火星包: rp_filter and martian packet

kernel_source/net/ipv4/route.c 
static void ip_handle_martian_source(struct net_device *dev,
				     struct in_device *in_dev,
				     struct sk_buff *skb,
				     __be32 daddr,
				     __be32 saddr)
{
	RT_CACHE_STAT_INC(in_martian_src);
#ifdef CONFIG_IP_ROUTE_VERBOSE
	if (IN_DEV_LOG_MARTIANS(in_dev) && net_ratelimit()) {
		/*
		 *	RFC1812 recommendation, if source is martian,
		 *	the only hint is MAC header.
		 */
		printk(KERN_WARNING "martian source %pI4 from %pI4, on dev %s\n",
			&daddr, &saddr, dev->name);
		if (dev->hard_header_len && skb_mac_header_was_set(skb)) {
			int i;
			const unsigned char *p = skb_mac_header(skb);
			printk(KERN_WARNING "ll header: ");
			for (i = 0; i < dev->hard_header_len; i++, p++) {
				printk("%02x", *p);
				if (i < (dev->hard_header_len - 1))
					printk(":");
			}
			printk("\n");
		}
	}
#endif
}

kernel_source/net/ipv4/fib_frontend.c
int fib_validate_source(){
    // 代码有选择性省略
    // 反转src和dst
	struct flowi fl = { .nl_u = { .ip4_u =
				      { .daddr = src,
					.saddr = dst,
					.tos = tos } },
			    .mark = mark,
			    .iif = oif };


	in_dev = __in_dev_get_rcu(dev);
    // 拿到收包的设备，顺便取出该设备上rp_filter的flag
	if (in_dev) {
		no_addr = in_dev->ifa_list == NULL;
		rpf = IN_DEV_RPFILTER(in_dev);
		if (mark && !IN_DEV_SRC_VMARK(in_dev))
			fl.mark = 0;
	}
	rcu_read_unlock();

	if (in_dev == NULL)
		goto e_inval;
	net = dev_net(dev);
    // 以src为dst，查fib
	if (fib_lookup(net, &fl, &res))
		goto last_resort;
	if (res.type != RTN_UNICAST)
		goto e_inval_res;
	*spec_dst = FIB_RES_PREFSRC(res);
	fib_combine_itag(itag, &res);

#ifdef CONFIG_IP_ROUTE_MULTIPATH
    // 以src作为dst，查到的发送dev和当前收包的dev相同，一切ok
	if (FIB_RES_DEV(res) == dev || res.fi->fib_nhs > 1)
#else
	if (FIB_RES_DEV(res) == dev)
#endif
	{
		ret = FIB_RES_NH(res).nh_scope >= RT_SCOPE_HOST;
		fib_res_put(&res);
		return ret;
	}
	fib_res_put(&res);
	if (no_addr)
		goto last_resort;
    // 如果dev不相同，并且rp_filter置为on，则检查失败
	if (rpf == 1)
		goto e_inval;

}