Linking containers together

Linking Containers Together

In the Using Docker section we touched on connecting to a service running inside a Docker container via a network port. This is one of the ways that you can interact with services and applications running inside Docker containers. In this section we're going to give you a refresher on connecting to a Docker container via a network port as well as introduce you to the concepts of container linking.

Network port mapping refresher

In the Using Docker section we created a container that ran a Python Flask application.

$ sudo docker run -d -P training/webapp python

Note: Containers have an internal network and an IP address (remember we used the docker inspect command to show the container's IP address in the Using Docker section). Docker can have a variety of network configurations. You can see more information on Docker networking here.

When we created that container we used the -P flag to automatically map any network ports inside that container to a random high port from the range 49000 to 49900 on our Docker host. When we subsequently ran docker ps we saw that port 5000 was bound to port 49155.

$ sudo docker ps nostalgic_morse
CONTAINER ID  IMAGE                   COMMAND       CREATED        STATUS        PORTS                    NAMES
bc533791f3f5  training/webapp:latest  python 5 seconds ago  Up 2 seconds>5000/tcp  nostalgic_morse

We also saw how we can bind a container's ports to a specific port using the -p flag.

$ sudo docker run -d -p 5000:5000 training/webapp python

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

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

$ sudo docker run -d -p training/webapp python

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

Or to bind port 5000 of the container to a dynamic port but only on the localhost we could:

$ sudo docker run -d -p training/webapp python

We can also bind UDP ports by adding a trailing /udp, for example:

$ sudo docker run -d -p training/webapp python

We also saw the useful docker port shortcut which showed us the current port bindings, this is also useful for showing us specific port configurations. For example if we've bound the container port to the localhost on the host machine this will be shown in the docker port output.

$ docker port nostalgic_morse 5000

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

Docker Container Linking

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 share connection information between them. Docker linking will create a parent child relationship where the parent container can see selected information about its child.

Container naming

To perform this linking Docker relies on the names of your containers. We've already seen that each container we create has an automatically created name, indeed we'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's 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 with a web application in it web.

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

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

$ sudo docker run -d -P --name web training/webapp python

You can see we've launched a new container and used the --name flag to call the container web. We can see the container's name using the docker ps command.

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

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

$ sudo docker inspect -f "{{ .Name }}" aed84ee21bde

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 the docker rm command 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 stops.

Container Linking

Links allow containers to discover and securely communicate with each other. To create a link you use the --link flag. Let's create a new container, this one a database.

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

Here we've created a new container called db using the training/postgres image, which contains a PostgreSQL database.

Now let's create a new web container and link it with our db container.

$ sudo docker run -d -P --name web --link db:db training/webapp python

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

--link name:alias

Where name is the name of the container we're linking to and alias is an alias for the link name. We'll see how that alias gets used shortly.

Let's look at our linked containers using docker ps.

$ docker ps
CONTAINER ID  IMAGE                     COMMAND               CREATED             STATUS             PORTS                    NAMES
349169744e49  training/postgres:latest  su postgres -c '/usr  About a minute ago  Up About a minute  5432/tcp                 db
aed84ee21bde  training/webapp:latest    python         16 hours ago        Up 2 minutes>5000/tcp  db/web,web

We can see our named containers, db and web, and we can see that the web containers also shows db/web in the NAMES column. This tells us that the web container is linked to the db container in a parent/child relationship.

So what does linking the containers do? Well we've discovered the link creates a parent-child relationship between the two containers. The parent container, here db, can access information on the child container web. To do this Docker creates a secure tunnel between the containers without the need to expose any ports externally on the container. You'll note when we started the db container we did not use either of the -P or -p flags. As we're linking the containers we don't need to expose the PostgreSQL database via the network.

Docker exposes connectivity information for the parent container inside the child container in two ways:

  • Environment variables,
  • Updating the /etc/hosts file.

Let's look first at the environment variables Docker sets. Let's run the env command to list the container's environment variables.

    $ sudo docker run --rm --name web2 --link db:db training/webapp env
    . . .
    . . .

Note: These Environment variables are only set for the first process in the container. Similarly, some daemons (such as sshd) will scrub them when spawning shells for connection.

We can see that Docker has created a series of environment variables with useful information about our db container. Each variable is prefixed with DB_ which is populated from the alias we specified above. If our 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, private and only the linked web container will be able to talk to the db container.

In addition to the environment variables Docker adds a host entry for the linked parent to the /etc/hosts file. Let's look at this file on the web container now.

root@aed84ee21bde:/opt/webapp# cat /etc/hosts  aed84ee21bde
. . .  db

We 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. Let's try to ping that host now via this host name.

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

Note: We had to install ping because our container didn't have it.

We've used the ping command to ping the db container using it's host entry which resolves to We can make use of this host entry to configure an application to make use of our db container.

Note: You can link multiple child containers to a single parent. For example, we could have multiple web containers attached to our db container.

Next step

Now we know how to link Docker containers together the next step is learning how to manage data, volumes and mounts inside our containers.

Go to Managing Data in Containers.