Hi, I was trying to figure out how the rp_filter does the source address validation. I traced out the location in source code(2.4.18) to fib_validate_source() in net/ipv4/fib_frontend.c But here the line where I think the rp filter comes in rpf = IN_DEV_RPFILTER(in_dev); But this looks for the enabling of rp_filter for the all the interfaces and for the individual interfaces. Then comes the line if (rpf) goto e_inval; That means if rp filtering is enabled return EINVAL!!!!!! I seem to be missing a lot of things Could some one help me out on this Regards KK - : send the line "unsubscribe linux-net" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html