text: /**
   * @file
   * This is the IPv4 layer implementation for incoming and outgoing IP traffic.
   *
   * @see ip_frag.c
   *
   */
  
  /*
   * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without modification,
   * are permitted provided that the following conditions are met:
   *
   * 1. Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   * 3. The name of the author may not be used to endorse or promote products
   *    derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
   * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
   * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
   * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
   * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
   * OF SUCH DAMAGE.
   *
   * This file is part of the lwIP TCP/IP stack.
   *
   * Author: Adam Dunkels <adam@sics.se>
   *
   */
  
  #include "lwip/opt.h"
  #include "lwip/ip.h"
  #include "lwip/def.h"
  #include "lwip/mem.h"
  #include "lwip/ip_frag.h"
  #include "lwip/inet.h"
  #include "lwip/inet_chksum.h"
  #include "lwip/netif.h"
  #include "lwip/icmp.h"
  #include "lwip/igmp.h"
  #include "lwip/raw.h"
  #include "lwip/udp.h"
  #include "lwip/tcp.h"
  #include "lwip/snmp.h"
  #include "lwip/dhcp.h"
  #include "lwip/stats.h"
  #include "arch/perf.h"
  
  #include <string.h>
  
  /**
   * The interface that provided the packet for the current callback
   * invocation.
   */
  struct netif *current_netif;
  
  /**
   * Header of the input packet currently being processed.
   */
  const struct ip_hdr *current_header;
  
  /**
   * Finds the appropriate network interface for a given IP address. It
   * searches the list of network interfaces linearly. A match is found
   * if the masked IP address of the network interface equals the masked
   * IP address given to the function.
   *
   * @param dest the destination IP address for which to find the route
   * @return the netif on which to send to reach dest
   */
  struct netif *
  ip_route(struct ip_addr *dest)
  {
    struct netif *netif;
  
    /* iterate through netifs */
    for(netif = netif_list; netif != NULL; netif = netif->next) {
      /* network mask matches? */
      if (netif_is_up(netif)) {
        if (ip_addr_netcmp(dest, &(netif->ip_addr), &(netif->netmask))) {
          /* return netif on which to forward IP packet */
          return netif;
        }
      }
    }
    if ((netif_default == NULL) || (!netif_is_up(netif_default))) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip_route: No route to 0x%"X32_F"\n", dest->addr));
      IP_STATS_INC(ip.rterr);
      snmp_inc_ipoutnoroutes();
      return NULL;
    }
    /* no matching netif found, use default netif */
    return netif_default;
  }
  
  #if IP_FORWARD
  /**
   * 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
   * @return the netif on which the packet was sent (NULL if it wasn't sent)
   */
  static struct netif *
  ip_forward(struct pbuf *p, struct ip_hdr *iphdr, struct netif *inp)
  {
    struct netif *netif;
  
    PERF_START;
    /* Find network interface where to forward this IP packet to. */
    netif = ip_route((struct ip_addr *)&(iphdr->dest));
    if (netif == NULL) {
      LWIP_DEBUGF(IP_DEBUG, ("ip_forward: no forwarding route for 0x%"X32_F" found\n",
                        iphdr->dest.addr));
      snmp_inc_ipoutnoroutes();
      return (struct netif *)NULL;
    }
    /* Do not forward packets onto the same network interface on which
     * they arrived. */
    if (netif == inp) {
      LWIP_DEBUGF(IP_DEBUG, ("ip_forward: not bouncing packets back on incoming interface.\n"));
      snmp_inc_ipoutnoroutes();
      return (struct netif *)NULL;
    }
  
    /* decrement TTL */
    IPH_TTL_SET(iphdr, IPH_TTL(iphdr) - 1);
    /* send ICMP if TTL == 0 */
    if (IPH_TTL(iphdr) == 0) {
      snmp_inc_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 (struct netif *)NULL;
    }
  
