Communication between two Docker containers on macOS 10.12

Question

I'm working with Docker 1.12.5 on macOS 10.12, and am setting up a development environment with with I have an application image, and a shared redis image which has some pre-populated configuration variables.

Even after following a few tutorials (and reading about how docker0 isn't available on Mac) I'm struggling to connect the two containers.

I start my redis image using:

docker run -d -p 6379:6379 (IMAGE ID)

In my redis image, I have:

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
dffb89854618        d59                 "docker-entrypoint.sh"   20 seconds ago      Up 19 seconds       0.0.0.0:6379->6379/tcp   drunk_williams

And from my Mac I can successfully connect via the redis-cli command without issue.

However when I start a simple ubuntu image, I can't seem to connect to this separate redis image:

root@2d4eda315f4f:/# ifconfig
eth0      Link encap:Ethernet  HWaddr 02:42:ac:11:00:03
          inet addr:172.17.0.3  Bcast:0.0.0.0  Mask:255.255.0.0
          inet6 addr: fe80::42:acff:fe11:3/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:20707 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11515 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:28252929 (28.2 MB)  TX bytes:635848 (635.8 KB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:12 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1
          RX bytes:680 (680.0 B)  TX bytes:680 (680.0 B)
root@2d4eda315f4f:/# telnet localhost 6379
Trying ::1...
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
root@2d4eda315f4f:/# telnet 172.17.0.3 6379
Trying 172.17.0.3...
telnet: Unable to connect to remote host: Connection refused

Is this a result of not having the docker0 interface available in the host? Is there some straightforward workaround for allowing these containers to communicate (when being run on the same host) in a development environment?

Update: Attempting to use named containers, I still can't connect.

docker run -d --name redis_server redis

Results in:

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
5d05820aa985        redis               "docker-entrypoint.sh"   43 hours ago        Up 1 seconds        6379/tcp                 redis_server

But then if I start a new Ubuntu container:

root@e92b47419bc4:/# redis-cli -h redis_server
Could not connect to Redis at redis_server:6379: Name or service not known

I'm not sure how to find/connect to the first redis_server container.

Answer 1

Each container has its own localhost

Each service runs in its own container. From the perspective of the Ubuntu container, redis is not listening on localhost.

Use Docker networks

To get your containers to communicate, they should be on the same Docker network. This consists of three steps:

1. Create a Docker network
2. Give your containers names
3. Attach your containers to the network you created

With this done, the containers can talk to each other using their names as if they were hostnames.

There's more than one way to skin this cat... I will look at two in this answer, but there are probably a few other ways to do it that I am not familiar with (like using Kubernetes or Swarm, for instance).

Doing it by hand

You can create a network for this application using docker network commands.

# Show the current list of networks
docker network ls

# Create a network for your app
docker network create my_redis_app

When you run the redis container, make sure it has a name, and is on this network. You can expose the ports externally (to macOS) if you want to (using -p), but that is not necessary just for other containers to talk to redis.

docker run -d -p 6379:6379 --name redis_server --network my_redis_app <IMAGE ID>

Now run your Ubuntu container. You can name it as well if you like, but I won't bother in this example because this one isn't running any services.

docker run -it --network my_redis_app ubuntu bash

Now from inside the Ubuntu container, you should be able to reach redis using the name redis_server, as if it were a DNS name.

Doing it using Compose

I tend to build setups like this using Compose, because it's easier to write it into a YAML file (IMO). Here's an example of the above, re-written in docker-compose.yml form:

version: '2'
services:
  redis:
    image: <IMAGE ID>
    networks:
      - my_redis_app
    ports: 6379:6379
  ubuntu:
    image: ubuntu:latest
    networks:
      - my_redis_app
networks:
  my_redis_app:
    driver: bridge

With this in place, you can run docker-compose up -d redis and have your redis service online using a specific Docker network. Compose will create the network for you, if it doesn't already exist.

It makes less sense to run the Ubuntu container that way... it is interactive, of course. But I assume once you have redis going, you will add some kind of application container, and perhaps a web proxy like nginx... just put the others under services as well, and you can manage them all together.

Since ubuntu is interactive, you can run it interactively:

# without -d, container is run interactively
docker-compose run ubuntu bash

And now in Ubuntu, you should be able to connect to redis using its name, which in this example is simply redis.