Default Gateway does not work

[Armin@netPI]

I setup my notebook (with internet connection) to work as a gateway using a second ethernet card.

I setup the 2nd card with exactly your modem settings and IP 169.254.100.97 and my netPI with 169.254.100.123 and subnet 255.255.0.0 and DNS to 169.254.100.97. I am having now an equivalent network setup like yours.

1. I can confirm that I am able to ping my notebook with 169.254.100.97 but I am not able to ping 8.8.8.8.
   
2. The I tried your settings you did in your second test ... 192.168.178.1 etc. With this test I can confirm that I am able to ping 8.8.8.8
   
3. Since I am using a notebook I am able to start Wireshark to make recordings. In non-working setup I don't see any ping command for 8.8.8.8 leaving netPI. I see ping commands leaving for 169.254.100.97
   

Then I setup my original raspberry Pi with Linux V4.19.57 very latest version and the following /etc/network/interfaces file

Code:

auto eth0
allow-hotplug eth0
iface eth0 inet static
address 169.254.100.123
netmask 255.255.0.0
gateway 169.254.100.97
dns-nameservers 169.254.100.97

and guess what ... with my original Raspberry Pi I am having exactly the same issue. I am not able to ping 8.8.8.8 and there are no Eth frames coming out from it if I start recording it with Wireshark.

So this an issue with Linux in general or a setting that I don't know right now. So it is not a specific netPI problem but a general problem. We have a network expert in the office, maybe he knows more.