Legacy container links
The information in this section explains legacy container links within the Docker default bridge. This is a bridge
network named bridge
created automatically when you install Docker.
Before the Docker networks feature, you could use the Docker link feature to allow containers to discover each other and securely transfer information about one container to another container. With the introduction of the Docker networks feature, you can still create links but they behave differently between default bridge
network and user defined networks
This section briefly discusses connecting via a network port and then goes into detail on container linking in default bridge
network.
Connect using network port mapping
In the Using Docker section, 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 section). Docker can have a variety of network configurations. You can see more information on Docker networking here.
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
Note: This section covers the legacy link feature in the default
bridge
network. Please refer to linking containers in user-defined networks for more information on links in user-defined networks.
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:
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
.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 containerdb
.
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 (withdocker rm
) before you can create a new container with the same name. As an alternative you can use the--rm
flag with thedocker 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 thedocker 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 exampleWEBDB_PORT_5432_TCP_ADDR=172.17.0.82
. - The
prefix_PORT
variable contains just the port number from the URL for exampleWEBDB_PORT_5432_TCP_PORT=5432
. - The
prefix_PROTO
variable contains just the protocol from the URL for exampleWEBDB_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, 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
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