Tools for zmodem/xmodem/ymodem file transfer Lrzsz is a cosmetically modified zmodem/ymodem/xmodem package built from the public-domain version of Chuck Forsberg's rzsz package. These programs use error correcting protocols ({z,x,y}modem) to send (sz, sx, sb) and receive (rz, rx, rb) files over a dial-in serial port from a variety of programs running under various operating systems.