Ping-monitor for OpenWrt behind an ISP router?

[doremifajb]

Hello! I am looking for a way to get alert in case the OpenWRT router dies or gets disconnected behind an ISP as double NAT.

For the system with OpenWRT+Modem (single NAT), I did this way:

Set ping at https://uptimerobot.com/ for the public address (not fixed).
Have a script on OpenWRT to send me an email if the public address changes, using
kdig +short myip.opendns.com @resolver1.opendns.com
or
. /lib/functions/network.sh; network_find_wan NET_IF; network_get_ipaddr NET_ADDR "${NET_IF}"; echo "${NET_ADDR}"

If I get an email from uptimerobot that my public address is not pingable, and I get an email from OpenWRT that the address changed, then I know that nothing happened. If I don't get any email from OpenWRT, then I know that either modem or OpenWRT is dead, I have to go check. This way, I don't have to pay for a fixed public IP.

Now, if I change the modem into an ISP router, it's not going to work: OpenWRT might be dead or the cable cut off and I won't get notified. Is there any way to get around it, other than using "exposed host" option for the router ? (Fritzbox 7530)

I will have two sets of this system (FB4020 with OpenWRT behind FB7530), so I might think of something like site-to-site wireguard if nothing else works, but I am wondering if there is any easier way to achieve this.

I will appreciate your hint!

[giuliomagnifico]

It isn't easier and faster to add a DDNS provider to the router? I'm doing the same to have my WireGuard tunnel always connected without even know my public ip. (Well, if you need the notification when the router is down, it's another story)

[lleachii]

Forward the ICMP Echo Request downstream to the device.

[doremifajb]

Thank you for your replies!
@giuliomagnifico
I didn't know much about DDNS. uptimerobot can ping to a host, so if DDNS can provide me a pingable host name for the OpenWRT behind FB7530, I should be getting notification. I will give it a try with a free one.

@lleachii
I looked at Fritzbox 7530 but I don't see any ICMP forwarding.... just port forwarding for various protocols like udp, tcp, etc.

[doremifajb]

@giuliomagnifico

Now I see, DDNS is just an alias for an IP address, so that you don't have to know when it changes. That's indeed useful, I'm going to install that on Raspberry Pi (with Wifi Controller), which is supposed to be always online.
but it's not going to help me pinging OpenWRT behind FB7530....

[lleachii]

@doremifajb

but I don't see any ICMP forwarding

Interesting...

[doremifajb]

@lleachii
That's indeed very interesting! Do you have a FB7530, too? (mine is ax, though). Mine shows, aside from tcp and udp, esp and gre. I don't know what these two are.