Building a Mix Protocol Apache Kafka Cluster

nbuesing profile image Neil Buesing ・6 min read


Goal: build a multi-protocol Apache Kafka Clusters for SSL Client Authentication for all clients while leveraging PLAINTEXT for inter broker communication.

There are many tutorials and articles on setting up Apache Kafka Clusters with different security options. However, I have not seen a mix-protocol cluster with SSL for encryption and client authentication and PLAINTEXT for broker communication; so I decided to build one.


  1. Setup broker configuration of super.users for SSL authenticated users, not for PLAINTEXT.
  2. Start brokers with SSL inter broker communication.
  3. create full-access ACLs for user User:ANONYMOUS, but only for the IPs of your brokers.
  4. Change brokers to use PLAINTEXT inter broker communication.
  5. Do a rolling restart.


The main challenges in setting up the cluster were:

  • Kafka broker authorizers are not protocol specific.

  • Unlike SASL, SSL authorization cannot be done over PLAINTEXT; it has to be over SSL encryption.

  • SSL Certificates are tricky, and those were addressed in What I learned about SSL Certificates when building a Secured Kafka Cluster.

  • While ACLs can allow resource access to a given principal from a given set of hosts, the broker's super.users is only a principal


  • SSL

Each broker needs to be configured with the truststore and keystore referencing the appropriate jks files.

    ssl.key.credentials = kafka.key
    ssl.keystore.credentials = kafka.key
    ssl.keystore.filename = broker-{ID}.keystore.jks
    ssl.truststore.credentials = kafka.key
    ssl.truststore.filename = kafka.server.truststore.jks

Note: Confluent platform docker images has minor changes over a standard
deployment; as in the use of ssl.*.credentials and files with those credentials instead of ssl.*.passwords.

  • Protocols and Listeners

The settings for protocols and listeners are standard. Both SSL and PLAINTEXT are needed.


The default value for this broker property is just fine, but if you prefer to only have configured what is needed;
consider setting it to:

listener.security.protocol.map = PLAINTEXT:PLAINTEXT,SSL:SSL


Each broker needs to established its list of listeners and advertises them in a way that the clients and other brokers can
directly access them. Standard 9092 and 9093 ports for PLAINTEXT and SSL used respectively.

  advertised.listeners = PLAINTEXT://broker-{ID}:9092,SSL://broker-{ID}:9093
  • Authorizer


To enable client authorization, set authorizer.class.name
to kafka.security.authorizer.AclAuthorizer, the provided implementation of org.apache.kafka.server.authorizer.Authorizer.

  authorizer.class.name = kafka.security.authorizer.AclAuthorizer
  • SSL Client Authentication


Set ssl.client.auth to required or requested.

  ssl.client.auth = required

While you could set this to requested that would still allow SSL clients to not support client authentication. Unless
you have a specific reason to need to do this, make sure you set this to required.

  • No ACL Found


Be sure to exclude the property allow.everyone.if.no.acl.found or set to false. If you set this to true,
it allows full access to a resource if no ACLs are found.

  allow.everyone.if.no.acl.found = false
  • Super Users


Do not add User:ANONYMOUS to super.users. There is no way to restrict privileges to a given host. Instead, only add the
users of the SSL certificates for your brokers and the user you will use to create ACLs. This ensures that the cluster
is always secure. In addition to each broker's certificate added here; add in the root certificate so kafka-acls can be used to create the proper ACL.

The super.users configuration it should not be used for
authentication for PLAINTEXT. When SSL authorization is enabled, the user presented over PLAINTEXT will always be User:ANONYMOUS.
If you add this user to super.users anyone will be able to access your cluster (unless network restrictions prevent it).

  super.users = User:CN=root;User:CN=broker-1;User:CN=broker-2;User:CN=broker-3;User:CN=broker-4
  • Inter Broker Protocol


This is a tricky setting. First, start the cluster with SSL.

security.inter.broker.protocol = SSL

Once the ACL for the brokers is created it change it to PLAINTEXT.

security.inter.broker.protocol = PLAINTEXT

The cluster will need to be restarted (following a proper rolling restart) for the change to take effect.

Creating the Secure Cluster

With all these pieces in place, the process is straightforward.

