<Linux Network 2.6.38> "where_to_go"
?
先来回顾一个潜在的结构?在skb的?pskb_copy，?skb_copy??等操作中起到了关键作用
?
#define?skb_shinfo(SKB)?????????????((struct?skb_shared_info?*)((SKB)->end))?
?
?
再回忆一下头部结构
?
?
好了现在就看看， 上前天说到的 ip_rcv 如何Ip处理包?
?
int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev){/*在sk_buff __netif_receive_skb eth_type_trans 中已经让包符合L3 */struct iphdr *iph;u32 len;/*如果是因为开了promiscuous 模式而让垃圾包到了L3 ，就丢弃不属于自己的 */if (skb->pkt_type == PACKET_OTHERHOST)goto drop;/*依旧SNMP 采集点*/IP_UPD_PO_STATS_BH(dev_net(dev), IPSTATS_MIB_IN, skb->len);/*如果这包在别的子系统也使用 就拷贝一份给自己专门用*/if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL) {IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS);goto out;/*拷贝失败就丢弃*/}/*如果是分包的话 ，用__pskb_pull_tail把skb_shared_info中frag_list找回来 */if (!pskb_may_pull(skb, sizeof(struct iphdr)))goto inhdr_error;/*重新指向L3头部*/iph = ip_hdr(skb);/* *RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum. * *Is the datagram acceptable? * *1.Length at least the size of an ip header *2.Version of 4 *3.Checksums correctly. [Speed optimisation for later, skip loopback checksums] *4.Doesn't have a bogus length *//*头部和版本检查*/if (iph->ihl < 5 || iph->version != 4)goto inhdr_error;/*同样的动作， 不过和上一次比这次是整个IP头部了*/if (!pskb_may_pull(skb, iph->ihl*4))goto inhdr_error;iph = ip_hdr(skb);/*校验*/if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))goto inhdr_error;/*长度完整性校验*/len = ntohs(iph->tot_len);/*skb buffer的真实长度只能比包头报告的长度大<因为可能被L2层填充了>或正好，小的话就有问题咯*/if (skb->len < len) {IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INTRUNCATEDPKTS);goto drop;} else if (len < (iph->ihl*4))/*包头可能比包体还长吗？ :)*/goto inhdr_error;/* Our transport medium may have padded the buffer out. Now we know it * is IP we can trim to the true length of the frame. * Note this now means skb->len holds ntohs(iph->tot_len). *//*被L2填充了吗？ 去掉！ 前面校验不算～*/if (pskb_trim_rcsum(skb, len)) {IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS);goto drop;}/* Remove any debris in the socket control block *//*看起来L3不需要 ip_options*/memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));/* Must drop socket now because of tproxy. *//*既然都被我处理过了，就跟我把，帮你净身*/skb_orphan(skb);/*接受Netfilter 的洗礼吧（LVS基于此），最后再执行 ip_rcv_finish，这是正常之旅*/return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING, skb, dev, NULL, ip_rcv_finish);inhdr_error:/*去MIB树上反应一下*/IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INHDRERRORS);drop:kfree_skb(skb);out:/*哎*/return NET_RX_DROP;}?
?接下来就是看看?ip_rcv_finish
?
?
?
static int ip_rcv_finish(struct sk_buff *skb){const struct iphdr *iph = ip_hdr(skb);struct rtable *rt;/* *如果包中没有dst_entry结构(不知如何转发)，就直接询问路由子系统看看有没人要它(后面就不处理了) */if (skb_dst(skb) == NULL) {int err = ip_route_input_noref(skb, iph->daddr, iph->saddr, iph->tos, skb->dev);if (unlikely(err)) {if (err == -EHOSTUNREACH)IP_INC_STATS_BH(dev_net(skb->dev),IPSTATS_MIB_INADDRERRORS);else if (err == -ENETUNREACH)IP_INC_STATS_BH(dev_net(skb->dev),IPSTATS_MIB_INNOROUTES);else if (err == -EXDEV)NET_INC_STATS_BH(dev_net(skb->dev), LINUX_MIB_IPRPFILTER);goto drop;}}/*更新该CPU的ip_rt_acct 统计，参考/proc/net/rt_acct*/#ifdef CONFIG_NET_CLS_ROUTEif (unlikely(skb_dst(skb)->tclassid)) {struct ip_rt_acct *st = this_cpu_ptr(ip_rt_acct);u32 idx = skb_dst(skb)->tclassid;st[idx&0xFF].o_packets++;st[idx&0xFF].o_bytes += skb->len;st[(idx>>16)&0xFF].i_packets++;st[(idx>>16)&0xFF].i_bytes += skb->len;}#endif/*如果有IP options就去处理*/if (iph->ihl > 5 && ip_rcv_options(skb))goto drop;/*根据目的地类型做一些统计*/rt = skb_rtable(skb);if (rt->rt_type == RTN_MULTICAST) {IP_UPD_PO_STATS_BH(dev_net(rt->dst.dev), IPSTATS_MIB_INMCAST,skb->len);} else if (rt->rt_type == RTN_BROADCAST)IP_UPD_PO_STATS_BH(dev_net(rt->dst.dev), IPSTATS_MIB_INBCAST,skb->len);/"where to go 见下面"/return dst_input(skb);drop:kfree_skb(skb);return NET_RX_DROP;}?
