Docker Container Linking

What is docker container linking?

Docker container linking is a feature to create a communication bride among containers. This bridge/linking is used to communicate with each other of containers and pass date securely from one container to another.

Show docker containers

• Run $ docker ps and we can see the following containers

CONTAINER ID        IMAGE                         COMMAND                  CREATED             STATUS              PORTS                  NAMES
376170ae9ae2        csemahadi/php7-docker-image   "/usr/sbin/apache2ct…"   15 seconds ago      Up 11 seconds       0.0.0.0:8080->80/tcp   test2
f577c140ee8c        csemahadi/php7-docker-image   "/usr/sbin/apache2ct…"   25 seconds ago      Up 22 seconds       0.0.0.0:8081->80/tcp   test1

How to create link?

Consider we have two docker container named test1 and test2 and we want to access the resource of test1 container from test2 container securely. For that we can follow the following steps

• Pull repo: $ git clone https://github.com/mh2k9/docker-container-linking.git.
• Go to test1 directory and run start-test1.sh. This bash command create a docker container named test1 by the following command

  #!/usr/bin/env bash
  
  docker run --name test1 -d -p 8081:80 -v $(pwd):/var/www/html csemahadi/php7-docker-image

• Now go to test2 directory and run start-test2.sh. This bash file contains the following docker command

  #!/usr/bin/env bash
  
  docker run --name test2 -p 8080:80 -v $(pwd):/var/www/html --link test1:tl csemahadi/php7-docker-image

  That command contains a --link flag which has the value test1:tl. Here test1 is the docker container which we created earlier and tl is the alice by which we can access the resource of test1 container from test2 container.

Resource access example:

Go to test2 directory and open the index.php file which contains the following PHP codes

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "tl/index.php");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
curl_setopt($ch, CURLOPT_TIMEOUT, 5);
$data = curl_exec($ch);
$httpcode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

print_r($data);

This PHP file do a cURL request to index.php of test1 container using the alice tl instead of using host/IP address. This cURL request return the contents of index.php of test1 container. It means container test2 is linked with container test1 with the alice tl and by this linking alice we can access all the resources of test1 container from test2 container.

Get the host/IP of test2 container

$ docker inspect --format '{{ .NetworkSettings.IPAddress }}' test2.

Possible IP is: 172.17.0.3. Now hit this IP on browser and you will see the page is showing a message: This is a message from test1 container.

That message is from index.php file of test1 container.