Want to Modify/MiTM for RTP traffic between two interface

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http://serverfault.com – I am running a linux machine between two IP Phones with Two NIC card's. All of the traffic passing through between eth0 and eth1 should move through TCP/IP stack. Currently,IP-forwarding is enabled in linux and traffic routes directly at Kernel level. If I am doing port forwarding using IPtables, I can get the traffic in TCP/IP stack but I dont want to perform port forwarding. Every time traffic is having different port, its difficult to map the port again with its original port. Is there any other mechanism, I can get every packet in tcp/IP stack without port forwarding either using some (HowTos)