How to simulate TCP connect timeout

timeout-server.py

#!/bin/env python3
#
# This script can be used as a server when you need to test the handling of
# TCP connection establishment timeouts of your client.
#
# The protocol can be HTTP, HTTPS or just about anything else, since connection
# will never be established.  It will hang in TCP handshake.
#

import socket

# Bind to server port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(("",8000))
s.listen(0)

# Connect one client to fill in the listen queue
c = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
c.connect(("127.0.0.1",8000))

# At this point, testing the client connect timeout handling can take place.
# Any attempt to establish connection from external clients will not complete.
# TCP tree-way handshake cannot be completed because TCP implementation at the
# server will not send SYN+ACK back until there is room in the listen queue.