Docker Network

Why do we need network?

  • To see why we need inter network communication, let's run two container (in background).
➜ docker run \
         -p 9090:8080
         -d amantuladhar/doc-kub-ws:web-basic
  • Let's try to access our app from different container
➜ docker run \
         -it amantuladhar/network-tools sh

network-tools image have few tools like curl, http, ping, nc.

  • From this container, let's call http localhost:8080/test.
  • It won't work because there is no connection between those two containers.
  • Consider scenario where our app needs to talk with container that is running mysql.

Listing network

docker network list
  • If you do docker network list, you will see three networks is already created
    • bridge
    • host
    • none
  • These are networks created by different network drivers docker supports. (We have few more drivers).
  • Default driver for network is bridge.

Create a network

  • To create a network you just issue docker network create @name.
docker network create myNet
  • This will create network with name myNet with driver bridge.

 

Using network

Define which network to connect to

  • --net=@network_name syntax.
  • To use the network we created we can use --net option.
➜ docker run \
         --name=myApp
         --net=myNet
         -p 9090:8080
         -d amantuladhar/doc-kub-ws:web-basic
  • Notice I am naming my container myself using --name option. You will know in a bit why I am doing this.
  • This will run our container in our network myNet.
  • Now lets run network-tools image on same network.
➜ docker run \
         --name=myTester \
         --net=myNet
         -it amantuladhar/network-tools sh
  • Try to execute curl on localhost:8080/test. or localhost:9090/test
  • It won't work. Why? We explicitly ran both of your container on same network.

Container Discovery

  • To access the app running on same network, we cannot use localhost.
  • I think this is obvious. From the application standpoint they are not running on same computer*.
  • If we can't use localhost what do we use?
  • Turns out we have to use container name i.e myApp in our case.
  • If you do curl on myApp:8080/test it will work.

Docker has builtin DNS server that converts container name to its IP.

Auto connect to specified container network

  • --net has a syntax that supports auto network discovery i.e --net=container:name.
  • If you use this syntax, docker will automatically connect network that is used by specified container.
  • If you use this syntax, you will be able to use localhost instead of container_name .
  • Run the image that has our app like before.
➜ docker run \
         --name=myApp
         --net=myNet
         -p 9090:8080
         -d amantuladhar/doc-kub-ws:web-basic
  • Run the tester again with container syntax.
docker run \
     --name=myTester \
     --net=container:myApp \
     -it \
     amantuladhar/network-tools \
     sh
  • Test using localhost, and it just works.
/ # curl localhost:8080/test | jq .
{
  "message": "Hello Fellas!!!",
  "host.address": "172.23.0.3",
  "app.version": "v1-web"
}
  • As exercise, you can try to create application that stores some values in database.
  • You can use amantuladhar/docker-kubernetes:v1-db image.
  • Simple Web App at branch initial-db
  • App expects that you will set DB_URL, DB_USER and DB_PASS environment variable, which is a full connection URL for mysql.