Docker Container Networking with Docker-in-Docker
I would like to network with a child docker container from a parent docker container, with a docker-in-docker setup.
Let's say I'm trying to connect to a simple Apache httpd server. When I run the httpd container on my host machine, everything works fine:
asnyder:~$ docker run -d -p 8080:80 httpd:alpine
asnyder:~$ curl localhost:8080
<html><body><h1>It works!</h1></body></html>
But when I do the same from a docker-in-docker setup, I get a Connection refused error:
asnyder:~$ docker run -d --name mydind --privileged docker:dind
asnyder:~$ docker run -it --link mydind:docker docker:latest sh
/ # docker run -d -p 8080:80 httpd:alpine
/ # curl localhost:8080
curl: (7) Failed to connect to localhost port 8080: Connection refused
I have tried a couple alterations without luck. Specifying the 0.0.0.0 interface:
asnyder:~$ docker run -d --name mydind --privileged docker:dind
asnyder:~$ docker run -it --link mydind:docker docker:latest sh
/ # docker run -d -p 0.0.0.0:8080:80 httpd:alpine
/ # curl 0.0.0.0:8080
curl: (7) Failed to connect to 0.0.0.0 port 8080: Connection refused
Using the host network:
asnyder:~$ docker run -d --name mydind --privileged docker:dind
asnyder:~$ docker run -it --link mydind:docker docker:latest sh
/ # docker run -d --network host httpd:alpine
/ # curl localhost:80
curl: (7) Failed to connect to localhost port 80: Connection refused
Surprisingly, I was unable to find any existing articles on this. Does anyone here have some insight?
Thanks!
Answer
There are pros and cons for both DinD and bind mounting the Docker socket and there are certainly use cases for both. As an example, check out this set of blog posts, which does a good job of explaining one of the use cases.
Given your example docker-in-docker setup above, you can access Apache httpd server in one of two ways:
1) From inside the docker:dind container, it will be available on localhost:8080.
2) From inside the docker:latest container, where you were trying to access it originally, it will be available on whatever hostname is set for the docker:dind container. In this case, you used --name mydind, therefore curl mydind:8080 would give you the standard Apache <html><body><h1>It works!</h1></body></html>.
Hope it makes sense!