RT-Thread-LWIP 的IP转发功能RT-Thread问答社区

LWIP 的IP转发功能

发布于 2023-05-19 15:48:41 浏览：678 订阅该版

在阅读LWIP的源码的时候，发现ip4_forward中的逻辑无法理解，希望大家能一起看看这里的逻辑，话不多说，先上代码：

```c
/**
 * Forwards an IP packet. It finds an appropriate route for the
 * packet, decrements the TTL value of the packet, adjusts the
 * checksum and outputs the packet on the appropriate interface.
 *
 * @param p the packet to forward (p->payload points to IP header)
 * @param iphdr the IP header of the input packet
 * @param inp the netif on which this packet was received
 */
static void
ip4_forward(struct pbuf *p, struct ip_hdr *iphdr, struct netif *inp)
{
  struct netif *netif;

PERF_START;
  LWIP_UNUSED_ARG(inp);

if (!ip4_canforward(p)) {
    goto return_noroute;
  }

/* RFC3927 2.7: do not forward link-local addresses */
  if (ip4_addr_islinklocal(ip4_current_dest_addr())) {
    LWIP_DEBUGF(IP_DEBUG, ("ip4_forward: not forwarding LLA %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                           ip4_addr1_16(ip4_current_dest_addr()), ip4_addr2_16(ip4_current_dest_addr()),
                           ip4_addr3_16(ip4_current_dest_addr()), ip4_addr4_16(ip4_current_dest_addr())));
    goto return_noroute;
  }

/* Find network interface where to forward this IP packet to. */
  netif = ip4_route_src(ip4_current_src_addr(), ip4_current_dest_addr());
  if (netif == NULL) {
    LWIP_DEBUGF(IP_DEBUG, ("ip4_forward: no forwarding route for %"U16_F".%"U16_F".%"U16_F".%"U16_F" found\n",
                           ip4_addr1_16(ip4_current_dest_addr()), ip4_addr2_16(ip4_current_dest_addr()),
                           ip4_addr3_16(ip4_current_dest_addr()), ip4_addr4_16(ip4_current_dest_addr())));
    /* @todo: send ICMP_DUR_NET? */
    goto return_noroute;
  }
#if !IP_FORWARD_ALLOW_TX_ON_RX_NETIF
  /* Do not forward packets onto the same network interface on which
   * they arrived. */
  if (netif == inp) {
    LWIP_DEBUGF(IP_DEBUG, ("ip4_forward: not bouncing packets back on incoming interface.\n"));
    goto return_noroute;
  }
#endif /* IP_FORWARD_ALLOW_TX_ON_RX_NETIF */

/* decrement TTL */
  IPH_TTL_SET(iphdr, IPH_TTL(iphdr) - 1);
  /* send ICMP if TTL == 0 */
  if (IPH_TTL(iphdr) == 0) {
    MIB2_STATS_INC(mib2.ipinhdrerrors);
#if LWIP_ICMP
    /* Don't send ICMP messages in response to ICMP messages */
    if (IPH_PROTO(iphdr) != IP_PROTO_ICMP) {
      icmp_time_exceeded(p, ICMP_TE_TTL);
    }
#endif /* LWIP_ICMP */
    return;
  }

/* Incrementally update the IP checksum. */
  if (IPH_CHKSUM(iphdr) >= PP_HTONS(0xffffU - 0x100)) {
    IPH_CHKSUM_SET(iphdr, (u16_t)(IPH_CHKSUM(iphdr) + PP_HTONS(0x100) + 1));
  } else {
    IPH_CHKSUM_SET(iphdr, (u16_t)(IPH_CHKSUM(iphdr) + PP_HTONS(0x100)));
  }

LWIP_DEBUGF(IP_DEBUG, ("ip4_forward: forwarding packet to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                         ip4_addr1_16(ip4_current_dest_addr()), ip4_addr2_16(ip4_current_dest_addr()),
                         ip4_addr3_16(ip4_current_dest_addr()), ip4_addr4_16(ip4_current_dest_addr())));

IP_STATS_INC(ip.fw);
  MIB2_STATS_INC(mib2.ipforwdatagrams);
  IP_STATS_INC(ip.xmit);

PERF_STOP("ip4_forward");
  /* don't fragment if interface has mtu set to 0 [loopif] */
  if (netif->mtu && (p->tot_len > netif->mtu)) {
    if ((IPH_OFFSET(iphdr) & PP_NTOHS(IP_DF)) == 0) {
#if IP_FRAG
      ip4_frag(p, netif, ip4_current_dest_addr());
#else /* IP_FRAG */
      /* @todo: send ICMP Destination Unreachable code 13 "Communication administratively prohibited"? */
#endif /* IP_FRAG */
    } else {
#if LWIP_ICMP
      /* send ICMP Destination Unreachable code 4: "Fragmentation Needed and DF Set" */
      icmp_dest_unreach(p, ICMP_DUR_FRAG);
#endif /* LWIP_ICMP */
    }
    return;
  }
  /* transmit pbuf on chosen interface */
  netif->output(netif, p, ip4_current_dest_addr());			// 这里为什么会调用  netif->output ????
  return;
return_noroute:
  MIB2_STATS_INC(mib2.ipoutnoroutes);
}
#endif /* IP_FORWARD */
```
我理解的IP转发功能是指，当netif上收到一个IP包的时候，发现目的IP并不是自己，然后调用ip_forward 尝试进行转发，但是代码中在找到对应的netif后，为什么会调用netif_arpout ?? 发出去后怎么能收到呢？