    /* Incrementally update the IP checksum. */
    if (IPH_CHKSUM(iphdr) >= htons(0xffff - 0x100)) {
      IPH_CHKSUM_SET(iphdr, IPH_CHKSUM(iphdr) + htons(0x100) + 1);
    } else {
      IPH_CHKSUM_SET(iphdr, IPH_CHKSUM(iphdr) + htons(0x100));
    }
  
    LWIP_DEBUGF(IP_DEBUG, ("ip_forward: forwarding packet to 0x%"X32_F"\n",
                      iphdr->dest.addr));
  
    IP_STATS_INC(ip.fw);
    IP_STATS_INC(ip.xmit);
    snmp_inc_ipforwdatagrams();
  
    PERF_STOP("ip_forward");
    /* transmit pbuf on chosen interface */
    netif->output(netif, p, (struct ip_addr *)&(iphdr->dest));
    return netif;
  }
  #endif /* IP_FORWARD */
  
  /**
   * This function is called by the network interface device driver when
   * an IP packet is received. The function does the basic checks of the
   * IP header such as packet size being at least larger than the header
   * size etc. If the packet was not destined for us, the packet is
   * forwarded (using ip_forward). The IP checksum is always checked.
   *
   * Finally, the packet is sent to the upper layer protocol input function.
   *
   * @param p the received IP packet (p->payload points to IP header)
   * @param inp the netif on which this packet was received
   * @return ERR_OK if the packet was processed (could return ERR_* if it wasn't
   *         processed, but currently always returns ERR_OK)
   */
  err_t
  ip_input(struct pbuf *p, struct netif *inp)
  {
    struct ip_hdr *iphdr;
    struct netif *netif;
    u16_t iphdr_hlen;
    u16_t iphdr_len;
  #if LWIP_DHCP
    int check_ip_src=1;
  #endif /* LWIP_DHCP */
  
    IP_STATS_INC(ip.recv);
    snmp_inc_ipinreceives();
  
    /* identify the IP header */
    iphdr = p->payload;
    if (IPH_V(iphdr) != 4) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_WARNING, ("IP packet dropped due to bad version number %"U16_F"\n", IPH_V(iphdr)));
      ip_debug_print(p);
      pbuf_free(p);
      IP_STATS_INC(ip.err);
      IP_STATS_INC(ip.drop);
      snmp_inc_ipinhdrerrors();
      return ERR_OK;
    }
  
    /* obtain IP header length in number of 32-bit words */
    iphdr_hlen = IPH_HL(iphdr);
    /* calculate IP header length in bytes */
    iphdr_hlen *= 4;
    /* obtain ip length in bytes */
    iphdr_len = ntohs(IPH_LEN(iphdr));
  
    /* header length exceeds first pbuf length, or ip length exceeds total pbuf length? */
    if ((iphdr_hlen > p->len) || (iphdr_len > p->tot_len)) {
      if (iphdr_hlen > p->len) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
          ("IP header (len %"U16_F") does not fit in first pbuf (len %"U16_F"), IP packet dropped.\n",
          iphdr_hlen, p->len));
      }
      if (iphdr_len > p->tot_len) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
          ("IP (len %"U16_F") is longer than pbuf (len %"U16_F"), IP packet dropped.\n",
          iphdr_len, p->tot_len));
      }
      /* free (drop) packet pbufs */
      pbuf_free(p);
      IP_STATS_INC(ip.lenerr);
      IP_STATS_INC(ip.drop);
      snmp_inc_ipindiscards();
      return ERR_OK;
    }
  
    /* verify checksum */
  #if CHECKSUM_CHECK_IP
    if (inet_chksum(iphdr, iphdr_hlen) != 0) {
  
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
        ("Checksum (0x%"X16_F") failed, IP packet dropped.\n", inet_chksum(iphdr, iphdr_hlen)));
      ip_debug_print(p);
      pbuf_free(p);
      IP_STATS_INC(ip.chkerr);
      IP_STATS_INC(ip.drop);
      snmp_inc_ipinhdrerrors();
      return ERR_OK;
    }
  #endif
  
