Ok.
So here is the thing, I am using a Dash button V2, which should use udp protocol as you said, so there must be no issue with ARP or UDP.
Then, you are running HA on a wired rpi, so you have the same situation as @looknsharp and dasshio is working fine.
Don’t know how to test. I will try to wire my rpi tomorrow and see if it works, but this seems a veryy particular problem @looknsharp… 
Damn, that’s unfortunate. Thank you for trying. I really appreciate it.
Ah… back when you first posted about your add-on, you mentioned it was using ARP so I assumed you had an original button. I’m guessing the button @looknsharp has actually is an original as the MAC address is so different, so maybe the issue is it that it doesn’t in fact work with ARP at all, only UDP?
Perhaps try @hijinx’s add-on as I suggested which may confirm this one way or the other, it only takes a few minutes to set up thanks to the wonders of hass.io 
BTW I just updated the dasher add-on to use the hass.io options.
Should be simple to copy the button config between dashio and dasher add-ons to check.
(although I would assume the config is slightly different and needs edits).
BTW all my testing of dasher has been via hass.io running on ubuntu on x86 architecture.
Ok. So, this morning I wired up my Raspberry 3 with dasshio and everything worked just fine.
My setup:
My Dasshio config:
{
"buttons": [
{
"name": "Gillette",
"address": "AC:63:BE:96:C4:1D",
"url": "https://my_ha_url.duckdns.org/api/services/light/toggle",
"headers": "{\"x-ha-access\": \"my_ha_password\"}",
"body": "{\"entity_id\": \"light.livingroom_lamp\"}"
}
]
}
Here you have a video with the test (note the bug: toggle is done twice per press -next dasshio version comming soon I guess! 
-) :
Sure I’ve seen posts before where people were suffering from double ‘presses’. Mine are ons or offs so not noticed.
Hey everyone! Thank you for helping. I gotta be honest, I’m feeling pretty intimidated by this. I got the button in April, looks like a JK29LP (so arp, I think).
I don’t really know where to go from here. I installed dasher, but am not sure how to use it ( is it in place of dassio?)
These scripts are all new to me. I put this in dasher, but am not confident about it. I added the “method” and changed the “x-ha-access” to “authorization” based on what was available here: hassio-addons/dasher/README.md at master · james-fry/hassio-addons · GitHub
"name": "Lamp", "address": "40:b4:cd:82:06:8f", "url": "http://MYURL.duckdns.org:8123/api/services/switch/off", "method": "POST", "headers": "{\"authorization\": \"MYPASSWORD\"}", "body": "{\"entity_id\": \"switch.lamp\"}"
As much as I would love it, at this point I don’t really expect to get it working.
Is it possible at all that this isn’t working because I used the button for an order before resetting it and trying this? Should I order a new button?
I don’t know how to run a script like that.
You’ve used the same format as dasshio, with all the extra \s etc it needs, rather than the one for dasher ie…
“{“authorization”: “MYPASSWORD”}”,
should be…
{“authorization”: “your_password”},
Just follow the example in the link you posted.
Regarding your version of button, that’ll be a UDP version that was introduced last year which does make your MAC address look suspect, do you have a firetv stick or anything else Amazon that may be confusing things. You can get it directly from the button if you hold the button down for more than six seconds to kick in the set up mode, look for the buttons wifi network on your phone and join it, then go to 192.168.0.1 to view it’s built in web page. You’ll need to set it up again in the Amazon app again to enter your network details again afterwards.
I used all my buttons once to trigger the £4.99 discount, disconnected it from my main account and then attached it to one that only has my buttons on it, running set up as far where it wants you to select a product.
It’s probably no consolation but dasshio should be ridiculously simple, only issue I had was before @danimtb shared a dummy config to follow 
and I was just guessing based on Dasher.
