Docker Container Access

Making http calls from Docker containers to applications in other containers.

Setup

Lets create containers that fire up a python http server.

~$ mkdir tmp && cd tmp
~/tmp$ echo ContainerA > indexA.html && echo ContainerB > indexB.html

Create the files a.Dockerfile

FROM python:3.5.9-alpine3.12

WORKDIR /app

COPY indexA.html index.html

CMD [ "python", "-m", "http.server" ]

and b.Dockerfile

FROM python:3.5.9-alpine3.12

WORKDIR /app

COPY indexB.html index.html

CMD [ "python", "-m", "http.server" ]

Create the images

docker build -t image-a -f a.Dockerfile . && docker build -t image-b -f b.Dockerfile .

Lets run image-a on port 8000

docker run -p 8000:8000 -it --rm --name image_a image-a

and image-b on port 9000

docker run -p 9000:8000 -it --rm --name image_b image-b

Issues

In the host machine we can open up a terminal and do

~$ curl http://localhost:8000
ContainerA
~$ curl http://localhost:9000
ContainerB

From within the container we should be able to do the same

~$ docker exec -it image_a sh
/app # apk add curl
fetch http://dl-cdn.alpinelinux.org/alpine/v3.12/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.12/community/x86_64/APKINDEX.tar.gz
(1/3) Installing nghttp2-libs (1.41.0-r0)
(2/3) Installing libcurl (7.69.1-r0)
(3/3) Installing curl (7.69.1-r0)
Executing busybox-1.31.1-r16.trigger
OK: 10 MiB in 28 packages
/app # curl http://localhost:8000
ContainerA
/app # curl http://localhost:9000
curl: (7) Failed to connect to localhost port 9000: Connection refused

How can we reach the server located in image_b container?

Use the IP address of the host

/app # curl http://192.168.1.21:9000
ContainerB

Mac OS X

The above does not work on mac. Funny enough, if we obtain the IP address from the OS X machine and curl port 9000 from a linux box we obtain the file we want but the containers running inside OS X cannot connect to any other container. Why?