· docker

Docker: Find the network for a container

If we want two Docker containers to communicate with each other they need to belong to the same network. In this post we’ll learn how to find out the network of existing containers so that we can attach new containers to that network.

All the containers mentioned in this post can be launched locally from Docker compose, using the following command:

git clone git@github.com:mneedham/ksql-kafka-neo4j-streams.git && cd ksql-kafka-neo4j-streams
docker-compose-up

Running this command will create four containers:

Starting zookeeper-blog ...
Starting broker-blog    ...
Starting ksql-server-blog ...
Starting neo4j-blog       ...

What if we want to add another container and have it be able to communicate with these containers? We want to know which network these containers are attached to, and a good place to start is the docker inspect command, which returns detailed information about containers. We’ll process the JSON document that the command returns using the jq library:

docker inspect neo4j-blog | jq

If we run that command, we’ll see the following output:

docker inspect

On the last line of this output we can see NetworkMode, which is described as follows in the documentation:

NetworkMode - Sets the networking mode for the container. Supported standard values are: bridge, host, none, and container:<name|id>. Any other value is taken as a custom network’s name to which this container should connect to.

— Docker Documentation

We can write the following command to extract that value:

$ docker inspect neo4j-blog --format='{{ .HostConfig.NetworkMode }}'
ksql-kafka-neo4j-streams_default

If we want to return the network for all running containers, we can just iterate over them and run the command for each one:

for container in `docker container ls --format "{{.Names}}"`; do
  network=`docker inspect $container --format="{{ .HostConfig.NetworkMode }}"`
  image=`docker inspect $container --format="{{ .Config.Image }}"`
  printf '%-20s %-35s %-15s\n' $container $image $network
done

ksql-server-blog     confluentinc/cp-ksql-server:5.2.1   ksql-kafka-neo4j-streams_default
neo4j-blog           neo4j:3.4.10                        ksql-kafka-neo4j-streams_default
broker-blog          confluentinc/cp-enterprise-kafka    ksql-kafka-neo4j-streams_default
zookeeper-blog       confluentinc/cp-zookeeper           ksql-kafka-neo4j-streams_default

If we want to extract more detailed information about the network, we can find that under NetworkSettings.Network. The following query returns this information for the neo4j-blog container:

$ docker inspect neo4j-blog --format='{{ json .NetworkSettings.Networks }}' | jq
{
  "ksql-kafka-neo4j-streams_default": {
    "IPAMConfig": null,
    "Links": null,
    "Aliases": [
      "c004bcd09652",
      "neo4j"
    ],
    "NetworkID": "f3bc43280a8a2ae764280e08fa9604097079d7020fac44d7d2012d2b2d353723",
    "EndpointID": "3ed6ac3c03e83e96fc56b57aedff2606f264237998194ff5215480744ce0b303",
    "Gateway": "172.27.0.1",
    "IPAddress": "172.27.0.5",
    "IPPrefixLen": 16,
    "IPv6Gateway": "",
    "GlobalIPv6Address": "",
    "GlobalIPv6PrefixLen": 0,
    "MacAddress": "02:42:ac:1b:00:05",
    "DriverOpts": null
  }
}

We can then write the following query to extract the name of the network:

$ docker inspect neo4j-blog --format='{{range $k,$v := .NetworkSettings.Networks}} {{$k}} {{end}}'
 ksql-kafka-neo4j-streams_default

And if we want to pull out a couple of properties, we can do that as well:

$ docker inspect neo4j-blog --format='{{range $k,$v := .NetworkSettings.Networks}} {{$k}} {{$v.IPAddress}} {{$v.Aliases}} {{end}}'
 ksql-kafka-neo4j-streams_default 172.27.0.5 [c004bcd09652 neo4j]
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