IP forwarding between NICs for Tailscale

[Europa2010AD]

I'm having a bit of trouble trying to enable IP forwarding between my NICs in order to do subnet routing via Tailscale.

Here is a brief overview of my local network:

• TrueNAS Scale host
  • NIC 1 has DHCP enabled, and has local IP 192.168.88.x, which is also the subnet of my router / gateway
  • NIC 2 is connected to my Mac Mini over 10Gbe ethernet, with local static IP 192.168.90.x. This is also where TrueNAS' webUI is pointed to (so I usually only log into my TrueNAS webUI from my Mac Mini).
  • A bridge has been set up properly with NIC 1 to allow Ubuntu VM running within TrueNAS to connect to host shares
• Ubuntu VM (running within the same TrueNAS system)
  • Ubuntu's VirtIO NIC has DHCP enabled and is on the same subnet as the host's NIC 1 on 192.168.88.x
  • Tailscale running inside the VM, with subnet routing enabled for both 192.168.88.0/24 and 192.168.90.0/24

To enable IP forwarding, I followed some instructions found in this forum and added net.ipv4.ip_forward to Sysctl with a value of 1, via the webUI. Then rebooted TrueNAS.

To test, I logged into the Ubuntu VM, and pinged 192.168.88.x, which was successful -- I believe this verifies that the network bridge is working properly.

Then from the Ubuntu VM again, I pinged 192.168.90.x, which failed -- does this indicate the IP forwarding isn't properly, and therefore traffic cannot reach NIC 2 of the host?

I checked my Tailscale admin page, and under "End Points", it only shows 192.168.88.x -- despite having both subnets routes advertised (and it shows both are enabled in the settings).

What am I doing wrong?