rp_filter is off by _kernel_ default.
(Some distibutions turn it on in startup scripts).

The folowing text from net/ipv4/Kconfig is confusing.(It seems one distribution 
maintainer thinks rp_filter is on by kernel default)

"config IP_ADVANCED_ROUTER
[..........]
If you turn on IP forwarding, you will also get the rp_filter, which
automatically rejects incoming packets if the routing table entry
for their source address doesn't match the network interface they're
arriving on. This has security advantages because it prevents the
so-called IP spoofing, however it can pose problems if you use
asymmetric routing (packets from you to a host take a different path
than packets from that host to you) or if you operate a non-routing
host which has several IP addresses on different interfaces. To turn
rp_filter off use:

echo 0 > /proc/sys/net/ipv4/conf/<device>/rp_filter
or
echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter"

In 2.4 the error is in Documentation/Configure/help. Please fix this too.

Thanks.