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Using SASL GSSAPI with librdkafka in a cross‐realm scenario with Windows SSPI and MIT Kerberos

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Emanuele Sabellico edited this page Aug 25, 2025 · 11 revisions

In this tutorial we'll see how to set up SASL/GSSAPI in a cross-realm scenario with Windows Active Directory and MIT Kerberos. On Windows librdkafka uses SSPI to automatically authenticate with current user. If cross-realm trust is set up, Windows users can directly authenticate as Kafka principals.

Requirements

Everything will be set up on a Windows Server instance, by using WSL2 for running Apache Kafka 4.0 on Ubuntu.

Configuration

Install Active Directory

  • Go to Server Manager
  • Select "Add roles and Features"
  • Check "Active Directory Domain Services"
  • Install it
  • After installation: "Promote this server to domain controller"
  • Add a new forest with name testwindomain.com (NetBIOS name: TESTWINDOMAIN)
  • Finish the configuration and restart
  • To login now you've to use TESTWINDOMAIN\<admin_user>

Install the required dependencies through WinGet

  • winget.exe install Microsoft.DotNet.SDK.8
  • winget.exe install Git.SDK

Install WSL2 with Ubuntu

  • Enable the WSL feature with this PowerShell command
    • Enable-WindowsOptionalFeature -Online -FeatureName Microsoft-Windows-Subsystem-Linux, VirtualMachinePlatform
  • Install the Ubuntu distribution
    • wsl --install Ubuntu
  • Install Linux dependencies
    • apt update && apt install -y krb5* openjdk-21-jdk
  • Get WSL IP address on PS
    • wsl hostname -I
  • Go to Windows DNS Manager and add a new Forward Lookup Zone (Primary) with name testkafkadomain.com
  • Add two hosts: kdc and kafka, both using the WSL IP address.

Configure MIT Kerberos on Linux

  • Edit /etc/krb5.conf
  • Change default realm in /etc/krb5.conf to TESTKAFKADOMAIN.COM
  • Add realm configuration:
TESTKAFKADOMAIN.COM = {
      kdc = kdc.testkafkadomain.com
      admin_server = kdc.testkafkadomain.com
      default_domain = testkafkadomain.com
}
  • Create the Kerberos DB
    • kdb5_util create -s
  • Create an ACL at /etc/krb5kdc/kadm5.acl with contents */[email protected] *
  • Restart the servers:
    • systemctl restart krb5-admin-server
    • systemctl restart krb5-kdc
  • Create the principal for Kafka brokers and add a keytab for it
    • mkdir /etc/security/keytabs
    • kadmin.local addprinc -randkey kafka/kafka.testkafkadomain.com
    • kadmin.local ktadd -k /etc/security/keytabs/kafka.testkafkadomain.com.keytab kafka/kafka.testkafkadomain.com

Install Kafka

  • Download and extract the binaries
    • wget -O kafka_2.13-4.0.0.tgz https://dlcdn.apache.org/kafka/4.0.0/kafka_2.13-4.0.0.tgz && \ tar -xvf kafka_2.13-4.0.0.tgz && cd kafka_2.13-4.0.0
  • Edit config/server.properties
  • Add SASL_PLAINTEXT://kafka.testkafkadomain.com:9094 to advertised.listeners and the corresponding value to listeners.
  • Add these new properties:
sasl.enabled.mechanisms=GSSAPI
sasl.kerberos.service.name=kafka
sasl.kerberos.principal.to.local.rules=RULE:[1:$1@$0](.*)s/(.*)@(.*)/$1_$2/,RULE:[2:$1@$0](.*)s/(.*)@(.*)/$1_$2/
authorizer.class.name=org.apache.kafka.metadata.authorizer.StandardAuthorizer
super.users=User:kafka_TESTKAFKADOMAIN.COM
  • The rules are respectively for primary@REALM and primary/instance@REALM, they strip the instance if present and replace @ with _ as it's not a valid character for Kafka principals. This allows to keep users coming from separate realm as different Kafka principals. The super user is an example of the result of this transformation.
  • Change log.dirs to <home_directory>/kafka_2.13-4.0.0/log-dirs
  • Change inter.broker.listener.name to SASL_PLAINTEXT
  • Change CONTROLLER security protocol map to SASL_PLAINTEXT in listener.security.protocol.map
  • Set advertised.listeners to PLAINTEXT://localhost:9092,SASL_PLAINTEXT://kafka.testkafkadomain.com:9094,CONTROLLER://kafka.testkafkadomain.com:9093
  • Edit kafka_server_jaas.conf inside kafka_2.13-4.0.0, use this configuration:
KafkaServer {
    com.sun.security.auth.module.Krb5LoginModule required
    useKeyTab=true
    storeKey=true
    keyTab="/etc/security/keytabs/kafka.testkafkadomain.com.keytab"
    principal="kafka/[email protected]";
};
  • Still inside kafka_2.13-4.0.0, create the logs directory
    • mkdir log-dirs
  • Create a cluster ID
    • KAFKA_CLUSTER_ID="$(bin/kafka-storage.sh random-uuid)"
  • Format the storage with the generated ID
    • bin/kafka-storage.sh format --standalone -t $KAFKA_CLUSTER_ID -c config/server.properties
  • Tell Kafka to use the jaas file
    • export KAFKA_OPTS="-Djava.security.auth.login.config=$PWD/kafka_server_jaas.conf"
  • Finally start Kafka
    • bin/kafka-server-start.sh config/server.properties

