> If someone used regular sockets deliberately, they could crash
> multiprocessing server code deliberately. Any chance of doing a real message
> length check against the embedded message length check?

You can do

    message = conn.recv_bytes(maxlength)

if you want a length check -- OSError will be raised if the message is too long.

But Listener() and Client() are *not* replacements for the normal socket API and I would not really advise using them for communication over a network.  They are mostly used internally by multiprocessing -- and then only with digest authentication.

All processes in the same program inherit the same randomly generated authentication key -- current_process().authkey.  If you create a listener by doing

    listener = Listener(address, authenticate=True)

then other processes from the same program can connect by doing

    conn = Client(address, authenticate=True)

Without knowing the correct authentication key it is not possible to connect and do a DOS like you describe.