ip_rcv()处理sk_buff数据分析

 *   Main IP Receive routine.
 */
int (struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev)
{
   const struct iphdr *iph;
 u32 len;
 /* When the interface is in promisc. mode, drop all the crap
  * that it receives, do not try to analyse it.
  */
  if (skb->pkt_type == PACKET_OTHERHOST) //该值在方法eth_type_trans()中确定，判断依据目标MAC地址
      goto drop;
   IP_UPD_PO_STATS_BH(dev_net(dev), IPSTATS_MIB_IN, skb->len);
   if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL) {
       IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS);
      goto out;
  }
    if (!pskb_may_pull(skb, sizeof(struct iphdr)))
        goto inhdr_error;
   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
     */
   if (iph->ihl < 5 || iph->version != 4)
        goto inhdr_error;
   if (!pskb_may_pull(skb, iph->ihl*4))
        goto inhdr_error;
   iph = ip_hdr(skb);
   if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
        goto csum_error;
    len = ntohs(iph->tot_len);
   if (skb->len < len) {
        IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INTRUNCATEDPKTS);
     goto drop;
 } else if (len < (iph->ihl*4))
      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).
    */
  if (pskb_trim_rcsum(skb, len)) {
       IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS);
      goto drop;
 }
    /* Remove any debris废墟 in the socket control block */
  memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
   /* Must drop socket now because of tproxy. */
  skb_orphan(skb);
 return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING, skb, dev, NULL,
            ip_rcv_finish);      //钩子函数ipv4_conntrack_in
csum_error:
  IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_CSUMERRORS);
inhdr_error:
  IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INHDRERRORS);
drop:
    kfree_skb(skb);
out:
 return NET_RX_DROP;
}
/*************************************
 *
 * (sk_buff)->pkt_type;
 *
 *************************************/
/* Packet types */
#define PACKET_BROADCAST   1       /* To all     */
#define PACKET_MULTICAST   2       /* To group       */
#define PACKET_OTHERHOST   3       /* To someone else    */
#define PACKET_OUTGOING        4       /* Outgoing of any type */
/* These ones are invisible by user level */
#define PACKET_LOOPBACK     5       /* MC/BRD frame looped back */
#define PACKET_FASTROUTE 6       /* Fastrouted frame   */