp env
    . . .
    DB_NAME=/web2/db
    DB_PORT=tcp://172.17.0.5:5432
    DB_PORT_5432_TCP=tcp://172.17.0.5:5432
    DB_PORT_5432_TCP_PROTO=tcp
    DB_PORT_5432_TCP_PORT=5432
    DB_PORT_5432_TCP_ADDR=172.17.0.5
    . . .
```

You can see that Docker has created a series of environment variables with
useful information about the source `db` container. Each variable is prefixed
with
`DB_`, which is populated from the `alias` you specified above. If the `alias`
were `db1`, the variables would be prefixed with `DB1_`. You can use these
environment variables to configure your applications to connect to the database
on the `db` container. The connection will be secure and private; only the
linked `web` container will be able to talk to the `db` container.

### Important notes on Docker environment variables

Unlike host entries in the [`/etc/hosts` file](#updating-the-etchosts-file),
IP addresses stored in the environment variables are not automatically updated
if the source container is restarted. We recommend using the host entries in
`/etc/hosts` to resolve the IP address of linked containers.

These environment variables are only set for the first process in the
container. Some daemons, such as `sshd`, will scrub them when spawning shells
for connection.

### Updating the `/etc/hosts` file

In addition to the environment variables, Docker adds a host entry for the
source container to the `/etc/hosts` file. Here's an entry for the `web`
container:

    $ docker run -t -i --rm --link db:webdb training/webapp /bin/bash
    root@aed84ee21bde:/opt/webapp# cat /etc/hosts
    172.17.0.7  aed84ee21bde
    . . .
    172.17.0.5  webdb 6e5cdeb2d300 db

You can see two relevant host entries. The first is an entry for the `web`
container that uses the Container ID as a host name. The second entry uses the
link alias to reference the IP address of the `db` container. In addition to 
the alias you provide, the linked container's name--if unique from the alias
provided to the `--link` parameter--and the linked container's hostname will
also be added in `/etc/hosts` for the linked container's IP address. You can ping
that host now via any of these entries:

    root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping
    root@aed84ee21bde:/opt/webapp# ping webdb
    PING webdb (172.17.0.5): 48 data bytes
    56 bytes from 172.17.0.5: icmp_seq=0 ttl=64 time=0.267 ms
    56 bytes from 172.17.0.5: icmp_seq=1 ttl=64 time=0.250 ms
    56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms

> **Note:** 
> In the example, you'll note you had to install `ping` because it was not included
> in the container initially.

Here, you used the `ping` command to ping the `db` container using its host entry,
which resolves to `172.17.0.5`. You can use this host entry to configure an application
to make use of your `db` container.

> **Note:** 
> You can link multiple recipient containers to a single source. For
> example, you could have multiple (differently named) web containers attached to your
>`db` container.

If you restart the source container, the linked containers `/etc/hosts` files
will be automatically updated with the source container's new IP address,
allowing linked communication to continue.

    $ docker restart db
    db
    $ docker run -t -i --rm --link db:db training/webapp /bin/bash
    root@aed84ee21bde:/opt/webapp# cat /etc/hosts
    172.17.0.7  aed84ee21bde
    . . .
    172.17.0.9  db

# Next step

Now that you know how to link Docker containers together, the next step is
learning how to take complete control over docker networking.

Go to [Docker Networking](dockernetworks.md).