All data exchanged between the telnet client and server may be captured by enabling debugging. To enable/disable debugging invoke the TelnetSession#setDebug method. By default debugging is disabled. When debugging is enabled all debugging output is directed to System.out. You may redirect this output to another stream using the TelnetSession#setDebugStream method
Example
// enable debugging
session.setDebug(true);
// redirect debugging to file
PrintStream ps = new PrintStream(new FileOutputStream("c:/log.txt"));
session.setDebugStream(ps);