Connect to Kafka running in Docker

When you first connect to a kafka node, it will give you back all the kafka node and the url where to connect. Then your application will try to connect to every kafka directly.

Issue is always what is the kafka will give you as url ? It's why there is the KAFKA_ADVERTISED_LISTENERS which will be used by kafka to tell the world how it can be accessed.

Now for your use-case, there is multiple small stuff to think about:

Let say you set plaintext://kafka:9092

• This is OK if you have an application in your docker compose that use kafka. This application will get from kafka the URL with kafka that is resolvable through the docker network.
• If you try to connect from your main system or from another container which is not in the same docker network this will fail, as the kafka name cannot be resolved.

==> To fix this, you need to have a specific DNS server like a service discovery one, but it is big trouble for small stuff. Or you set manually the kafka name to the container ip in each /etc/hosts

If you set plaintext://localhost:9092

• This will be ok on your system if you have a port mapping ( -p 9092:9092 when launching kafka)
• This will fail if you test from an application on a container (same docker network or not) (localhost is the container itself not the kafka one)

==> If you have this and wish to use a kafka client in another container, one way to fix this is to share the network for both container (same ip)

Last option : set an IP in the name: plaintext://x.y.z.a:9092 ( kafka advertised url cannot be 0.0.0.0 as stated in the doc https://kafka.apache.org/documentation/#brokerconfigs_advertised.listeners )

This will be ok for everybody... BUT how can you get the x.y.z.a name ?

The only way is to hardcode this ip when you launch the container: docker run .... --net confluent --ip 10.x.y.z .... Note that you need to adapt the ip to one valid ip in the confluent subnet.

Disclaimer

tl;dr - A simple port forward from the container to the host will not work, and no hosts files should be modified. What exact IP/hostname + port do you want to connect to? Make sure that value is set as advertised.listeners on the broker. Make sure that address and the servers listed as part of bootstrap.servers are actually resolvable (ping an IP/hostname, use netcat to check ports...)

To verify the ports are mapped correctly on the host, ensure that docker ps shows the kafka container is mapped from 0.0.0.0:<host_port> -> <advertised_listener_port>/tcp. The ports must match if trying to run a client from outside the Docker network.

If you are interested in using a simple port mapping, try to find a container that supports Kafka "KRaft" mode (the bitnami/kafka image, for example)

---

The below answer uses confluentinc docker images to address the question that was asked, not wurstmeister/kafka. More specifically, the latter images are not well-maintained despite being the one of the most popular Kafka docker image.

The following sections try to aggregate all the details needed to use another image. For other, commonly used Kafka images, it's all the same Apache Kafka running in a container.
You're just dependent on how it is configured. And which variables make it so.

wurstmeister/kafka

Refer their README section on listener configuration, Also read their Connectivity wiki.

bitnami/kafka

If you want a small container, try these. The images are much smaller than the Confluent ones and are much more well maintained than wurstmeister. Refer their README for listener configuration.

debezium/kafka

Docs on it are mentioned here.

Note: advertised host and port settings are deprecated. Advertised listeners covers both. Similar to the Confluent containers, Debezium can use KAFKA_ prefixed broker settings to update its properties.

Others

spotify/kafka is deprecated and outdated.
fast-data-dev or lensesio/box are great for an all in one solution, but are bloated if you only want Kafka
Your own Dockerfile - Why? Is something incomplete with these others? Start with a pull request, not starting from scratch.

For supplemental reading, a fully-functional docker-compose, and network diagrams, see this blog by @rmoff

Answer

The Confluent quickstart (Docker) document assumes all produce and consume requests will be within the Docker network.

You could fix the problem of connecting to kafka:9092 by running your Kafka client code within its own container as that uses the Docker network bridge, but otherwise you'll need to add some more environment variables for exposing the container externally, while still having it work within the Docker network.

