When I recently updated to RTCS 18.104.22.168 (and RTCS 22.214.171.124). I have the OPT_KEEPALIVE property enabled on one of my "control" sockets that I am using for a home brew FTP server.
In RTCS 4.1, no issues, seems to work as intended.
In RTCS 4.2, when the control socket connection sits idle for a few minutes the server side (MQX/RTCS) the first KEEPALIVE sent ends up resetting the socket connection. KEEPALIVE should not do this unless the remote side is gone. Two keep alive probes from RTCS (ID=80 and 81), one keepalive response from the host , then RTCS resets the connection.