Lwip--ARP协议（ARP数据包处理）

一、ARP数据包的处理

ARP数据包的处理函数为etharp_arp_input，他完成两个任务：第一、如果收到的是应答包，说明是本机之前发出的ARP请求包，更新自身的ARP缓存表；第二、如果收到的是请求包，如果与自身IP匹配，记录原主机的IP与MAC地址，更新自身的ARP表外，还要发送一个应答包，如果不匹配，则记录原主机的IP与MAC地址，更新自身的ARP表。

二、对于ARP表的更新：（下一文章具体讲解）

其中更新ARP表的主要函数为：etharp_update_arp_entry(netif,&sipaddr, &(hdr->shwaddr),for_us ? ETHARP_FLAG_TRY_HARD :ETHARP_FLAG_FIND_ONLY); 根据ARP包（IP地址）是否是给本机的设置标志，如果设置标志为ETHARP_FLAG_TRY_HARD，则无论如何都要更新ARP表，如果表项已满，则删除年龄较老的表项；如果设置为ETHARP_FLAG_FIND_ONLY，先遍历整个表项，如果有空则插入，如果没有则舍弃。此函数后面讲

以下为etharp_arp_input函数的源码

/**
 * Responds to ARP requests to us. Upon ARP replies to us, add entry to cache  
 * send out queued IP packets. Updates cache with snooped address pairs.
 *
 * Should be called for incoming ARP packets. The pbuf in the argument
 * is freed by this function.
 *
 * @param netif The lwIP network interface on which the ARP packet pbuf arrived.
 * @param ethaddr Ethernet address of netif.
 * @param p The ARP packet that arrived on netif. Is freed by this function.
 *
 * @return NULL
 *
 * @see pbuf_free()
 */
static void
etharp_arp_input(struct netif *netif, struct eth_addr *ethaddr, struct pbuf *p)
{
  struct etharp_hdr *hdr;
  struct eth_hdr *ethhdr;
  /* these are aligned properly, whereas the ARP header fields might not be */
  ip_addr_t sipaddr, dipaddr;
  u8_t for_us;
  /* drop short ARP packets: we have to check for p->len instead of p->tot_len here
     since a struct etharp_hdr is pointed to p->payload, so it musn't be chained! */
  if (p->len < SIZEOF_ETHARP_PACKET) {
    pbuf_free(p);
    return;
  }
  ethhdr = (struct eth_hdr *)p->payload;
  hdr = (struct etharp_hdr *)((u8_t*)ethhdr + SIZEOF_ETH_HDR);
  /* RFC 826 "Packet Reception": */
  if ((hdr->hwtype != PP_HTONS(HWTYPE_ETHERNET)) ||
      (hdr->hwlen != ETHARP_HWADDR_LEN) ||
      (hdr->protolen != sizeof(ip_addr_t)) ||
      (hdr->proto != PP_HTONS(ETHTYPE_IP)))  {
    pbuf_free(p);
    return;
  }
  /* Copy struct ip_addr2 to aligned ip_addr, to support compilers without
   * structure packing (not using structure copy which breaks strict-aliasing rules). */
  IPADDR2_COPY(&sipaddr, &hdr->sipaddr);
  IPADDR2_COPY(&dipaddr, &hdr->dipaddr);

  /* this interface is not configured? */
  if (ip_addr_isany(&netif->ip_addr)) {
    for_us = 0;
  } else {
    /* ARP packet directed to us? */
    for_us = (u8_t)ip_addr_cmp(&dipaddr, &(netif->ip_addr));
  }

  /* ARP message directed to us?
      -> add IP address in ARP cache; assume requester wants to talk to us,
         can result in directly sending the queued packets for this host.
     ARP message not directed to us?
      ->  update the source IP address in the cache, if present */
  etharp_update_arp_entry(netif, &sipaddr, &(hdr->shwaddr),
                   for_us ? ETHARP_FLAG_TRY_HARD : ETHARP_FLAG_FIND_ONLY);

  /* now act on the message itself */
  switch (hdr->opcode) {
  /* ARP request? */
  case PP_HTONS(ARP_REQUEST):
    /* ARP request. If it asked for our address, we send out a
     * reply. In any case, we time-stamp any existing ARP entry,
     * and possiby send out an IP packet that was queued on it. */

    /* ARP request for our address? */
    if (for_us) {

      /* Re-use pbuf to send ARP reply.
         Since we are re-using an existing pbuf, we can't call etharp_raw since
         that would allocate a new pbuf. */
      hdr->opcode = htons(ARP_REPLY);

      IPADDR2_COPY(&hdr->dipaddr, &hdr->sipaddr);
      IPADDR2_COPY(&hdr->sipaddr, &netif->ip_addr);

      ETHADDR16_COPY(&hdr->dhwaddr, &hdr->shwaddr);
      ETHADDR16_COPY(ðhdr->dest, &hdr->shwaddr);
      ETHADDR16_COPY(&hdr->shwaddr, ethaddr);
      ETHADDR16_COPY(ðhdr->src, ethaddr);

      /* hwtype, hwaddr_len, proto, protolen and the type in the ethernet header
         are already correct, we tested that before */

      /* return ARP reply */
      netif->linkoutput(netif, p);
    /* we are not configured? */
    } else if (ip_addr_isany(&netif->ip_addr)) {
      /* { for_us == 0 and netif->ip_addr.addr == 0 } */
      LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_arp_input: we are unconfigured, ARP request ignored.\n"));
    /* request was not directed to us */
    } else {
      /* { for_us == 0 and netif->ip_addr.addr != 0 } */
      LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_arp_input: ARP request was not for us.\n"));
    }
    break;
  case PP_HTONS(ARP_REPLY):
    break;
  default:
    break;
  }
  /* free ARP packet */
  pbuf_free(p);
}