This code example is given in the book "CPython Internals".
from queue import Queue
import socket
import time
timeout = 1.0
def check_port(host: str, port: int, results: Queue):
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(timeout)
result = sock.connect_ex((host, port))
if result == 0:
results.put(port)
sock.close()
if __name__ == '__main__':
start = time.time()
host = "localhost"
results = Queue()
for port in range(80, 100):
check_port(host, port, results)
while not results.empty():
print("Port {0} is open".format(results.get()))
print("Completed scan in {0} seconds".format(time.time() - start))
If host == 'localhost', then this script works very quickly (all ports (form 1 to 65535) checked for about 1.5 secs!), moreover, the duration of the timeout does not play any role.
If I set the host "8.8.8.8" or some other external one, then the script execution time looks correct. For example, checking ports from 440 to 444 with timeout == 1 on 8.8.8.8 takes about 4 secs.
Why is checking the availability of local ports so fast? (I use Ubuntu if it's important)
I understand that there is no practical need for this. I'm just wondering why the timeout value does not affect the script execution time when checking local ports. I guess this question is more about the operating system than programming, so I'm here.
