I'm not a Mac user, but I would think from the perspective of the Linux box, it shouldn’t require any special handling. The Mac is doing all the routing. The only thing the Linux box has to do is behave normally; make its DHCP request over the ethernet cable between itself and the Mac. IOW, the Mac is doing all the heavy lifting and behaving like a WAN.
If it’s still not working, and your Mac machine is using a wireless router, then you don’t even need routing. Again, leaving the Linux machine as-is (w/ DHCP enabled), you just need to bridge the Mac and Linux network connections on the Mac.