4.8 KiB
page_title: Link Containers page_description: How to create and use both links and names page_keywords: Examples, Usage, links, linking, docker, documentation, examples, names, name, container naming
Link Containers
Introduction
From version 0.6.5 you are now able to name
a
container and link
it to another container by
referring to its name. This will create a parent -> child relationship
where the parent container can see selected information about its child.
Container Naming
New in version v0.6.5.
You can now name your container by using the --name
flag. If no name is provided, Docker will automatically
generate a name. You can see this name using the docker ps
command.
# format is "sudo docker run --name <container_name> <image_name> <command>"
$ sudo docker run --name test ubuntu /bin/bash
# the flag "-a" Show all containers. Only running containers are shown by default.
$ sudo docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
2522602a0d99 ubuntu:12.04 /bin/bash 14 seconds ago Exit 0 test
Links: service discovery for docker
New in version v0.6.5.
Links allow containers to discover and securely communicate with each
other by using the flag -link name:alias
.
Inter-container communication can be disabled with the daemon flag
-icc=false
. With this flag set to
false
, Container A cannot access Container B
unless explicitly allowed via a link. This is a huge win for securing
your containers. When two containers are linked together Docker creates
a parent child relationship between the containers. The parent container
will be able to access information via environment variables of the
child such as name, exposed ports, IP and other selected environment
variables.
When linking two containers Docker will use the exposed ports of the container to create a secure tunnel for the parent to access. If a database container only exposes port 8080 then the linked container will only be allowed to access port 8080 and nothing else if inter-container communication is set to false.
For example, there is an image called crosbymichael/redis
that exposes the port 6379 and starts the Redis server. Let’s
name the container as redis
based on that image
and run it as daemon.
$ sudo docker run -d -name redis crosbymichael/redis
We can issue all the commands that you would expect using the name
redis
; start, stop, attach, using the name for our
container. The name also allows us to link other containers into this
one.
Next, we can start a new web application that has a dependency on Redis
and apply a link to connect both containers. If you noticed when running
our Redis server we did not use the -p
flag to
publish the Redis port to the host system. Redis exposed port 6379 and
this is all we need to establish a link.
$ sudo docker run -t -i -link redis:db -name webapp ubuntu bash
When you specified -link redis:db
you are telling
Docker to link the container named redis
into this
new container with the alias db
. Environment
variables are prefixed with the alias so that the parent container can
access network and environment information from the containers that are
linked into it.
If we inspect the environment variables of the second container, we would see all the information about the child container.
$ root@4c01db0b339c:/# env
HOSTNAME=4c01db0b339c
DB_NAME=/webapp/db
TERM=xterm
DB_PORT=tcp://172.17.0.8:6379
DB_PORT_6379_TCP=tcp://172.17.0.8:6379
DB_PORT_6379_TCP_PROTO=tcp
DB_PORT_6379_TCP_ADDR=172.17.0.8
DB_PORT_6379_TCP_PORT=6379
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
SHLVL=1
HOME=/
container=lxc
_=/usr/bin/env
root@4c01db0b339c:/#
Accessing the network information along with the environment of the child container allows us to easily connect to the Redis service on the specific IP and port in the environment.
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.
You can work around this by storing the initial env
in a file, or looking at /proc/1/environ
.
Running docker ps
shows the 2 containers, and the
webapp/db
alias name for the Redis container.
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
4c01db0b339c ubuntu:12.04 bash 17 seconds ago Up 16 seconds webapp
d7886598dbe2 crosbymichael/redis:latest /redis-server --dir 33 minutes ago Up 33 minutes 6379/tcp redis,webapp/db