renewip: when the router keeps disconnecting

June 15th, 2008

So we now all have broadband connections and everything is great, right? Well, not quite. Some providers have better services than others. My connection seems rather fragile at times and tends to die about once in three-four days. When that happens, no amount of resetting the equipment helps to get it working again. It's an upstream issue that I have no control over.

But there is another problem. Once the cable modem starts working again, the router (which receives an IP address from my provider, and serves LAN and wifi locally) doesn't seem to know this and doesn't automatically re-establish a connection. Or I'm not really sure what it does, it's a black box and there is a web interface to it, where there's a button to press to do this, which sometimes works. But what really is happening, who knows. There seems to be a weird timing problem to the whole thing, where if I kill the power for both the modem and the router and they both come back at the same time, it generally works. However, if the modem is taking longer to negotiate a link, the router will be disconnected. And apparently doesn't try to reconnect on its own, so I've been stuck rebooting the two a few times until the timing is right. Resetting them separately for some reason doesn't seem to work.

So what can be done about it? Well, the router does have that stupid web interface, so it's possible to make those clicks automatically if we're disconnected. Python's urllib makes this very easy to do. First we login with router_login, which submits a form with POST. Then we check the state of the internet connection with check_router_state, which just reads out the relevant information from the page. And if it's disconnected we run renew_router_connection to submit another form (ie. simulating the button click on the web page).

Testing connectivity

More than just testing if the router has a connection to the provider, broadband connections sometimes have connectivity problems. Even if you can get a connection, the provider sometimes has problems on his network, meaning your connection doesn't work anyway.

So I came up with a test to see how well the connection is working. It's an optimistic test, so that first we assume we have a fully functional connection and ping It doesn't matter what host we use here, just some internet host that is known to be reliable and "always" available. For this to work these conditions must be met:

  1. We have to reach the gateway of the subnet where our broadband IP address lives.
  2. We have to reach the provider's nameserver (known as dns1 in the code) to look up the host "".
  3. We have to reach (we have their IP address now).

So first we ping If that fails, it could be because dns lookup failed. So we ping the provider's nameserver. If that fails, the provider's internal routing is probably screwed up, so we ping the gateway. And if that fails too then we know that although we have an IP address, the connection is dead (or very unstable).

#!/usr/bin/env python
# Author: Martin Matusiak <>
# Licensed under the GNU Public License, version 3.

import os
import re
import sys
import time
import urllib

ip_factory = ""
password = ""

inet_host = ""

def write(s):

def grep(needle, haystack):
    if needle and haystack:
        m =, haystack)
        if m and m.groups(): return m.groups()[0]

def invoke(cmd):
    (sin, sout) = os.popen2(cmd)

def ping(host):
    cmd = 'ping -c1 -n -w2 ' + host + ' 2>&1'
    res = invoke(cmd)
    v = grep("rtt min/avg/max/mdev = [0-9.]+/([0-9.]+)/[0-9.]+/[0-9.]+ ms", res)
    if v: return int(float(v))

def find_lan_gateway():
    cmd = "route -n"
    res = invoke(cmd)
    v = grep("[0-9.]+\s+([0-9.]+)\s+[0-9.]+\s+UG", res)
    if v: return v

def load_url(url, params=None):
    data = None
    if params: data = urllib.urlencode(params)
    f = urllib.urlopen(url, data)

def router_login():
    form = {"page": "login", "pws": password}
    load_url("http://%s/login.htm" % ip, form)

def check_router_state():
    state = { "conn": None, "gateway": None, "dns1": None }
    s = load_url("http://%s/js/js_status_main.htm" % ip)
    if s:
        v = grep("var bWanConnected=([0-9]);", s)
        if v == "1": state['conn'] = True
        elif v == "0": state['conn'] = False
        if state['conn']:
            g = grep('writit\("([0-9.]+)","GATEWAY"\);', s)
            if g and g != "": state['gateway'] = g
            g = grep('writit\("([0-9.]+)","DNSIP"\);', s)
            if g and g != "": state['dns1'] = g
    return state
def renew_router_connection():
    form = {"page": "status_main", "button": "dhcprenew"}
    s = load_url("http://%s/status_main.htm" % ip, form)
    return s

ip = find_lan_gateway()
if not ip:
    ip = ip_factory
    write("LAN gateway detection failed, using factory ip %s for router\n" % ip_factory)
    write("Router ip: %s\n" % ip)

while True:
        router = check_router_state()
        t = time.strftime("%H:%M:%S", time.localtime())
        if router['conn']:
            hosts = [(inet_host, inet_host),
                ("dns1", router['dns1']), ("gateway", router['gateway'])]
            connectivity = ""
            write("[%s] Connected  " % t)
            for (name, host) in hosts:
                delay = ping(host)
                if delay:
                    write("(%s: %s) " % (name, delay))
                    write("(%s !!) " % name)

            write("[%s] NOT CONNECTED, attempting reconnect\n" % t)
    except Exception, e:
        cls = grep("<type 'exceptions.(.*)'", str(e.__class__))
        write("%s: %s\n" % (cls, e))

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