moby--moby/docs/sources/use/working_with_links_names.md

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

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

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 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

Resolving Links by Name

Note: New in version v0.11.

Linked containers can be accessed by hostname. Hostnames are mapped by appending entries to '/etc/hosts' using the linked container's alias.

For example, linking a container using '--link redis:db' will generate the following '/etc/hosts' file:

root@6541a75d44a0:/# cat /etc/hosts
172.17.0.3  6541a75d44a0
172.17.0.2  db

127.0.0.1   localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0     ip6-localnet
ff00::0     ip6-mcastprefix
ff02::1     ip6-allnodes
ff02::2     ip6-allrouters
root@6541a75d44a0:/#

Using this mechanism, you can communicate with the linked container by name:

root@6541a75d44a0:/# echo PING | redis-cli -h db
PONG
root@6541a75d44a0:/#