First add a protocol mapping of PLAINTEXT_HOST:PLAINTEXT that will map the listener protocol to a Kafka protocol

Key: KAFKA_LISTENER_SECURITY_PROTOCOL_MAP
Value: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT

Then setup two advertised listeners on different ports. (kafka here refers to the docker container name; it might also be named broker, so double check your service + hostnames).

Key: KAFKA_ADVERTISED_LISTENERS
Value: PLAINTEXT://kafka:9092,PLAINTEXT_HOST://localhost:29092

Notice the protocols here match the left-side values of the protocol mapping setting above

When running the container, add -p 29092:29092 for the host port mapping, and advertised PLAINTEXT_HOST listener.

---

tl;dr

(with the above settings)

If something still doesn't work, KAFKA_LISTENERS can be set to include <PROTOCOL>://0.0.0.0:<PORT> where both options match the advertised setting and Docker-forwarded port

Client on same machine, not in a container

Advertising localhost and the associated port will let you connect outside of the container, as you'd expect.

In other words, when running any Kafka Client outside the Docker network (including CLI tools you might have installed locally), use localhost:29092 for bootstrap servers and localhost:2181 for Zookeeper (requires Docker port forwarding)

Client on another machine

If trying to connect from an external server, you'll need to advertise the external hostname/ip of the host as well as/in place of localhost.
Simply advertising localhost with a port forward will not work because Kafka protocol will still continue to advertise the listeners you've configured.

This setup requires Docker port forwarding and router port forwarding (and firewall / security group changes) if not in the same local network, for example, your container is running in the cloud and you want to interact with it from your local machine.

Client in a container, on the same host

This is the least error-prone configuration; you can use DNS service names directly.

When running an app in the Docker network, use kafka:9092 (see advertised PLAINTEXT listener config above) for bootstrap servers and zookeeper:2181 for Zookeeper, just like any other Docker service communication (doesn't require any port forwarding)

If you use separate docker run commands, or Compose files, you need to define a shared network manually

See the example Compose file for the full Confluent stack or more minimal one for a single broker.

Related question

Connect to Kafka on host from Docker (ksqlDB)

Appendix

For anyone interested in Kubernetes deployments:

• Operators (recommended): https://operatorhub.io/?keyword=Kafka
• Helm Artifact Hub: https://artifacthub.io/packages/search?ts_query_web=kafka&sort=stars&page=1

before zookeeper

1. docker container run --name zookeeper -p 2181:2181 zookeeper

after kafka

2. docker container run --name kafka -p 9092:9092 -e KAFKA_ZOOKEEPER_CONNECT=192.168.8.128:2181 -e KAFKA_ADVERTISED_LISTENERS=PLAINTEXT://ip_address_of_your_computer_but_not_localhost!!!:9092 -e KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR=1 confluentinc/cp-kafka

in kafka consumer and producer config

@Bean
public ProducerFactory<String, String> producerFactory() {
    Map<String, Object> configProps = new HashMap<>();
    configProps.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "192.168.8.128:9092");
    configProps.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
    configProps.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
    return new DefaultKafkaProducerFactory<>(configProps);
}

@Bean
public ConsumerFactory<String, String> consumerFactory() {
    Map<String, Object> props = new HashMap<>();
    props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "192.168.8.128:9092");
    props.put(ConsumerConfig.GROUP_ID_CONFIG, "group_id");
    props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
    props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
    return new DefaultKafkaConsumerFactory<>(props);
}

I run my project with these regulations. Good luck dude.

the simplest way to solve this is adding a custom hostname to your broker using -h option

docker run -d \
    --net=confluent \
    --name=kafka \
    -h broker-1 \
    -p 9092:9092 \
    -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
    -e KAFKA_ADVERTISED_LISTENERS=PLAINTEXT://kafka:9092 \
    -e KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR=1 \
    confluentinc/cp-kafka:4.1.0

and edit your /etc/hosts

127.0.0.1   broker-1

and use:

props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "broker-1:9092");