Why I cannot connect to Kafka from outside?

In the Kafka FAQ (updated for new properties) you can read:

When a broker starts up, it registers its ip/port in ZK. You need to make sure the registered ip is consistent with what's listed in bootstrap.servers in the producer config. By default, the registered ip is given by InetAddress.getLocalHost.getHostAddress(). Typically, this should return the real ip of the host. However, sometimes (e.g., in EC2), the returned ip is an internal one and can't be connected to from outside. The solution is to explicitly set the host ip and port to be registered in ZK by setting the advertised.listeners property in server.properties.

I solved this problem, by setting advertised.host.name in server.properties and metadata.broker.list in producer.properties to public IP address and host.name to 0.0.0.0.

The easiest way how to reach your Kafka server (version kafka_2.11-1.0.0) on EC2 from consumer in external network is to change the properties file

kafka_2.11-1.0.0/config/server.properties

And modify the following line

listeners=PLAINTEXT://ec2-XXX-XXX-XXX-XXX.eu-central-1.compute.amazonaws.com:9092

Using your public address

Verified on 2.11-2.0.0