Working with Links and Names

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

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

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.

# Example: there is an image called redis-2.6 that exposes the port 6379 and starts redis-server.
# Let's name the container as "redis" based on that image and run it as daemon.
$ sudo docker run -d -name redis redis-2.6

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.

# Linking the redis container as a child
$ 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.

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Working with Links and Names

Container Naming

Links: service discovery for docker

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