    /* Trim pbuf. This should have been done at the netif layer,
     * but we'll do it anyway just to be sure that its done. */
    pbuf_realloc(p, iphdr_len);
  
    /* match packet against an interface, i.e. is this packet for us? */
  #if LWIP_IGMP
    if (ip_addr_ismulticast(&(iphdr->dest))) {
      if ((inp->flags & NETIF_FLAG_IGMP) && (igmp_lookfor_group(inp, &(iphdr->dest)))) {
        netif = inp;
      } else {
        netif = NULL;
      }
    } else
  #endif /* LWIP_IGMP */
    {
      /* start trying with inp. if that's not acceptable, start walking the
         list of configured netifs.
         'first' is used as a boolean to mark whether we started walking the list */
      int first = 1;
      netif = inp;
      do {
        LWIP_DEBUGF(IP_DEBUG, ("ip_input: iphdr->dest 0x%"X32_F" netif->ip_addr 0x%"X32_F" (0x%"X32_F", 0x%"X32_F", 0x%"X32_F")\n",
            iphdr->dest.addr, netif->ip_addr.addr,
            iphdr->dest.addr & netif->netmask.addr,
            netif->ip_addr.addr & netif->netmask.addr,
            iphdr->dest.addr & ~(netif->netmask.addr)));
  
        /* interface is up and configured? */
        if ((netif_is_up(netif)) && (!ip_addr_isany(&(netif->ip_addr)))) {
          /* unicast to this interface address? */
          if (ip_addr_cmp(&(iphdr->dest), &(netif->ip_addr)) ||
              /* or broadcast on this interface network address? */
              ip_addr_isbroadcast(&(iphdr->dest), netif)) {
            LWIP_DEBUGF(IP_DEBUG, ("ip_input: packet accepted on interface %c%c\n",
                netif->name[0], netif->name[1]));
            /* break out of for loop */
            break;
          }
        }
        if (first) {
          first = 0;
          netif = netif_list;
        } else {
          netif = netif->next;
        }
        if (netif == inp) {
          netif = netif->next;
        }
      } while(netif != NULL);
    }
  
  #if LWIP_DHCP
    /* Pass DHCP messages regardless of destination address. DHCP traffic is addressed
     * using link layer addressing (such as Ethernet MAC) so we must not filter on IP.
     * According to RFC 1542 section 3.1.1, referred by RFC 2131).
     */
    if (netif == NULL) {
      /* remote port is DHCP server? */
      if (IPH_PROTO(iphdr) == IP_PROTO_UDP) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE, ("ip_input: UDP packet to DHCP client port %"U16_F"\n",
          ntohs(((struct udp_hdr *)((u8_t *)iphdr + iphdr_hlen))->dest)));
        if (ntohs(((struct udp_hdr *)((u8_t *)iphdr + iphdr_hlen))->dest) == DHCP_CLIENT_PORT) {
          LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE, ("ip_input: DHCP packet accepted.\n"));
          netif = inp;
          check_ip_src = 0;
        }
      }
    }
  #endif /* LWIP_DHCP */
  
    /* broadcast or multicast packet source address? Compliant with RFC 1122: 3.2.1.3 */
  #if LWIP_DHCP
    /* DHCP servers need 0.0.0.0 to be allowed as source address (RFC 1.1.2.2: 3.2.1.3/a) */
    if (check_ip_src && (iphdr->src.addr != 0))
  #endif /* LWIP_DHCP */
    {  if ((ip_addr_isbroadcast(&(iphdr->src), inp)) ||
           (ip_addr_ismulticast(&(iphdr->src)))) {
        /* packet source is not valid */
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("ip_input: packet source is not valid.\n"));
        /* free (drop) packet pbufs */
        pbuf_free(p);
        IP_STATS_INC(ip.drop);
        snmp_inc_ipinaddrerrors();
        snmp_inc_ipindiscards();
        return ERR_OK;
      }
    }
  
