My current ISP provides an internet connection over a copper wire. To use it, I have a crappy modem (Technicolor CGA2121, DOCSIS 3.0). It’s running in bridge mode, meaning that all it does is convert the signal running over the coax cable into plain old Ethernet.

My main networking device is a TP-Link Archer C7 v5. It runs OpenWRT. This router/Wi-Fi AP box connects to the modem and handles everything, including getting a public IPv4 address from the ISP.1

After a power outage or my ISP doing maintenance, the public IP address has usually changed. This wouldn’t be a problem if I just stuck to the ISP-approved modem.2

With my setup, there was a problem. The OpenWRT box would try to operate with the IPv4 address that it was given since the DHCP lease had not yet expired. However, this meant that there was no internet connectivity. A reboot of the OpenWRT box would resolve the issue.

This manual workaround wasn’t good enough for me. It would be quite problematic if this issue happened while I was away from home because I’d still like to access my home server.

After traversing OpenWRT forums and consulting the Slack workspace of my local hackerspace, I found that bringing up the WAN interface again would result in the OpenWRT box getting a new public IPv4 address. Problem solved!

To automate this workaround, I created a single crontab entry in the OpenWRT box. This is also configurable in a graphical user interface as long as you have LuCI installed.

The crontab entry looks like this:

*/5 * * * * /bin/ash -c '/bin/ping -c 3 8.8.8.8 > /dev/null || /sbin/ifup wan'

Every 5 minutes, the router pings Google’s DNS server. If that command succeeds, then the internet connection works and that’s it. If the ping fails, then the other half of the shell command is executed, which brings up the wan interface on my router.

Feel free to use a different IP address to test with. Your WAN network interface might also have a different name.

The downside of this solution is that if the server you’re using to verify your internet connection is down or refuses pings, then you’ll be causing interruptions in your home network every 5 minutes.

Talking to the ISP about this issue was something I considered as well. Then I remembered that it took me 1.5 months of fighting chatbots and repeating the same information to different customer care agents to use my own modem that’s identical to the one the ISP uses. That’s a hell no from me.

  1. I really should get around to drilling those holes in the apartment building to get access to a fiber connection. ↩︎

  2. using the ISP-approved box would introduce a whole other set of problems because they are surprisingly low quality. ↩︎