?
这个决定L3包走向的函数指针在那里设置的呢，跟着我看一下吧
接着上一篇讲过的inet_init()->ip_init()->ip_rt_init()
-->
rtnl_register(PF_INET,?RTM_GETROUTE,?inet_rtm_getroute,?NULL);
<route.c?__rtnl_register登记?>
-->
?
?
int ip_route_input_common(struct sk_buff *skb, __be32 daddr, __be32 saddr, u8 tos, struct net_device *dev, bool noref){//...if (our ) {int res = ip_route_input_mc(skb, daddr, saddr, tos, dev, our);//...}//...}//...res = ip_route_input_slow(skb, daddr, saddr, tos, dev);}?inet_rtm_getroute()->ip_route_input()->ip_route_input_common()-
?
A：ip_route_input_slow()
->ip_mkroute_input()->__mkroute_input()
"ip_forward;"
B<our>:?ip_route_input_mc()
B1?"ip_local_deliver"
B2?"init_net.loopback_dev";
这是几个likely的不考虑异常分支，别的就先不介绍了
?
?
?
int ip_forward(struct sk_buff *skb){struct iphdr *iph;/* Our header */struct rtable *rt;/* Route we use */struct ip_options * opt= &(IPCB(skb)->opt);if (skb_warn_if_lro(skb))goto drop;if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))goto drop;/*Router Alert option 的处理，后面会分析 ip_call_ra_chain()函数*/if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))return NET_RX_SUCCESS;/*pkt_type在L2层处理的时候有无设置PACKET_HOST*/if (skb->pkt_type != PACKET_HOST)goto drop;/*什么都没做 直接让pass了*/skb_forward_csum(skb);/*rfc 规定防止路由循环等等用的*/if (ip_hdr(skb)->ttl <= 1)goto too_many_hops;/*VPN IPSec交给xfrm 框架处理转发*/if (!xfrm4_route_forward(skb))goto drop;rt = skb_rtable(skb);/*如果IP包选项指明了要用自己提供的路由来走<Strict Source Routing >，而又不能满*足就失败*/if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)goto sr_failed;/*如果包长度大于了目的地的MTU 却禁止分包 就发送一个ICMP<这块参考 TCP/IP详解卷一 9章>*/if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) && (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(dst_mtu(&rt->dst)));goto drop;}/* We are about to mangle packet. Copy it! 这个之前解释过了 */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 协议要求*/ip_decrease_ttl(iph);/* *如果包允许走别的路由而且他也表示希望走一个更好的，就重新计算路由，当然也会* 引起一个ICMP ，这个函数后面分析 */if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))ip_rt_send_redirect(skb);/*根据 IP包头部的TOS 设置包优先级 给后面的 Traffic Control 用<ps: 路由器设计很重视这个选项>*/skb->priority = rt_tos2priority(iph->tos);/*之前分析过，经过NetFilter后调用ip_forward_finish*/return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev, rt->dst.dev, ip_forward_finish);sr_failed:/* *Strict routing permits no gatewaying 协议规定的参考 TCP/IP-I 1 */ 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;}?好吧?，下面就是
?
?
?
static int ip_forward_finish(struct sk_buff *skb){struct ip_options * opt= &(IPCB(skb)->opt);IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);/* ip_forward里面已经处理了2个IP options <Router Alert 和Strict Source Routing>，* 下面就是把剩下的处理完，由于几乎很少会用到IP-O <看看这里的unlikely也知道概率了吧* >所以就不分析了*/if (unlikely(opt->optlen))ip_forward_options(skb);/*最终归属，根据 消息包类型去调用相应的函数指针 ip_mc_output ..ip_finish_output */return dst_output(skb);}?
?
?
=========================
补充上面说的 ip_call_ra_chian
?
{struct ip_ra_chain *ra;u8 protocol = ip_hdr(skb)->protocol;struct sock *last = NULL;struct net_device *dev = skb->dev;/*这里遍历了整个raw sock 链表 */for (ra = rcu_dereference(ip_ra_chain); ra; ra = rcu_dereference(ra->next)) {struct sock *sk = ra->sk;/* If socket is bound to an interface, only report * the packet if it came from that interface. *//*包头端口号和该raw sock 的端口匹配 设备接口序号也匹配*/if (sk && inet_sk(sk)->inet_num == protocol && (!sk->sk_bound_dev_if || sk->sk_bound_dev_if == dev->ifindex) && net_eq(sock_net(sk), dev_net(dev))) {/*如果分段过 就去重组整个IP包 ip_fragment.c*/if (ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)) {if (ip_defrag(skb, IP_DEFRAG_CALL_RA_CHAIN))return 1;}if (last) {/*关键就是这里，把包复制一遍然后传给上层*放入该sock的sk_receive_queue*/struct sk_buff *skb2 = skb_clone(skb, GFP_ATOMIC);if (skb2)raw_rcv(last, skb2);}/*下一个raw sock */last = sk;}}if (last) {raw_rcv(last, skb);return 1;}