    /* packet not for us? */
    if (netif == NULL) {
      /* packet not for us, route or discard */
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE, ("ip_input: packet not for us.\n"));
  #if IP_FORWARD
      /* non-broadcast packet? */
      if (!ip_addr_isbroadcast(&(iphdr->dest), inp)) {
        /* try to forward IP packet on (other) interfaces */
        ip_forward(p, iphdr, inp);
      } else
  #endif /* IP_FORWARD */
      {
        snmp_inc_ipinaddrerrors();
        snmp_inc_ipindiscards();
      }
      pbuf_free(p);
      return ERR_OK;
    }
    /* packet consists of multiple fragments? */
    if ((IPH_OFFSET(iphdr) & htons(IP_OFFMASK | IP_MF)) != 0) {
  #if IP_REASSEMBLY /* packet fragment reassembly code present? */
      LWIP_DEBUGF(IP_DEBUG, ("IP packet is a fragment (id=0x%04"X16_F" tot_len=%"U16_F" len=%"U16_F" MF=%"U16_F" offset=%"U16_F"), calling ip_reass()\n",
        ntohs(IPH_ID(iphdr)), p->tot_len, ntohs(IPH_LEN(iphdr)), !!(IPH_OFFSET(iphdr) & htons(IP_MF)), (ntohs(IPH_OFFSET(iphdr)) & IP_OFFMASK)*8));
      /* reassemble the packet*/
      p = ip_reass(p);
      /* packet not fully reassembled yet? */
      if (p == NULL) {
        return ERR_OK;
      }
      iphdr = p->payload;
  #else /* IP_REASSEMBLY == 0, no packet fragment reassembly code present */
      pbuf_free(p);
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("IP packet dropped since it was fragmented (0x%"X16_F") (while IP_REASSEMBLY == 0).\n",
        ntohs(IPH_OFFSET(iphdr))));
      IP_STATS_INC(ip.opterr);
      IP_STATS_INC(ip.drop);
      /* unsupported protocol feature */
      snmp_inc_ipinunknownprotos();
      return ERR_OK;
  #endif /* IP_REASSEMBLY */
    }
  
  #if IP_OPTIONS_ALLOWED == 0 /* no support for IP options in the IP header? */
  
  #if LWIP_IGMP
    /* there is an extra "router alert" option in IGMP messages which we allow for but do not police */
    if((iphdr_hlen > IP_HLEN &&  (IPH_PROTO(iphdr) != IP_PROTO_IGMP)) {
  #else
    if (iphdr_hlen > IP_HLEN) {
  #endif /* LWIP_IGMP */
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("IP packet dropped since there were IP options (while IP_OPTIONS_ALLOWED == 0).\n"));
      pbuf_free(p);
      IP_STATS_INC(ip.opterr);
      IP_STATS_INC(ip.drop);
      /* unsupported protocol feature */
      snmp_inc_ipinunknownprotos();
      return ERR_OK;
    }
  #endif /* IP_OPTIONS_ALLOWED == 0 */
  
    /* send to upper layers */
    LWIP_DEBUGF(IP_DEBUG, ("ip_input: \n"));
    ip_debug_print(p);
    LWIP_DEBUGF(IP_DEBUG, ("ip_input: p->len %"U16_F" p->tot_len %"U16_F"\n", p->len, p->tot_len));
  
    current_netif = inp;
    current_header = iphdr;
  
