[tproxy] Tproxy

Good morning, Actually I am trying to understand the modifications that Tproxy does in the processed packets sent by a proxy with two interfaces. This proxy redirect the traffic from a local LAN to Internet. When I analyze the outgoing packets with Wireshark, I can see that the proxy has added the source's IP (client). I think that I should not see it. Where can I find some information about these modifications? I have been looking on Google for days but I didn't find anything. Thank you in advance. Regards, Alvaro