Basic Kerberos Test Class in Java

Luke Hebert

- 2 minutes read - 362 words

When Kerberos authentication fails inside a large application, the noise from logging frameworks, connection pools, and middleware can make it hard to tell whether the problem is in your code or in the Kerberos exchange itself. A small, standalone test class strips away all of that and lets you confirm that the ticket-granting process works before you go back to debugging the full stack.

Why Test Kerberos in Isolation

Kerberos involves several moving parts: the client keytab or password, the KDC address, the realm name, and the service principal. A single mismatch in any of those will produce an opaque LoginException buried under application logs. Running a bare-bones LoginContext against the same KDC tells you immediately whether the credentials and network path are sound.

What the Test Class Does

The Java class below uses only standard JDK classes (javax.security.auth.login.LoginContext and javax.security.auth.Subject). It reads a JAAS configuration file, attempts a Kerberos login, and prints the resulting Subject so you can verify that a TGT was acquired.

📎 Download kerberostest.java

JAAS Configuration

The accompanying jaas.conf file defines the KerberosLogin login module with the required options: useTicketCache, storeKey, principal, keyTab, and debug. Adjust the principal and keyTab values to match your environment before running.

📎 Download jaas.conf

Compile and Run

  1. Save the Java source as KerberosTest.java and the config as jaas.conf in the same directory.

  2. Compile: javac KerberosTest.java

  3. Run with the JAAS config on the classpath:

    java -Djava.security.auth.login.config=./jaas.conf KerberosTest

If the login succeeds you will see the Subject printed to stdout, including the KerberosTicket entry. If it fails, the exception message will point to the specific step (DNS lookup, AS-REQ, TGS-REQ) that broke.

Quick Troubleshooting Checklist

  • Wrong realm or KDC address: Confirm the realm in krb5.conf matches the KDC you expect.
  • Clock skew: Kerberos rejects tickets when the client and KDC clocks differ by more than 5 minutes.
  • Keytab permissions: The keytab file must be readable by the user running the Java process.
  • DNS resolution: Make sure the KDC hostname resolves to the correct IP; a stale DNS entry is a common culprit.

Running this small test first saves a lot of back-and-forth when the real application is the one that needs to authenticate.