ARP weirdness on local networks

[mbbrutman]

Host machine: Linux (Ubuntu 20.04) with the primary Ethernet disabled, leaving the esp_slip_router device as the only network connection. I'm using the pre-built firmware that is current as of yesterday.

What works:

• Pinging to my home router (192.168.2.1)
• Pinging the outside world (8.8.8.8 or brutman.com)

What fails:

• Pinging any other machine on my home network from the Ubuntu machine.
• Pinging the esp_slip_router from any device on my home network on it's WiFi (external) address.

Tcpdump on the Ubuntu machine shows me the request is going to the esp_slip_router device. Tcpdump on a target machine (a Linux laptop) shows no ICMP packets arriving from the esp_slip_router device, and the esp_slip_router device is also not even responding to ARP.

NAT should be properly routing ICMP originating from the Ubuntu machine, and it's doing so for external addresses. And of course you should be able to ARP it.

Here is what I think is going wrong:

The esp_slip_router has a problem responding to ARP. It doesn't seem to be responding at all.

• Getting an initial address works because it has to broadcast the DHCP request, allowing the home router to harvest the MAC address. Which then allows the router to talk to the esp_slip_router device, enabling access to the outside world.
• Internal machines do not respond to ARP and can not ping the esp_slip_router device if it does not send or receive ARP.
• I can make a specific machine respond to pings from the Ubuntu machine or be able to ping the esp_slip_router device directly if I prime that machine's ARP cache with the MAC address of the esp_slip_router. (Priming the ARP cache allows the ICMP packet to get to the esp_slip_router device, and it has the return MAC address, so that gets around the need to explicitly ARP. The esp_slip_router device is luckily caching the hardware address.)

So the short story is that I think ARP caching is working, but something is preventing the esp_slip_router device from ARPing or responding to ARP requests.