this post was submitted on 16 Aug 2023
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Im rusty. So don't trust this.
It looks like you have a routing issue with your default route. The fact that a ping gets the IP to start working, tells me that you need a broadcast packet on the local network, that broadcast excites the other computer to send a message out, that updates the IP to Mac table, which then makes the machine routable because now there is a direct ethernet pathway.
So I think the magic here is the initial broadcast ping is doing.
Ideally this isn't necessary, ethernet should be sending out a broadcast packet for the Mac to IP table anyway for your TCP traffic. I don't know why that's not happening. I would do an TCP dump in both scenarios, and see if the broadcast is going out.
My intuition is that there's something fucky going on with your default route, that ping is not being affected by. I bet you don't send out a broadcast address discovery packet in the TCP scenario but you do in the ping scenario
But any IP packet should trigger an arp “who has?”.
Yes. It should. Hence the fucky mystery. Good to check with a network dump to rule it out.
When this happened to me, the broadcast would trigger the ARP update; the camera would respond ever so slightly slower and since it was the last device to claim it was at the IP the ARP table would be updated with it. It would work until the conflicting device would send a packet which would update the ARP table again, back to the original device. The services I expected to respond would no longer receive the packets, they’d go to the wrong machine and it of course wouldn’t respond to requests for services it was not running.
That’s how you end up in this situation of “works for a bit then stops responding”
Either this or a firewall issue.