看LWIP源码关于发送局域网外IP包其ip地址置为网关地址疑问？？

henu四叶草

最近在看llwip基于arp部分实现源码，想咨询下LWIP关于其ip数据包发送局域网外单播包的一点疑惑？就是lwip如果发现是非本地单播包会将发送的目的ip改为网关ip，那么我的疑惑是发给网关之后，那么网关是怎么知道数据是要发给谁的？小弟拜谢了

//ip层包的发送是借助etharp_output()接口实现，单播包会调用etharp_query()给指定的ip地址处发送ip数据包或arp请求，下面是etharp_output()部分代码
err_t
etharp_output(struct netif *netif, struct pbuf *q, ip_addr_t *ipaddr)
{
  struct eth_addr *dest;
  struct eth_addr mcastaddr;
  ip_addr_t *dst_addr = ipaddr;


  LWIP_ASSERT("netif != NULL", netif != NULL);
  LWIP_ASSERT("q != NULL", q != NULL);
  LWIP_ASSERT("ipaddr != NULL", ipaddr != NULL);


  /* make room for Ethernet header - should not fail */
  if (pbuf_header(q, sizeof(struct eth_hdr)) != 0) {
    /* bail out */
    LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
      ("etharp_output: could not allocate room for header.\n"));
    LINK_STATS_INC(link.lenerr);
    return ERR_BUF;
  }


  /* Determine on destination hardware address. Broadcasts and multicasts
   * are special, other IP addresses are looked up in the ARP table. */


  /* broadcast destination IP address? */
  if (ip_addr_isbroadcast(ipaddr, netif)) {
    /* broadcast on Ethernet also */
    dest = (struct eth_addr *)&ethbroadcast;
  /* multicast destination IP address? */
  } else if (ip_addr_ismulticast(ipaddr)) {
    /* Hash IP multicast address to MAC address.*/
    mcastaddr.addr[0] = LL_MULTICAST_ADDR_0;
    mcastaddr.addr[1] = LL_MULTICAST_ADDR_1;
    mcastaddr.addr[2] = LL_MULTICAST_ADDR_2;
    mcastaddr.addr[3] = ip4_addr2(ipaddr) & 0x7f;
    mcastaddr.addr[4] = ip4_addr3(ipaddr);
    mcastaddr.addr[5] = ip4_addr4(ipaddr);
    /* destination Ethernet address is multicast */
    dest = &mcastaddr;
  /* unicast destination IP address? */
  } else {
    s8_t i;
    /* outside local network? if so, this can neither be a global broadcast nor
       a subnet broadcast. */
    if (!ip_addr_netcmp(ipaddr, &(netif->ip_addr), &(netif->netmask)) &&
        !ip_addr_islinklocal(ipaddr)) {
#if LWIP_AUTOIP
      struct ip_hdr *iphdr = (struct ip_hdr*)((u8_t*)q->payload +
        sizeof(struct eth_hdr));
      /* According to RFC 3297, chapter 2.6.2 (Forwarding Rules), a packet with
         a link-local source address must always be "directly to its destination
         on the same physical link. The host MUST NOT send the packet to any
         router for forwarding". */
      if (!ip_addr_islinklocal(&iphdr->src))
#endif /* LWIP_AUTOIP */
      {
        /* interface has default gateway? */
        if (!ip_addr_isany(&netif->gw)) {
          /* send to hardware address of default gateway IP address */
          dst_addr = &(netif->gw);                                      //这部分将i局域网外目的ip地址设置为网关ip，
        /* no default gateway available */
        } else {
          /* no route to destination error (default gateway missing) */
          return ERR_RTE;
        }
      }
    }
#if LWIP_NETIF_HWADDRHINT
    if (netif->addr_hint != NULL) {
      /* per-pcb cached entry was given */
      u8_t etharp_cached_entry = *(netif->addr_hint);
      if (etharp_cached_entry < ARP_TABLE_SIZE) {
#endif /* LWIP_NETIF_HWADDRHINT */
        if ((arp_table[etharp_cached_entry].state >= ETHARP_STATE_STABLE) &&
            (ip_addr_cmp(dst_addr, &arp_table[etharp_cached_entry].ipaddr))) {
          /* the per-pcb-cached entry is stable and the right one! */
          ETHARP_STATS_INC(etharp.cachehit);
          return etharp_output_to_arp_index(netif, q, etharp_cached_entry);
        }
#if LWIP_NETIF_HWADDRHINT
      }
    }
#endif /* LWIP_NETIF_HWADDRHINT */


    /* find stable entry: do this here since this is a critical path for
       throughput and etharp_find_entry() is kind of slow */
    for (i = 0; i < ARP_TABLE_SIZE; i++) {
      if ((arp_table.state >= ETHARP_STATE_STABLE) &&
          (ip_addr_cmp(dst_addr, &arp_table.ipaddr))) {
        /* found an existing, stable entry */
        ETHARP_SET_HINT(netif, i);
        return etharp_output_to_arp_index(netif, q, i);
      }
    }
    /* no stable entry found, use the (slower) query function:
       queue on destination Ethernet address belonging to ipaddr */
    return etharp_query(netif, dst_addr, q);             //单播包会调用etharp_query()实现单播包发送，那么网关怎么知道收到的数据包是发给谁的？如何转发的呢？
  }


  /* continuation for multicast/broadcast destinations */
  /* obtain source Ethernet address of the given interface */
  /* send packet directly on the link */
  return etharp_send_ip(netif, q, (struct eth_addr*)(netif->hwaddr), dest);
}

henu四叶草

看懂了，是这样的，如果ip包发现发送的数据是局域网外的数据包，那么ip包会先查询网卡mac是否在缓存表中，没有，则发送arp请求，并将要发送到局域网外的数据包挂到arp缓存队列中。数据包的数据ip信息并没有修改，只是发送的目的是网关。当收到网关的arp应答后，etharp_uodate_entry()接口会将缓存队列的数据包发给网卡，整个过程中数据包并未发生改变。