Sending a reset in TCP/IP Socket connection

Question

I am using python’s socket.py to create a connection to an ftp-server. Now I want to reset the connection (send a RST Flag) and listen to the response of the ftp-server. (FYI using socket.send('','R') does not work as the OS sends FIN flag instead of RST.)

Germaun

Why do you want to reset the connection? – Tronna

replicating a malicious ftpclient – Germaun

Related: resetting a TCP connection from the server side – Hudis

Answer 1

Turn the SO_LINGER socket option on and set the linger time to 0 seconds. This will cause TCP to abort the connection when it is closed, flush the data and send a RST. See section 7.5 and example 15.21 in UNIX Network Programming (UNP).

In python:

def client(host, port):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0)
    s.connect((host, port))
    l_onoff = 1                                                                                                                                                           
    l_linger = 0                                                                                                                                                          
    s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER,                                                                                                                     
                 struct.pack('ii', l_onoff, l_linger))
    # send data here
    s.close()

Effloresce

It will cause TCP to abort the connection when closed, throw away any pending data, and send an RST. – Tronna

Just some more info on this for those curious: SO_LINGER with a time of 0 seconds means the TCP stack won't "linger around" on close(): it will discard any unsent data and send a RST instead of a FIN. For more info read this and this. – Cuprous

Note that if you are using this code in the Microsoft Windows environment then you need to replace the 'ii' in the call to struct.pack() with 'hh'. Windows uses shorts in struct linger, where Linux uses ints. – Confiteor

Answer 2

If you want to implement your own behavior over connections I think you should try using Scapy. It is a really useful library/tool. It lets you play with IP/TCP/UDP/ICMP packages.

Testify

Answer 3

To send an RST on a TCP connection, set the SO_LINGER option to true with a zero timeout, then close the socket. This resets the connection. I have no idea how to do that in Python, or indeed whether you can even do it.

Tronna

Answer 4

Connect to a server, send data and then RST (instead of FIN): (I tested it on Windows)

import struct
from socket import socket, AF_INET, SOCK_STREAM
from _socket import SOL_SOCKET, SO_LINGER

ip="192.168.1.10" #change it to real IP address.
port=8084 #change it to real port number.
s = socket(AF_INET, SOCK_STREAM)
s.setsockopt(SOL_SOCKET, SO_LINGER, struct.pack('ii', 1, 0))
s.connect((ip, port))
#send data to server (optional)
s.close()

Oneself