# A simple FTP client.## The information to write this program was gathered from RFC 959,# but this is not a complete implementation!  Yet it shows how a simple# FTP client can be built, and you are welcome to extend it to suit# it to your needs...## How it works (assuming you've read the RFC):## User commands are passed uninterpreted to the server.  However, the# user never needs to send a PORT command.  Rather, the client opens a# port right away and sends the appropriate PORT command to the server.# When a response code 150 is received, this port is used to receive# the data (which is written to stdout in this version), and when the# data is exhausted, a new port is opened and a corresponding PORT# command sent.  In order to avoid errors when reusing ports quickly# (and because there is no s.getsockname() method in Python yet) we# cycle through a number of ports in the 50000 range.  import sys, posix, stringfrom socket import *  BUFSIZE = 1024 # Default port numbers used by the FTP protocol.#FTP_PORT = 21FTP_DATA_PORT = FTP_PORT - 1 # Change the data port to something not needing root permissions.#FTP_DATA_PORT = FTP_DATA_PORT + 50000  # Main program (called at the end of this file).#def main():    hostname = sys.argv[1]    control(hostname)  # Control process (user interface and user protocol interpreter).#def control(hostname):    #    # Create control connection    #    s = socket(AF_INET, SOCK_STREAM)    s.connect((hostname, FTP_PORT))    f = s.makefile('r') # Reading the replies is easier from a file...    #    # Control loop    #    r = None    while 1:        code = getreply(f)        if code in ('221', 'EOF'): break        if code == '150':            getdata(r)            code = getreply(f)            r = None        if not r:            r = newdataport(s, f)        cmd = getcommand()        if not cmd: break        s.send(cmd + '\r\n')  # Create a new data port and send a PORT command to the server for it.# (Cycle through a number of ports to avoid problems with reusing# a port within a short time.)#nextport = 0#def newdataport(s, f):    global nextport    port = nextport + FTP_DATA_PORT    nextport = (nextport+1) % 16    r = socket(AF_INET, SOCK_STREAM)    r.bind((gethostbyname(gethostname()), port))    r.listen(1)    sendportcmd(s, f, port)    return r  # Send an appropriate port command.#def sendportcmd(s, f, port):    hostname = gethostname()    hostaddr = gethostbyname(hostname)    hbytes = string.splitfields(hostaddr, '.')    pbytes = [repr(port/256), repr(port%256)]    bytes = hbytes + pbytes    cmd = 'PORT ' + string.joinfields(bytes, ',')    s.send(cmd + '\r\n')    code = getreply(f)  # Process an ftp reply and return the 3-digit reply code (as a string).# The reply should be a line of text starting with a 3-digit number.# If the 4th char is '-', it is a multi-line reply and is# terminate by a line starting with the same 3-digit number.# Any text while receiving the reply is echoed to the file.#def getreply(f):    line = f.readline()    if not line: return 'EOF'    print line,    code = line[:3]    if line[3:4] == '-':        while 1:            line = f.readline()            if not line: break # Really an error            print line,            if line[:3] == code and line[3:4] != '-': break    return code  # Get the data from the data connection.#def getdata(r):    print '(accepting data connection)'    conn, host = r.accept()    print '(data connection accepted)'    while 1:        data = conn.recv(BUFSIZE)        if not data: break        sys.stdout.write(data)    print '(end of data connection)' # Get a command from the user.#def getcommand():    try:        while 1:            line = raw_input('ftp.py> ')            if line: return line    except EOFError:        return ''  # Call the main program.#main()