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I'm trying to make my Pi listen for ARP broadcasts on my wireless network, to detect an Amazon Dash Button button press. When I push the button, it broadcasts an ARP request to find the network gateway. I can see this request using tcpdump on my laptop, but not on my Pi. Here's the output on each when I push the button and filter on the button's MAC address:

Laptop: (expected output)

$ sudo tcpdump ether host fc:a6:67:1d:ab:cd -n
listening on wlp2s0, link-type EN10MB (Ethernet), capture size 262144 bytes
08:23:51.025847 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fc:a6:67:1d:ab:cd, length 261
08:23:51.028398 ARP, Request who-has 192.168.1.1 tell 192.168.1.11, length 28

Pi:

$ sudo tcpdump ether host fc:a6:67:1d:ab:cd -n
listening on wlan0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:23:50.950669 EAPOL key (3) v2, len 95
12:23:50.952511 EAPOL key (3) v2, len 117
12:23:50.953982 EAPOL key (3) v2, len 151
12:23:50.964768 EAPOL key (3) v2, len 95
12:23:51.036188 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fc:a6:67:1d:ab:cd, length 261

Apparently both are seeing some traffic from the button press, including the BOOTP/DHCP request, but the Pi sees only some EAPOL traffic, while the laptop sees only the ARP request.

Other notes:

  • I have the same issue using scapy scanner in Python, so I assume this is a problem with the wireless driver or configuration.

  • The Pi can see its own ARP traffic in tcpdump, i.e., when the Pi itself sends out a who-has request or gets a response to one of its own requests, those ARP packets do show up in tcpdump output. The Pi fails to capture all ARP broadcasts that don't directly concern it.

  • I'm sure the broadcast is physically reaching the Pi.

What can I do to capture ARP traffic on my Pi?

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sudo tcpdump -ni eth0 arp. It will periodically spew out captured arp packets. If your Pi doesn't receive arp traffic from a local area network, you are offline.

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