Sending files to client from server via socket/ftp python

Question

I am new to python. Using Ubuntu 18.04, python 3.6.

Tried writing a script to send ANY File from server to client (currently trying to send .pdf file) and vice versa.

Earlier i used sockets to send file, but was facing problems with extensions (like sending .pdf file but received .txt file - could not figure out why this was happening). Then used ftp, but stuck at one point right now.

SERVER SCRIPT:

import socket
import os

s = socket.socket()
host = ""
port = 9999

s.bind((host, port))
s.listen(5)
print("Binding Done\n")

socket_object, address = s.accept()
print("Connection Established\n")

print("Sending file...")
f = open("cds.pdf", 'rb')
while f.read(1024):
    socket_object.send(f.read(1024))

print("Files Send")
f.close()
socket_object.close()
s.close()

CLIENT SCRIPT:

import socket
import os
from ftplib import FTP

ftp = FTP()
s = socket.socket()
host = "192.168.43.16"
port = 9999

s.connect((host, port))
#ftp.connect(host = "192.168.43.16", port = 9999)

print("Receiving data...")

f = open("cds_copy.pdf", 'wb')
while True:
    ftp.retrbinary('RETR cds_copy.pdf', f.write, 1024)

ftp.storbinary('STOR cds_copy.pdf', open('cds_copy.pdf', 'rb'))
print("File Collected")
ftp.quit()
f.close()
s.close()

ERROR:

$python3 client.py
Receiving data...
Traceback (most recent call last):
  File "client.py", line 17, in <module>
    ftp.retrbinary('RETR cds_copy.pdf', f.write, 1024)
  File "/usr/lib/python3.6/ftplib.py", line 441, in retrbinary
    self.voidcmd('TYPE I')
  File "/usr/lib/python3.6/ftplib.py", line 277, in voidcmd
    self.putcmd(cmd)
  File "/usr/lib/python3.6/ftplib.py", line 199, in putcmd
    self.putline(line)
  File "/usr/lib/python3.6/ftplib.py", line 194, in putline
    self.sock.sendall(line.encode(self.encoding))
AttributeError: 'NoneType' object has no attribute 'sendall'

Unable to figure out the error.

Any suggestions will be helpful. Thanking You.

Answer 1

FTP is an application protocol as defined in RFC 959 . If you want to use it in your client you must have a server speaking that FTP protocol. Your server does not speak the FTP protocol but simply dumps the content of a file to the client. In this case the client should expect exactly this and not speak FTP, ie

import socket 
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.43.16", 9999))
f = open("out.pdf","wb")
while True:
    buf = s.recv(1024)
    if buf == "":
        break
    f.write(buf)

Apart from that your server is broken: it first reads 1024 bytes from the file, discards it and then reads the next 1024 byte and sends it to the client:

while f.read(1024):                           # read 1024 byte from f but discard
    socket_object.send(f.read(1024))          # read another 1024 byte from f and send

This is likely not what you've intended. Instead it should look more like this:

while True:
    buf = f.read(1024)              # read bytes from f
    if buf == "":                   # check that not done
        break
    socket_object.sendall(buf)     # write previously read bytes to client

Note that this also uses sendall instead of send since only sendall will take care to actually send all given data. A simple send instead might only send part of the data and you have to check the return value to find out how much was send.