  #if LWIP_RAW
    /* raw input did not eat the packet? */
    if (raw_input(p, inp) == 0)
  #endif /* LWIP_RAW */
    {
  
      switch (IPH_PROTO(iphdr)) {
  #if LWIP_UDP
      case IP_PROTO_UDP:
  #if LWIP_UDPLITE
      case IP_PROTO_UDPLITE:
  #endif /* LWIP_UDPLITE */
        snmp_inc_ipindelivers();
        udp_input(p, inp);
        break;
  #endif /* LWIP_UDP */
  #if LWIP_TCP
      case IP_PROTO_TCP:
        snmp_inc_ipindelivers();
        tcp_input(p, inp);
        break;
  #endif /* LWIP_TCP */
  #if LWIP_ICMP
      case IP_PROTO_ICMP:
        snmp_inc_ipindelivers();
        icmp_input(p, inp);
        break;
  #endif /* LWIP_ICMP */
  #if LWIP_IGMP
      case IP_PROTO_IGMP:
        igmp_input(p,inp,&(iphdr->dest));
        break;
  #endif /* LWIP_IGMP */
      default:
  #if LWIP_ICMP
        /* send ICMP destination protocol unreachable unless is was a broadcast */
        if (!ip_addr_isbroadcast(&(iphdr->dest), inp) &&
            !ip_addr_ismulticast(&(iphdr->dest))) {
          p->payload = iphdr;
          icmp_dest_unreach(p, ICMP_DUR_PROTO);
        }
  #endif /* LWIP_ICMP */
        pbuf_free(p);
  
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("Unsupported transport protocol %"U16_F"\n", IPH_PROTO(iphdr)));
  
        IP_STATS_INC(ip.proterr);
        IP_STATS_INC(ip.drop);
        snmp_inc_ipinunknownprotos();
      }
    }
  
    current_netif = NULL;
    current_header = NULL;
  
    return ERR_OK;
  }
  
  /**
   * Sends an IP packet on a network interface. This function constructs
   * the IP header and calculates the IP header checksum. If the source
   * IP address is NULL, the IP address of the outgoing network
   * interface is filled in as source address.
   * If the destination IP address is IP_HDRINCL, p is assumed to already
   * include an IP header and p->payload points to it instead of the data.
   *
   * @param p the packet to send (p->payload points to the data, e.g. next
              protocol header; if dest == IP_HDRINCL, p already includes an IP
              header and p->payload points to that IP header)
   * @param src the source IP address to send from (if src == IP_ADDR_ANY, the
   *         IP  address of the netif used to send is used as source address)
   * @param dest the destination IP address to send the packet to
   * @param ttl the TTL value to be set in the IP header
   * @param tos the TOS value to be set in the IP header
   * @param proto the PROTOCOL to be set in the IP header
   * @param netif the netif on which to send this packet
   * @return ERR_OK if the packet was sent OK
   *         ERR_BUF if p doesn't have enough space for IP/LINK headers
   *         returns errors returned by netif->output
   *
   * @note ip_id: RFC791 "some host may be able to simply use
   *  unique identifiers independent of destination"
   */
  err_t
  ip_output_if(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
               u8_t ttl, u8_t tos,
               u8_t proto, struct netif *netif)
  {
  #if IP_OPTIONS_SEND
    return ip_output_if_opt(p, src, dest, ttl, tos, proto, netif, NULL, 0);
  }
  
  /**
   * Same as ip_output_if() but with the possibility to include IP options:
   *
   * @ param ip_options pointer to the IP options, copied into the IP header
   * @ param optlen length of ip_options
   */
  err_t ip_output_if_opt(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
         u8_t ttl, u8_t tos, u8_t proto, struct netif *netif, void *ip_options,
         u16_t optlen)
  {
  #endif /* IP_OPTIONS_SEND */
    struct ip_hdr *iphdr;
    static u16_t ip_id = 0;
  
    snmp_inc_ipoutrequests();
  
