Python socket connect_ex() seems to get stuck depending on OS

Question

I'm running a port scanner on my home network. If I run it against devices on my home network, it only works on other Linux machines (physical machines not VMs). By "works" I mean it finds what few ports are actually open (ssh, mysql, sunrpc, and a couple others).

When scanning Windows and other miscellaneous iot devices it just hangs and never finishes. I can't seem to figure out where it gets stuck.

I think this might be the most relevant part of the code:

for port in range(begin, end):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    try:
        status = s.connect_ex((self.target_ip, port))
    except Exception:
        continue
    finally:
        s.close()
    if status != 0:
        continue
    try:
        service = socket.getservbyport(port)
    except Exception:
        service = "unknown"
    print("SERVICE: %-15s\tPORT: %-8d" % (service, port))

Answer 1

I modified your code so that I could simulate a run on my machine and it seems like it hangs because that very last print statement is not reached. But this is because your continue line within the if status != 0 always comes back as "not 0", at least on my Windows 10 Pro machine.

for port in range(begin, end):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    try:
        status = s.connect_ex((self.target_ip, port))
        print("DEBUG: ", status) # add this
    except Exception as err:
        print("Error: ", err) # add this
        continue
    finally:
        s.close()
    if status != 0:
        print("DEBUG: status was not 0") # add this
        # if this line is reached, we start at the top of the loop
        # skipping the try except and print below entirely.
        continue
    try:
        service = socket.getservbyport(port)
    except Exception:
        service = "unknown"
    print("SERVICE: %-15s\tPORT: %-8d" % (service, port))

Could you give this a shot and see if will shine some light on what might be wrong or let us know what kind of output you get. I was not able to currently run this on any Linux machine or IoT devices. I assume you are able to read the console output on each of your devices to have determined that it seemed like it was "hanging".

Edit 1: More context for the hanging issue

Let me give you some examples of what happens with this script on my network.

CASE 1: '192.168.1.0' This address exists, but generally isn't used or assigned to anything. Hangs for about 20 seconds per port for me.

CASE 2: '192.168.1.1' This is usually the router on the network. Responds immediately. Hangs for about 1 second per port that is scanned.

CASE 3: '192.168.1.3' This device is in the network range, but no device is actively using it. Hangs for about 20 seconds per port (just like case 1).

So the long "hang" isn't that it is not working. It can basically mean the IP is wrong or there is no device being reached so the timeout limit is being reached for that socket connection attempt instead of raising an exception it just moves on.

Edit 2

Before iterating over a huge range of IP addresses and a wide range of ports per each IP address being tested. Sometimes it can be helpful to test assumptions 1 at a time by defining more control over a few ports and a specific IP address.

This is what I converted your code into to do that. If the conn is not 0 I am just considering that port closed no matter which error code came back for this case. We only care about the ones that respond with 0 because that indicates the operation succeeded.

import socket

# Lets test just a few custom ports.
ports = [21, 22, 80, 8080]

# And test a specific IP address to test, 1 at a time.
ip = '192.168.1.1' # change this if needed

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

for port in ports:
    # loop through our small custom port range.
    try:
        conn = sock.connect_ex((ip, port))
        if conn == 0:
            print("Port [{}] open.".format(port))
        else:
            print("Port [{}] closed.".format(port))
    except Exception as err:
        print(err)
        continue

    try:
        service = socket.getservbyport(port)
    except Exception:
        service = "unknown"
    print("SERVICE: %-15s\tPORT: %-8d" % (service, port))

sock.close()