Hey i am having a serious problem with running ettercap on a 2 nic gateway my issue is that ettercap cannot run with a filter.The manual states that ettercap takes over routing and disables kernel ip forwarding but it doesn't route the packets to the default gateway ppp0. Also with unoffensive mode i am not able to modify or replace html codes. Arp poisoning my lan with another box isn't the proper solution since it destabilizes the lan. Do we have any other easy to use packet injection tools that run on linux?

here are the interfaces i am working with:
eth0 Link encap:Ethernet HWaddr 00:04:7A:23:77:5C
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

lan0 Link encap:Ethernet HWaddr 00:41:37:E4:2B:C1
inet addr:192.168.0.1 Bcast:192.168.0.255 Mask:255.255.255.0

ppp0 Link encap:Point-to-Point Protocol
inet addr:x.x.x.x P-t-P:x.x.x.xMask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1492 Metric:1