[问答]

LWIP ip数据包发送局域网外单播包的数据网关怎么知道要发给谁？

1932 LwIP 数据包网关

问答对人有帮助，内容完整，我也想知道答案 0 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_i*roadcast(ipaddr, netif)) { / broadcast on Ethernet also / dest = (struct eth_addr )ðbroadcast;* / 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);* } 0
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