This is the documentation for CDH 4.7.0.
Documentation for other versions is available at Cloudera Documentation.

Appendix D - Enabling Debugging Output for the Sun Kerberos Classes

Initially getting a secure Hadoop cluster configured properly can be tricky, especially for those who are not yet familiar with Kerberos. To help with this, it can be useful to enable debugging output for the Sun Kerberos classes. To do so, set the HADOOP_OPTS environment variable to the following:

HADOOP_OPTS="-Dsun.security.krb5.debug=true"