Here are the two write ups that I have been working from. Easy Raspberry Pi WiFi Bridge https://pimylifeup.com/raspberry-pi-wifi-bridge/ Uses a RPi 2, not sure what version of Raspbian, dnsmasq
Using a Pi 3 as a Ethernet to WiFi router https://medium.com/linagora-engineering/using-a-pi-3-as-a-ethernet-to-wifi-router-2418f0044819 Uses a RPi 3, /w Jessie, isc-dhcp
What I am looking for is eth0 with a fixed IPaddress and serves as a DHCP host for one client (even if IP range is 192.168.10.10 to 192.168.10.50). wlan0 can be either fixed or set by local network (can be 192.168.0.4. gateway 192.168.0.1) I can connect to the RPi B3 via local network and wlan0. The device (a meteorology smart hub) that I want to connect on the eth0 side is internally set as a dhcp client. Since the device can only tell me go/no go. I am using a laptop to test the eth0 side for client-side access.
Setting up as “Easy Raspberry Pi WiFi Bridge” with dnsmasq, the laptop is assigned a 169…. IP address. This is apparently an error code from the network chip on the RPi. I tried starting the RPi first and then connecting, starting the laptop first and then the RPi. No difference. No connection. I can connect to the RPi from the wlan0 side. With the eth0 connected I see it’s fixed IP address with ifconfig. Disconnect eth0, and ifconfig does not show the IP Address.
Setting up as “Using a Pi 3 as a Ethernet to WiFi router” with isc-dhcp, the daemon will not start without a connection to eth0. Same as above, with the eth0 connected I see it’s fixed IP address with ifconfig. I was able to get the RPi to pass through to the local network to access local web servers, but not to the internet. The RPi shows up on my local network router. Both the test laptop and device will pick up an IP Address from the local router and work fine. I can take a sd card with a fixed address on either eth0 or wlan0 and connect to the local router with two different RPis (mac addresses). The router does not care.
I am continuing to research and found this: [SOLVED] Bridge wlan0 and eth0 - help needed https://www.raspberrypi.org/forums/viewtopic.php?t=193770 This thread raises a question about Stretch. However, not sure it applies, since I don’t need to access the device from the local side, just need to let it get out.