Bridging between two Ethernet ports

I am aware there is a built in utility for bridging connections, but I was trying to implement the same using a Python Script.
This is the how the set-up looks like, I am unable to send a DHCP ACK from ETH0 to the device because somehow even though I try it is not visible at ETH1, but the rest of the packets are. Anyone knows what can be the issue with this?