    /* Should the IP header be generated or is it already included in p? */
    if (dest != IP_HDRINCL) {
      u16_t ip_hlen = IP_HLEN;
  #if IP_OPTIONS_SEND
      u16_t optlen_aligned = 0;
      if (optlen != 0) {
        /* round up to a multiple of 4 */
        optlen_aligned = ((optlen + 3) & ~3);
        ip_hlen += optlen_aligned;
        /* First write in the IP options */
        if (pbuf_header(p, optlen_aligned)) {
          LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip_output_if_opt: not enough room for IP options in pbuf\n"));
          IP_STATS_INC(ip.err);
          snmp_inc_ipoutdiscards();
          return ERR_BUF;
        }
        MEMCPY(p->payload, ip_options, optlen);
        if (optlen < optlen_aligned) {
          /* zero the remaining bytes */
          memset(((char*)p->payload) + optlen, 0, optlen_aligned - optlen);
        }
      }
  #endif /* IP_OPTIONS_SEND */
      /* generate IP header */
      if (pbuf_header(p, IP_HLEN)) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip_output: not enough room for IP header in pbuf\n"));
  
        IP_STATS_INC(ip.err);
        snmp_inc_ipoutdiscards();
        return ERR_BUF;
      }
  
      iphdr = p->payload;
      LWIP_ASSERT("check that first pbuf can hold struct ip_hdr",
                 (p->len >= sizeof(struct ip_hdr)));
  
      IPH_TTL_SET(iphdr, ttl);
      IPH_PROTO_SET(iphdr, proto);
  
      ip_addr_set(&(iphdr->dest), dest);
  
      IPH_VHLTOS_SET(iphdr, 4, ip_hlen / 4, tos);
      IPH_LEN_SET(iphdr, htons(p->tot_len));
      IPH_OFFSET_SET(iphdr, 0);
      IPH_ID_SET(iphdr, htons(ip_id));
      ++ip_id;
  
      if (ip_addr_isany(src)) {
        ip_addr_set(&(iphdr->src), &(netif->ip_addr));
      } else {
        ip_addr_set(&(iphdr->src), src);
      }
  
      IPH_CHKSUM_SET(iphdr, 0);
  #if CHECKSUM_GEN_IP
      IPH_CHKSUM_SET(iphdr, inet_chksum(iphdr, ip_hlen));
  #endif
    } else {
      /* IP header already included in p */
      iphdr = p->payload;
      dest = &(iphdr->dest);
    }
  
    IP_STATS_INC(ip.xmit);
  
    LWIP_DEBUGF(IP_DEBUG, ("ip_output_if: %c%c%"U16_F"\n", netif->name[0], netif->name[1], netif->num));
    ip_debug_print(p);
  
  #if ENABLE_LOOPBACK
    if (ip_addr_cmp(dest, &netif->ip_addr)) {
      /* Packet to self, enqueue it for loopback */
      LWIP_DEBUGF(IP_DEBUG, ("netif_loop_output()"));
      return netif_loop_output(netif, p, dest);
    }
  #endif /* ENABLE_LOOPBACK */
  #if IP_FRAG
    /* don't fragment if interface has mtu set to 0 [loopif] */
    if (netif->mtu && (p->tot_len > netif->mtu)) {
      return ip_frag(p,netif,dest);
    }
  #endif
  
    LWIP_DEBUGF(IP_DEBUG, ("netif->output()"));
    return netif->output(netif, p, dest);
  }
  
  /**
   * Simple interface to ip_output_if. It finds the outgoing network
   * interface and calls upon ip_output_if to do the actual work.
   *
   * @param p the packet to send (p->payload points to the data, e.g. next
              protocol header; if dest == IP_HDRINCL, p already includes an IP
              header and p->payload points to that IP header)
   * @param src the source IP address to send from (if src == IP_ADDR_ANY, the
   *         IP  address of the netif used to send is used as source address)
   * @param dest the destination IP address to send the packet to
   * @param ttl the TTL value to be set in the IP header
   * @param tos the TOS value to be set in the IP header
   * @param proto the PROTOCOL to be set in the IP header
   *
   * @return ERR_RTE if no route is found
   *         see ip_output_if() for more return values
   */
  err_t
  ip_output(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
            u8_t ttl, u8_t tos, u8_t proto)
  {
    struct netif *netif;
  
