Connecting Docker Containers - networking

Question

Hello Helpful Developers,
I'm having issues connecting docker containers. I have built a subversion docker container and a mongo docker container.
docker run -d -p 3343:3343 -p 4434:4434 -p 18080:18080 --name svn-server mamohr/subversion-edge
docker run -p 27017:27017 --name my-mongo -d mongo
I'm able to hit http://x.x.x.x:18080/ from a browser, but unable to curl from the my-mongo instance. I can talk to each container from my development machine, but unable to talk from container to container.
I see things like --net=bridge, host, ????, but I'm getting confused.
Please help.....

Answer 1

Borrowing this schema from SDN hub, imagine that C1 is your SVN container and C2 is your Mongo container:
Both containers are connected to docker0 bridge and NATed to external 192.168.50.16 network.
To connect from your Mongo container, check the bridge0 IP address of the SVN container:
# docker inspect <svn-container-name>
"Networks": {
"bridge0": {
"IPAddress": "172.17.0.19",
}
then CURL directly to it's bridge0 IP address:
curl http://172.17.0.19:18080/

Answer 2

To get you immediately going, you can start your hosts with --net=host and then both containers and host will be able to communicate.
Or you can use link( --link ) between from mongo to the other container.
There is lot to explain about docker networking and the docker documentation will be good point to start.
Read the documentation at https://docs.docker.com/engine/userguide/networking/dockernetworks/

Answer 3

I would advice you to take a look at docker compose. I think it's the best way to manage a system, which is composed of many containers.
Here is the official guide: https://docs.docker.com/compose/

Answer 4

Docker containers by default start attached to a bridge network called default. You can do docker network ls and see the networks you have available. You can also create networks with different attributes etc...
So in your case, both your containers are being started on the same default network, which means they should be able to communicate with each other just fine. In fact, if you only want your SVN server to be able to talk to Mongo (and don't need to connect to mongo from your host) you don't even need to expose ports on the Mongo container. Containers on the same network as each can communicate with each other just fine without ports being exposed. Exposing ports is to allow host > container connectivity.
So, what hostname / port are you using when you try to curl from the mongo instance to your SVN instance? You should be using svn-server as that will resolve to the SVN container (using Docker's built-in DNS resolution).

Answer 5

Direct container to container networking via container name can be achieved with a user defined network.
docker network create mynet
docker run -d --net=mynet --name svn-server mamohr/subversion-edge
docker run -d --net=mynet --name my-mongo mongo
docker exec <svn-id> ping my-mongo
docker exec <mongo-id> ping svn-server
You should always be able to connect to mapped ports though, even in your current setup. The hosts runs a process that listens on that port so any host IP should do.
$ docker run -d -p 8080:80 --net=mynet --name sleep busybox nc -lp 80 -e echo here!
63115ef88664f1186ea012e41138747725790383c741c12ca8675c3058383e68
$ ss -lntp | grep 8080
LISTEN 0 128 :::8080 :::* users:(("exe",pid=6287,fd=4))
$ docker run busybox nc <any_host_ip> 8080
here!

Answer 6

Please remember, container is not available by default to the ourside world.
When you running the svn-server container, you published the container's 18080 port and mapped it from the host's 18080 port. So you can access it by http://your_host_IP:18080.
From your two docker run commands, both svn-server container and my-mongo container are on the default bridge network. These two containers are connected by docker0, so they can communicate each other directly by localhost.
But if you tried to access http://your_host_IP:18080 from within your my-mongo container, that means your request would first be send to docker0, but docker0 will drop your request because you're trying to access the host, not the svn-server container.
So try this curl http://localhost:18080 or curl http://svn-server_IP:18080 from my-mongo container to access svn-server container.