How Can I Reset A TCP Socket In Python?

I have a socket proxy written in Python which when it receives a RST from a pair of communicating peers will close the connection to both peers by letting the sockets be garbage co

Solution 1:

Looking at the current socket package implementation in CPython, there is really no pythonic way (to connect a socket to an AF_UNSPEC address, as of 2019-01 (i.e. to reset the connection on Linux).

The next best thing is to set the SO_LINGER option on the accepted socket (either directly or via inheritance). When lingering is enabled (and set to a zero timeout) closing the socket yields a reset of the connection.

You have to be careful to set the SO_LINGER option on the right sockets API level and to use the right encoding for the option value (it's a struct).

Example:

import socket
import struct
import time

s = socket.socket(socket.AF_INET6)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 0)
# if we want to inherit this option:
#s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', 1, 0))
s.bind(('', 2323))
s.listen()
con, addr = s.accept()
con.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', 1, 0))
time.sleep(1)
con.close()
time.sleep(3)

Connecting to this port with curl:

$ curl localhost:2323
curl: (56) Recv failure: Connection reset by peer

Connecting to this port without sending anything:

$ socat - tcp:localhost:2323

When dumping the packets with e.g.

$ tshark -i lo -f 'tcp port 2323'

the last packet should be a RST (sent from server to client), in both cases - for example:

39 9758.478140247    127.0.0.1 → 127.0.0.1    TCP 66 2323 → 34494 [RST, ACK]
        Seq=1 Ack=1 Win=43776 Len=0 TSval=2787120418 TSecr=2787119417

Solution 2:

You can try to use the SO_LINGER socket option ( setsockopt ) with linger time set to 0. close on socket with SO_LINGER set with 0 seconds lingering time will result in RST instead of FIN.