No dice. I did this:
"name": "Lamp",
"address": "40:b4:cd:82:06:8f",
"url": "http://MYIPADDRESS:8123/api/services/switch/off",
"method": "POST",
"headers": "{'authorization': 'MYPASSWORD'}",
"body": "{'entity_id': 'switch.lamp'}"
Hass.io added the quotes to the outside of the brackets and changed the inner quotes to single quotes after saving and restarting the service.
I only have an echo dot and 2 kindles, but the kindles are in airplane mode and not in my router’s client list. I’m positive that is the MAC address for my button. I printed out my client list and labeled all of my devices. I’ve ordered a new button for the sake of troubleshooting. What makes my MAC address suspect?
This is whats killing me. It seems I’ve done everything right. 
The first six characters are completely different to any new version button I’ve seen quoted, that’s why I originally thought it was an old one.
If you do the little routine I suggested you’ll know for sure once and for all, would certainly try that before ordering another even if it doesn’t cost anything in reality.
I think I misunderstood this when I read it this morning. So this would create a connection between the phone and the button as opposed to connecting the button to my network. Is that correct? I can do that when I get home from work.
Yep, it does it automatically when you use the app but keep away from the app and connect manually in your wifi menu, stick that IP into your browser and you’ll get a page like this…
Probably find it’s what you’re already using but at least we know we can discount that as a problem 
Cool, I will do that when I get home. Thank you for your help.
Yup, just did it and it showed the MAC I am using.
Doh, well at least we know that’s not the problem!
I made a fresh build of hass.io today on a new SD card. Decided to give it another go. I got one successful response, then it stopped working. I restarted Home Assistant and got a few more, but wildly inconsistent responses.
starting version 3.2.2
2017-08-27 13:27:46,664 | INFO | Reading config file: /data/options.json
2017-08-27 13:27:46,668 | INFO | Starting sniffing...
2017-08-27 13:31:38,515 | INFO | Button button pressed!
2017-08-27 13:31:38,516 | INFO | Request: http://MYIP:8123/api/services/switch/toggle
2017-08-27 13:31:39,497 | INFO | Status Code: 200
2017-08-27 13:31:39,502 | INFO | Successful request
2017-08-27 13:31:39,569 | INFO | Button button pressed!
2017-08-27 13:31:39,572 | INFO | Request: http://MYIP:8123/api/services/switch/toggle
2017-08-27 13:31:40,005 | INFO | Status Code: 200
2017-08-27 13:31:40,007 | INFO | Successful request
2017-08-27 13:31:40,176 | INFO | Button button pressed!
2017-08-27 13:31:40,179 | INFO | Request: http://MYIP:8123/api/services/switch/toggle
2017-08-27 13:31:40,704 | INFO | Status Code: 200
2017-08-27 13:31:40,707 | INFO | Successful request
2017-08-27 13:34:38,276 | INFO | Button button pressed!
2017-08-27 13:34:38,277 | INFO | Request: http://MYIP:8123/api/services/switch/toggle
2017-08-27 13:34:38,724 | INFO | Status Code: 200
2017-08-27 13:34:38,725 | INFO | Successful request
Seems to have stopped working again. I also got occasional responses from Dasher. I’m beginning to wonder if it is my hardware now. Could it be that the Pi 2 is too slow to run this? I’m not sure how they work.
Finally managed to get my Dash Button to work 
Held the button on my Dash Button to get it to Device info as suggested through WiFi.
Was using the wrong address (Too many Kindles and FireSticks)
Entered the correct MAC Address, but still no joy.
Until … I entered the MAC address WITHOUT using Capitals.
00:x0:00:xx:xx:x0
And
BINGO !!! Cue Happy Dance !!!
Now have 5 Buttons working
I don’t know why, but Capitals in my case don’t work.
Thanks guys ! I would have been plugging the wrong MAC Address in for days without your help.
Glad you’ve had partial success, on the right track now
Wouldn’t have thought it was too slow but you could possibly check by turning off every addon bar dasshio and see if that helps.
Whelp, I think I’ve gotten as far as my skill set currently allows. I got all set up on an rpi 3 and a new button. I got one response from the old button and some inconsistent responses from the new button.