Thanks for your responses!
The weather station is supposed to output at port 80, but you're right better to verify. So I checked with tcpdump -i eth0 but no traffic from 192.168.31 (weather station) seems to be reaching 192.168.1.27 (weewx install). No luck there.
Below is the output for that...
Hi all! I'm trying to mirror data sent from a Bresser weather station to a WeeWX install (so I can intercept the data sent to rtupdate.wunderground.com). Unfortunately failing to get results :( Can you help?
Running these commands on AsusWrt (RT-AC68U on Merlin 384.11_2):
modprobe xt_TEE...