Configure Kafka ACLs

  • Create a command.properties file with these contents:
security.protocol=SASL_PLAINTEXT
sasl.mechanism=GSSAPI
sasl.kerberos.service.name=kafka
sasl.jaas.config=com.sun.security.auth.module.Krb5LoginModule required \
   useKeyTab=true \
   storeKey=true \
   keyTab="/etc/security/keytabs/kafka.testkafkadomain.com.keytab" \
   principal="kafka/[email protected]";
  • Allow the Windows producer to produce to Kafka topic test1 
bin/kafka-acls.sh --bootstrap-server kafka.testkafkadomain.com:9094 --command-config command.properties --add --allow-principal "User:kafka_client_TESTWINDOMAIN.COM" --topic test1 --resource-pattern-type LITERAL --operation Write

Establish mutual trust

This is the key part, we must add a shared principal with same password to obtain a cross realm TGT for a Windows realm principal to MIT Kerberos. Replace the example password.

  • On MIT Kerberos add the krbtgt primary from TESTWINDOMAIN.COM realm to TESTKAFKADOMAIN.COM one
  • On Windows add the corresponding trust with same password
    • netdom trust TESTKAFKADOMAIN.COM /Domain:TESTWINDOMAIN.COM /add /realm /passwordt:example
  • On Windows add a kdc for TESTKAFKADOMAIN.COM
    • ksetup /addkdc TESTKAFKADOMAIN.COM kdc.testkafkadomain.com
  • On Windows add the mapping for hostnames like .testkafkadomain.com to the TESTKAFKADOMAIN.COM realm
    • ksetup /addhosttorealmmap .testkafkadomain.com TESTKAFKADOMAIN.COM

Create a Producer on Windows

This is the final part. We now start a .NET producer on Windows using the logged in user and authenticating to Kafka using MIT Kerberos. The .NET client uses the prebuilt binaries in librdkafka.redist as it uses SSPI and doesn't need libsasl2 that is only for Linux.

libsasl2 isn't included in prebuilt binaries for Linux because it dynamically loads other plugins that need to call its functions and it cannot be done from the already dynamically loaded shared library without adding the RTLD_GLOBAL flag to dlopen. The simplest and most secure way of using libsasl2 on Linux is to install librdkafka with the provided Debian or Red Hat Linux packages and dynamically link it.

  • Create new Active Directory user TESTWINDOWDOMAIN\kafka_client
    • New-ADUser -Name "kafka_client" -SamAccountName "kafka_client" -UserPrincipalName "[email protected]" -AccountPassword (ConvertTo-SecureString "Example@!" -AsPlainText -Force) -Enabled $true
  • Add it to the Administrators group to allow to login locally (only for this example):
    • Add-ADGroupMember -Identity "Administrators" -Members "kafka_client"
  • Start a new shell with user TESTWINDOMAIN\kafka_client
    • runas /user:TESTWINDOMAIN\kafka_client cmd
  • Clone confluent-kafka-dotnet
    • cd %USERPROFILE% && C:\git-sdk-64\cmd\git.exe clone https://github.com/confluentinc/confluent-kafka-dotnet.git
  • Go to the Producer example
    • cd %USERPROFILE%\confluent-kafka-dotnet\examples\Producer
  • Edit Program.cs and change the config to:
    • var config = new ProducerConfig { BootstrapServers = brokerList, SecurityProtocol = SecurityProtocol.SaslPlaintext, SaslMechanism = SaslMechanism.Gssapi };
  • Start the Producer and you can produce some messages
    • dotnet run kafka.testkafkadomain.com:9094 test1
  • You've now setup cross-realm trust between the two realms. If you want to try a consumer too you can these additional ACLs and use the Consumer example after editing its configuration.
    • bin/kafka-acls.sh --bootstrap-server kafka.testkafkadomain.com:9094 --command-config command.properties --add --allow-principal "User:kafka_client_TESTWINDOMAIN.COM" --topic test1 --resource-pattern-type LITERAL --operation Write
    • bin/kafka-acls.sh --bootstrap-server kafka.testkafkadomain.com:9094 --command-config command.properties --add --allow-principal "User:kafka_client_TESTWINDOMAIN.COM" --group csharp-consumer --resource-pattern-type LITERAL --operation All
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