The documentation says :
Note: By default the property ssl.endpoint.identification.algorithm is not defined, so hostname verification is not performed. In order to enable hostname verification, set the following properties
But with Flume 1.9.0 this is not true anymore because since Kafka 2.0.0 hostname verification is enable by default.
The default value for ssl.endpoint.identification.algorithm was changed to https, which performs hostname verification (man-in-the-middle attacks are possible otherwise). Set ssl.endpoint.identification.algorithm to an empty string to restore the previous behaviour.
The problem is that it is impossible to disable hostname verification since flume does not support empty values in configuration (cf FlumeConfiguration.addRawProperty)