上一篇分析报文接收，IP层提交本地处理的流程，本篇分析报文转发场景的处理过程，在ip_rcv_finish函数中，会根据IP地址决定是提交给本机处理，还是报文转发，报文转发的入口函数为ip_forward，本篇将从ip_forward函数入手分析转发过程。
1、ip_forward函数
int ip_forward(struct sk_buff *skb)2、ip_forward_finish函数
{
u32 mtu;
struct iphdr *iph; /* Our header */
struct rtable *rt; /* Route we use */
struct ip_options *opt = &(IPCB(skb)->opt);
/* that should never happen */
if (skb->pkt_type != PACKET_HOST) //不允许处理非本host的报文，即报文目的mac是本机
goto drop;
if (unlikely(skb->sk))
goto drop;
if (skb_warn_if_lro(skb)) //报文为非线性，gso_size不为零，但是gso_type为零，丢弃此类报文
goto drop;
if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) //ipset安全规则检测
goto drop;
if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
return NET_RX_SUCCESS;
skb_forward_csum(skb);
/*
* 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减到0 ，丢弃报文
goto too_many_hops;
if (!xfrm4_route_forward(skb)) //ipset路由安全规则检测，得到路由信息
goto drop;
rt = skb_rtable(skb); //得到路由表项
if (opt->is_strictroute && rt->rt_uses_gateway)
goto sr_failed;
IPCB(skb)->flags |= IPSKB_FORWARDED; //flag中田间forward标记，
mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
if (!ip_may_fragment(skb) && ip_exceeds_mtu(skb, mtu)) {
IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, //报文长度超过mtu且不允许ip分片，发送icmp消息给发送者
htonl(mtu));
goto drop;
}
/* We are about to mangle packet. Copy it! */
if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len)) //扩展报文，以填充mac头
goto drop;
iph = ip_hdr(skb);
/* Decrease ttl after skb cow done */
ip_decrease_ttl(iph); //ip头的ttl减一
/*
* 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); //通知发送端，路由重定向
skb->priority = rt_tos2priority(iph->tos); //根据tos值计算priority值
return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, NULL, skb,
skb->dev, rt->dst.dev, ip_forward_finish); //调用netfilter，实现iptables功能，通过后调用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_BH(dev_net(skb_dst(skb)->dev), 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 sock *sk, struct sk_buff *skb)ip_forward流程中主要功能包括：根据报文信息得到路由、ipset安全检测、转发的基本逻辑（ttl减少、mtu判断等），ipset安全检测，后续统一分偶析，本篇不再涉及，ip_output之前已经有分析，不再赘述。
{
struct ip_options *opt = &(IPCB(skb)->opt);
IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS); //报文统计
IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
if (unlikely(opt->optlen))
ip_forward_options(skb);
skb_sender_cpu_clear(skb);
return dst_output_sk(sk, skb); //此时会进入xfrm4_outpot处理，最终会调用ip_output
}