Flink will use the environment variable HADOOP_CLASSPATH to augment the classpath that is used when starting Flink components such as the Client, JobManager, or TaskManager. Most Hadoop distributions and cloud environments will not set this variable by default so if the Hadoop classpath should be picked up by Flink the environment variable must be exported on all machines that are running Flink components.
When running on YARN, this is usually not a problem because the components running inside YARN will be started with the Hadoop classpaths, but it can happen that the Hadoop dependencies must be in the classpath when submitting a job to YARN. For this, it's usually enough to do a
{% highlight bash %} export HADOOP_CLASSPATH=hadoop classpath {% endhighlight %}
in the shell. Note that hadoop is the hadoop binary and that classpath is an argument that will make it print the configured Hadoop classpath.