I am taking packet data and using libpcap pcap inject method to pass the packet onto another network. This works fine but the mac address in the packet changes to that of the local device been used to relay the packet. Tracing through the libpcap code the inject method calls the standard send() socket method, but from there I loose track of the calls which are updating the mac address. Where in the Linux code is the send() method? and does anybody know what I would need to do to prevent the mac address been over written in the packet.

Thanks in advance

You can try tcpreplay tool, following link, to replay captured packet. If MAC address issue still come out, you can change interface MAC to match with correct MAC address, in captured packet.
http://tcpreplay.appneta.com/