  1. Start Cluster
  2. Create ACLs for a given superuser, User:ANONYMOUS only from IPs of brokers.
  3. Change inter broker protocol on all the brokers from SSL to PLAINTEXT
  4. Perform a rolling restart of the cluster

An alternate approach considered was temporarily adding User:ANONYMOUS to the super.users, and remove it after the ACLs
are configured; to use this approach, ensure the cluster is not accessible until User:ANONYMOUS is part of super.users.

Example Cluster

The Kafka SSL Cluster repository provides an example cluster. It deploys on
a single machine leveraging docker-compose. It showcases this configuration, it is not a production-ready cluster.

See Confluent's quickstart documentation to understand
the differences in some of the configurations leveraging the Confluent Platform docker images.

Use the jumphost container to access the cluster. Scripts to easily create the ACLs and test the cluster are mounted to this

Connect to that cluster using docker exec.

docker exec -it kafka_jumphost bash
cd /opt/bin

Example Code

An Example configuration is available at the GitHub repository Kafka SSL Cluster.
It includes scripts to create certificates, script to swap changing protocol from SSL to PLAINTEXT, a
rolling restart script, and a dashboard. Please see its documentation for additional details.

To run commands to create ACLs, you will need a configuration like the following.


Super User ACL

Once the cluster is up and running with the security inter broker protocol of SSL, create an ACL based superuser
that can only be used by the hosts of the brokers. With this, the unauthenticated user of User:ANONYMOUS has full operational
access to the cluster, but only from the hosts hosting the brokers.

kafka-acls \
 --bootstrap-server broker-1:9093,broker-2:9093,broker-3:9093,broker-4:9093 \
 --command-config ./config/adminclient-config.conf \
 --add \
 --force \
 --allow-principal User:ANONYMOUS \
 --allow-host \
 --allow-host \
 --allow-host \
 --allow-host \
 --operation All \
 --topic '*' \

Performance Testing

Now that we know it is feasible, is a mixed protocol cluster worth it? While using docker-compose to run a cluster on
one machine is not the best way to property test; it does give insights on where it could lead.


With acks=1 theoretically, I would expect no change. However, with less encryption going on between brokers, the CPU
load would go down, so a small improvement is expected. My tests show a 7% improvement.

security.inter.broker.protocol records/sec MB/sec ms avg latency
SSL 29,338.56 27.98 970.06
PLAINTEXT 31,512.17 30.06 926.85


With acls=all a larger improvement is expected. This is because the time for the insync replicates to replicate should be faster.
The test shows a 44% improvement; which is greater than what I was expecting. I will need to move this to a realistic
configuration and do additional testing. This result is rather promising and justifies doing more realistic cluster testing.

security.inter.broker.protocol records/sec MB/sec ms avg latency
SSL 12,097.04 11.54 2,557.13
PLAINTEXT 17,418.82 16.62 1,732.00

The Test

Here is the performance script I ran with the ACLs generated for the topic.

performance script

  export BOOTSTRAP_SERVERS=broker-1:9093,broker-2:9093,broker-3:9093,broker-4:9093

  kafka-producer-perf-test \
    --num-records 500000 \
    --record-size 1000 \
    --throughput -1 \
    --producer.config ./config/producer-config.conf \
    --producer-props \
      bootstrap.servers=${BOOTSTRAP_SERVERS} \
      acks=all \
      request.timeout.ms=60000 \
      retries=2147483647 \
    --topic $1


  kafka-acls \
   --bootstrap-server broker-1:9093,broker-2:9093,broker-3:9093,broker-4:9093 \
   --command-config ./config/adminclient-config.conf \
   --add \
   --allow-principal User:CN=jumphost \
   --allow-host '*' \
   --operation READ \
   --operation WRITE \
   --topic $1

  kafka-acls \
   --bootstrap-server broker-1:9093,broker-2:9093,broker-3:9093,broker-4:9093 \
   --command-config ./config/adminclient-config.conf \
   --add \
   --allow-principal User:CN=jumphost \
   --allow-host '*' \
   --operation ALL \
   --group '*' 


I hope this gives you some insights into Kafka security and possible configurations you could explore for performance
improvements. For me it was the journey to create a cluster configuration I hadn't seen before; giving me the opportunity
to fully understand the security configurations of Apache Kafka.

Additional Information

This article was originally published on my professional blog at buesing.dev.

Posted on Jun 1 by:

nbuesing profile

Neil Buesing


Apache Kafka Enthusiast; opinions are my own; he/him


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