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Let's play with a simple TCP server with SO_REUSEPORT
import socket
import sys
from contextlib import contextmanager
import click
so_reuseport = socket.SO_REUSEPORT
@contextmanager
def reserve_sock_addr(reuseport: bool, predefined_port: int=0):
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
if reuseport:
print('enable SO_REUSEPORT')
sock.setsockopt(socket.SOL_SOCKET, so_reuseport, 1)
sock.bind(("", predefined_port if predefined_port else 0))
_, port = sock.getsockname()
print(f'reserve following socket {socket.getfqdn()} {port}')
yield (socket.getfqdn(), port)
print('closing the first socket')
# https://pymotw.com/2/socket/tcp.html
def simple_tcp_echo(port: int, reuseport: bool):
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
if reuseport:
print('enable SO_REUSEPORT')
sock.setsockopt(socket.SOL_SOCKET, so_reuseport, 1)
sock.bind(("", port))
_, port = sock.getsockname()
print(f'listening to {socket.getfqdn()} {port}')
# Listen for incoming connections
sock.listen()
while True:
# Wait for a connection
print('waiting for a connection')
connection, client_address = sock.accept()
with connection:
try:
print('connection from', client_address)
# Receive the data in small chunks and retransmit it
while True:
data = connection.recv(8)
print(f'received "{data}"')
if data:
print('sending data back to the client')
connection.sendall(data)
else:
print('no more data from')
break
finally:
# Clean up the connection
connection.close()
print('clean up the connection')
print('closing the second socket')
@click.command()
@click.option("--port", default=0, type=int,
help="TCP port to use")
@click.option("--reuseport", is_flag=True, type=bool,
help="Enable SO_REUSEPORT")
def start_server(port: int, reuseport: bool):
if not port:
with reserve_sock_addr(reuseport) as (_, port):
port = port
simple_tcp_echo(port, reuseport)
print('this is the end')
@click.command()
@click.option("--predefined_port", default=0, type=int,
help="TCP port to use")
@click.option("--target_port", type=int,
help="target TCP port to hijack")
@click.option("--reuseport", is_flag=True, type=bool,
help="Enable SO_REUSEPORT")
def tcp_bind_loop(predefined_port: int,
target_port: int,
reuseport: bool):
from collections import Counter
all_ports = Counter()
n_attempts = 0
while(True):
n_attempts += 1
with reserve_sock_addr(reuseport, predefined_port=predefined_port) as (_, port):
port = port
all_ports[port] += 1
if port < target_port:
print(f'binding to {socket.getfqdn()} {port} less then {target_port}')
elif target_port == port:
print(f'hijacking successful! {socket.getfqdn()} {port}')
break
elif port > target_port:
print(f'binding to {socket.getfqdn()} {port} greater then {target_port}')
print(f'there were {len(all_ports)} unique ports after {n_attempts} attempts')
print(f'the top-25 most common were {all_ports.most_common(25)}')
print('sad but true')
@click.command()
@click.option("--reuseport", is_flag=True, type=bool,
help="Enable SO_REUSEPORT")
def create_reuseport_and_wait(reuseport: bool):
with reserve_sock_addr(reuseport) as (_, port):
port = port
input("#1 Press Enter to continue...")
input("#2 Press Enter to continue...")
print('this is the end')
@click.command()
@click.option("--reuseport", is_flag=True, type=bool,
help="Enable SO_REUSEPORT")
def create_reuseport_and_reuseagain(reuseport: bool):
with reserve_sock_addr(reuseport) as (_, port):
port = port
input("#1 Press Enter to continue...")
with reserve_sock_addr(reuseport, predefined_port=port) as (_, port):
port = port
input("#1.1 Press Enter to continue...")
input("#2 Press Enter to continue...")
print('this is the end')
@click.command()
@click.option("--reuseport", is_flag=True, type=bool,
help="Enable SO_REUSEPORT")
def create_reuseport_and_explicit_exit(reuseport: bool):
context_manager_instance = reserve_sock_addr(reuseport)
(_, port) = context_manager_instance.__enter__()
explicit_close = lambda: context_manager_instance.__exit__(None, None, None)
input("#1 Press Enter to continue...")
explicit_close()
with reserve_sock_addr(reuseport, predefined_port=port) as (_, port):
print('reserved 2nd time')
port = port
input("#1.1 Press Enter to continue...")
input("#2 Press Enter to continue...")
print('this is the end')
@click.group()
def cli():
"""simple TCP server to reproduce port binding issues"""
cli.add_command(start_server)
cli.add_command(tcp_bind_loop)
cli.add_command(create_reuseport_and_wait)
cli.add_command(create_reuseport_and_reuseagain)
cli.add_command(create_reuseport_and_explicit_exit)
if __name__ == "__main__":
cli()
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