Routing traffic internally from Ethernet-Thunderbolt adapter to Ethernet I have  Mac1 connected via ethernet to a router.
The Mac1 has the IP 10.0.1.5
On this IP I have a Python-server listening to incoming UDP packets.

Now there is another Mac2 connected to Mac1 via a Thunderbolt-Ethernet Adapter. The IP of Mac1 on this adapter is 192.168.20.1
Mac2 sends UPD packets to this address.

How can i route this traffic internally on Mac1 from  192.168.20.1 to 10.0.1.5?

Both machines are on MacOS 10.10

Thanks a lot!
There is no need to route anything. Just let the python-server also listen to incoming UDP packets at eth2 (192.168.20.1) assuming eth2 is the TB-Ethernet interface.