Apologies if duplicated posting
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Jurgen, I have replicated your setup on my network “faking” a wifi PWS so hopefully near enough to the real thing. By doing this I found a couple of incorrect steps in my comments. I had forgotten to ask you to edit /etc/sysctl.conf in order to uncomment the line net.ipv4.ip_forward=1. I also didn’t realise the iptable command for a bridged connection is different:
sudo iptables -t nat -A PREROUTING -m physdev --physdev-in wlan0 -s <PWS IP> -p tcp --dport 80 -j DNAT --to-destination <Weather Service IP:PORT>
I have created a full instruction document for this scenario and checked it into the OpenSprinkler github [HERE].
Could I ask a favor ? Would you be willing to start afresh with the Raspberry Pi Zero and see if you can follow the instructions in the link provided. I am keen to get something written down for this use-case so that others can more easily set this up. Let me know if you find any issues.