Not Even My Wife Reads This

I have a TrendNet TE100-P1P print server. It's a little $30 gizmo with an Ethernet jack on one end and a parallel printer port on the other, so that you can turn a printer into a network printer without hanging it off a computer. Bought it because my new motherboard didn't have a parallel port. The other benefit is that now the printer works when my main desktop computer is off.

Anyway, we had a power failure, and the printer stopped working. No "lexmark" found in local DNS. No DHCP lease for the TrendNet gizmo.

The gizmo was still there, but I couldn't find it. There's no UI on the device, just the network jack. So I wrote a little script (below) to portscan my LAN for anything that answered on port 631. That found it.

Turns out that when the power goes out, the print server reconfigures its network settings from DHCP to fixed. It keeps its last network settings, but the DHCP / DNS server no longer has any record of them, so you can't find it unless you remember what its last IP was.

Here's the script. Because I had 700+ addresses to check, and it sometimes takes a few seconds for a failed socket connection to timeout, I used 250 threads to make it faster. (I initally tried using a separate thread for each IP, but I hit an OS limit at 255 threads, and decided to use a fixed-size pool and two shared work queues.)

#!/usr/bin/env python

"""Hunting for the TrendNet print server"""

import socket
import threading
import Queue

socket.setdefaulttimeout(5)

port = 631
num_threads = 250

class ConnectThread(threading.Thread):
    def __init__(self, in_queue, out_queue):
        threading.Thread.__init__(self)
        self.in_queue = in_queue
        self.out_queue = out_queue
        self.setDaemon(True)

    def run(self):
        while True:
            ip = self.in_queue.get()
            try:
                sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
                sock.connect((ip, port))
            except socket.error:
                self.out_queue.put((ip, False))
            else:
                self.out_queue.put((ip, True))
            self.in_queue.task_done()

def main():
    job_queue = Queue.Queue()
    result_queue = Queue.Queue()
    count = 0
    for net in range(3):
        for addr in range(1, 255 + 1):
            ip = "192.168.%d.%d" % (net, addr)
            count += 1
            job_queue.put(ip)
    for unused in xrange(num_threads):
        ConnectThread(job_queue, result_queue).start()
    job_queue.join()

    for unused in xrange(count):
        ip, status = result_queue.get()
        if status:
            print ip
    

if __name__ == "__main__":
    main()