One of the many desirable features built directly into Docker is networking. Docker’s networking feature can be accessed by using a
--link flag which allows to connect any number of Docker containers without the need to expose a container’s internal ports to the outside world.
In this guide, you will learn how to network two or more Docker containers together on a Linux system through command line instructions. This will work on any Linux distribution. Check out the step by step instructions below to find out how.
In this tutorial you will learn:
- How to network Docker containers together
|Category||Requirements, Conventions or Software Version Used|
|System||Any Linux distro|
|Other||Privileged access to your Linux system as root or via the
# – requires given linux commands to be executed with root privileges either directly as a root user or by use of
$ – requires given linux commands to be executed as a regular non-privileged user
Connecting Docker containers through networking
- In this config you will learn how to link two or more Docker containers together using a simple Docker networking technique. We can start by the deployment of our first Docker container, named
sanbox1, to which we’ll later create a network link:
# docker run -it --name sandbox1 -h sanbox1 linuxconfig/sandbox /bin/bash
There is nothing new about the above command except to note that we are not exposing any network ports even though our intention is to access services (SSH, database connection, etc.) via their relevant port numbers.
- The following Linux command will deploy a second and this time a parent docker container named
sandbox2. We will also use a
--linkflag which will create a so called parent-child relationship with previously deployed container
sandbox1. Furthermore, the
--linkflag will enable the parent container to access any services running on
sandbox1container via its corresponding ports numbers without the child container’s need to expose any ports to outside world.
# docker run -it --name sandbox2 -h sandbox2 --link sandbox1:sandbox1 linuxconfig/sandbox /bin/bash
The above docker command used the
--linkflag which expects two arguments separated by a colon. The first argument is expected to be a container ID or as in our case a supplied
sandbox1container name we would like to link to. The second argument, also
sandbox1, is an internal alias used by
sandbox1‘s network configuration using the
# grep sandbox1 /etc/hosts 172.17.0.41 sandbox1
- Depending on your child container’s ports configuration in use you can also extract
sandbox1‘s configuration from system environment variables. For example:
# env HOSTNAME=sandbox2 TERM=xterm SANDBOX1_PORT=tcp://172.17.0.37:7555 SANDBOX1_PORT_7555_TCP=tcp://172.17.0.37:7555 PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin PWD=/ SANDBOX1_PORT_7555_TCP_ADDR=172.17.0.37 SANDBOX1_PORT_7555_TCP_PROTO=tcp SHLVL=1 HOME=/root SANDBOX1_NAME=/sandbox2/sandbox1 SANDBOX1_PORT_7555_TCP_PORT=7555 _=/usr/bin/env
- This way we can simply use our child container’s alias to connect to it from a parent container
sandbox2without the need to hardcode its IP address:
# ping -c 1 sandbox1 PING sandbox1 (172.17.0.41): 56 data bytes 64 bytes from 172.17.0.41: icmp_seq=0 ttl=64 time=0.071 ms --- sandbox1 ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max/stddev = 0.071/0.071/0.071/0.000 ms
and also access any ports ports and services:
# nmap -p 22 sandbox1 Starting Nmap 6.47 ( http://nmap.org ) at 2015-05-18 08:58 UTC Nmap scan report for sandbox1 (172.17.0.41) Host is up (0.000090s latency). PORT STATE SERVICE 22/tcp open ssh MAC Address: 02:42:AC:11:00:29 (Unknown) Nmap done: 1 IP address (1 host up) scanned in 0.50 seconds
That’s all there is to it. Our two containers can now communicate with each other and services like SSH work between them. You can apply this same configuration to any two or more Docker containers that you need to network together.
In this guide, we saw how to link Docker containers together on a Linux system. This is a relatively basic task that is facilitated through Docker’s
--link option. Having the ability to network two or more Docker containers together increases the power and usefulness of Docker substantially.