    if ((netif = ip_route(dest)) == NULL) {
      LWIP_DEBUGF(IP_DEBUG, ("ip_output: No route to 0x%"X32_F"\n", dest->addr));
      IP_STATS_INC(ip.rterr);
      return ERR_RTE;
    }
  
    return ip_output_if(p, src, dest, ttl, tos, proto, netif);
  }
  
  #if LWIP_NETIF_HWADDRHINT
  /** Like ip_output, but takes and addr_hint pointer that is passed on to netif->addr_hint
   *  before calling ip_output_if.
   *
   * @param p the packet to send (p->payload points to the data, e.g. next
              protocol header; if dest == IP_HDRINCL, p already includes an IP
              header and p->payload points to that IP header)
   * @param src the source IP address to send from (if src == IP_ADDR_ANY, the
   *         IP  address of the netif used to send is used as source address)
   * @param dest the destination IP address to send the packet to
   * @param ttl the TTL value to be set in the IP header
   * @param tos the TOS value to be set in the IP header
   * @param proto the PROTOCOL to be set in the IP header
   * @param addr_hint address hint pointer set to netif->addr_hint before
   *        calling ip_output_if()
   *
   * @return ERR_RTE if no route is found
   *         see ip_output_if() for more return values
   */
  err_t
  ip_output_hinted(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
            u8_t ttl, u8_t tos, u8_t proto, u8_t *addr_hint)
  {
    struct netif *netif;
    err_t err;
  
    if ((netif = ip_route(dest)) == NULL) {
      LWIP_DEBUGF(IP_DEBUG, ("ip_output: No route to 0x%"X32_F"\n", dest->addr));
      IP_STATS_INC(ip.rterr);
      return ERR_RTE;
    }
  
    netif->addr_hint = addr_hint;
    err = ip_output_if(p, src, dest, ttl, tos, proto, netif);
    netif->addr_hint = NULL;
  
    return err;
  }
  #endif /* LWIP_NETIF_HWADDRHINT*/
  
  #if IP_DEBUG
  /* Print an IP header by using LWIP_DEBUGF
   * @param p an IP packet, p->payload pointing to the IP header
   */
  void
  ip_debug_print(struct pbuf *p)
  {
    struct ip_hdr *iphdr = p->payload;
    u8_t *payload;
  
    payload = (u8_t *)iphdr + IP_HLEN;
  
    LWIP_DEBUGF(IP_DEBUG, ("IP header:\n"));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|%2"S16_F" |%2"S16_F" |  0x%02"X16_F" |     %5"U16_F"     | (v, hl, tos, len)\n",
                      IPH_V(iphdr),
                      IPH_HL(iphdr),
                      IPH_TOS(iphdr),
                      ntohs(IPH_LEN(iphdr))));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|    %5"U16_F"      |%"U16_F"%"U16_F"%"U16_F"|    %4"U16_F"   | (id, flags, offset)\n",
                      ntohs(IPH_ID(iphdr)),
                      ntohs(IPH_OFFSET(iphdr)) >> 15 & 1,
                      ntohs(IPH_OFFSET(iphdr)) >> 14 & 1,
                      ntohs(IPH_OFFSET(iphdr)) >> 13 & 1,
                      ntohs(IPH_OFFSET(iphdr)) & IP_OFFMASK));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |    0x%04"X16_F"     | (ttl, proto, chksum)\n",
                      IPH_TTL(iphdr),
                      IPH_PROTO(iphdr),
                      ntohs(IPH_CHKSUM(iphdr))));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  | (src)\n",
                      ip4_addr1(&iphdr->src),
                      ip4_addr2(&iphdr->src),
                      ip4_addr3(&iphdr->src),
                      ip4_addr4(&iphdr->src)));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  | (dest)\n",
                      ip4_addr1(&iphdr->dest),
                      ip4_addr2(&iphdr->dest),
                      ip4_addr3(&iphdr->dest),
                      ip4_addr4(&iphdr->dest)));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
  }
  #endif /* IP_DEBUG */
  