IP 层收发报文简要剖析6--ip_forward 报文转发

//在函数ip_route_input_slow->ip_mkroute_input注册，
/*
 * IP数据包的转发是由ip_forward()处理，该函数在ip_rcv_finish()
 * 通过输入路由缓存被调用。
 */
int ip_forward(struct sk_buff *skb)
{
	u32 mtu;
	struct iphdr *iph;	/* Our header */
	struct rtable *rt;	/* Route we use */
	struct ip_options *opt	= &(IPCB(skb)->opt);
	struct net *net;

	/* that should never happen */
	if (skb->pkt_type != PACKET_HOST)
		goto drop;

	if (unlikely(skb->sk))
		goto drop;

	if (skb_warn_if_lro(skb))
		goto drop;

	if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))/*查找ipsec 策略路由*/
		goto drop;

	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))//存在路由警告选项
		return NET_RX_SUCCESS;

	skb_forward_csum(skb);
	net = dev_net(skb->dev);

	/*
	 *	According to the RFC, we must first decrease the TTL field. If
	 *	that reaches zero, we must reply an ICMP control message telling
	 *	that the packet‘s lifetime expired.
	 */
	if (ip_hdr(skb)->ttl <= 1)//ttl 减少了
		goto too_many_hops;

	if (!xfrm4_route_forward(skb))//ipsec 策略路由转发处理
		goto drop;

	rt = skb_rtable(skb);

	if (opt->is_strictroute && rt->rt_uses_gateway)//如果数据包启用了严格路由处理，且下一跳不是网关
		goto sr_failed;//发送ICMP_SR_FAILED icmp

	IPCB(skb)->flags |= IPSKB_FORWARDED;
	mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
	if (ip_exceeds_mtu(skb, mtu)) {
		IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
			  htonl(mtu));
		goto drop;
	}

	/* We are about to mangle packet. Copy it! 确保空间足够*/
	if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
		goto drop;
	iph = ip_hdr(skb);

	/* Decrease ttl after skb cow done */
	ip_decrease_ttl(iph);

	/*
	 *	We now generate an ICMP HOST REDIRECT giving the route
	 *	we calculated.
	 */
	if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
	    !skb_sec_path(skb))
		ip_rt_send_redirect(skb);//数据报文输出路由存在重定向标志，且该数据报中不存在源路由选项，就发送重定向icmp报文

	skb->priority = rt_tos2priority(iph->tos);

	return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
		       net, NULL, skb, skb->dev, rt->dst.dev,
		       ip_forward_finish);

sr_failed:
	/*
	 *	Strict routing permits no gatewaying
	 */
	 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
	 goto drop;

too_many_hops:
	/* Tell the sender its packet died... */
	__IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
	icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
drop:
	kfree_skb(skb);
	return NET_RX_DROP;
}
static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
{
	struct ip_options *opt	= &(IPCB(skb)->opt);

	__IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
	__IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);

	if (unlikely(opt->optlen))
		ip_forward_options(skb);//IP数据报文选项 记录路由选项 时间戳等

	return dst_output(net, sk, skb);//指向ip_output ip_mc_output
}