I have configured routing on BackTrack 5r3 (it's a kind of Ubuntu). But it doesn't work.

To set up router I made following steps:
1. Clean install BT5r3 from its Live CD.
2. Configured eth0 statically as 172.16.3.3/16 and eth1 by DHCP (leased 10.0.2.15/24, gw 10.0.2.2). Everything is up and running. Checked by ifconfig, route, and ping.
3. Set net.ipv4.ip_forward=1. Checked by sysctl, and cat /proc/... .
4. Checked iptables - everything is in ACCEPT policy, and no other rules.

After all traffic is not forwarded - host in 172.16.0.0 configured to have 172.16.3.3 as default gateway, and ping 10.0.2.2 comes to the router but not forwarded. Traffic checked with wireshark on both interfaces of the router.

Maybe I missed something well-known?

I think I found roots of this issue. I didn't mentioned that BT5 runs on an VirtualBox. Never thought it matters... VM uses bridged interface. After continuous sniffing I recognize that destination MAC-addresses of packets to be routed differs from router's MAC. It seems like VirtualBox changes MAC of VM NIC in outgoing packets (including ARP) to MAC of host NIC.

I believe I set up routing just fine.