*/  if (netif == NULL) {    /* remote port is DHCP server? */    if (IPH_PROTO(iphdr) == IP_PROTO_UDP) {      LWIP_DEBUGF(IP_DEBUG | DBG_TRACE | 1, ("ip_input: UDP packet to DHCP client port %u\n",        ntohs(((struct udp_hdr *)((u8_t *)iphdr + iphdrlen))->dest)));      if (ntohs(((struct udp_hdr *)((u8_t *)iphdr + iphdrlen))->dest) == DHCP_CLIENT_PORT) {        LWIP_DEBUGF(IP_DEBUG | DBG_TRACE | 1, ("ip_input: DHCP packet accepted.\n"));        netif = inp;      }    }  }#endif /* LWIP_DHCP */  /* packet not for us? */  if (netif == NULL) {    /* packet not for us, route or discard */    LWIP_DEBUGF(IP_DEBUG | DBG_TRACE | 1, ("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_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%04x tot_len=%u len=%u MF=%u offset=%u), 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 | 2, ("IP packet dropped since it was fragmented (0x%x) (while IP_REASSEMBLY == 0).\n",      ntohs(IPH_OFFSET(iphdr))));    IP_STATS_INC(ip.opterr);    IP_STATS_INC(ip.drop);    snmp_inc_ipunknownprotos();    return ERR_OK;#endif /* IP_REASSEMBLY */  }#if IP_OPTIONS == 0 /* no support for IP options in the IP header? */  if (iphdrlen > IP_HLEN) {    LWIP_DEBUGF(IP_DEBUG | 2, ("IP packet dropped since there were IP options (while IP_OPTIONS == 0).\n"));    pbuf_free(p);    IP_STATS_INC(ip.opterr);    IP_STATS_INC(ip.drop);    snmp_inc_ipunknownprotos();    return ERR_OK;  }#endif /* IP_OPTIONS == 0 */  /* send to upper layers */  LWIP_DEBUGF(IP_DEBUG, ("ip_input: \n"));  ip_debug_print(p);  LWIP_DEBUGF(IP_DEBUG, ("ip_input: p->len %d p->tot_len %d\n", p->len, p->tot_len));#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:  case IP_PROTO_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 */  case IP_PROTO_ICMP:    snmp_inc_ipindelivers();    icmp_input(p, inp);    break;  default:    /* 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);    }    pbuf_free(p);    LWIP_DEBUGF(IP_DEBUG | 2, ("Unsupported transport protocol %d\n", IPH_PROTO(iphdr)));    IP_STATS_INC(ip.proterr);    IP_STATS_INC(ip.drop);    snmp_inc_ipunknownprotos();  }#if LWIP_RAW  } /* LWIP_RAW */#endif  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. */err_tip_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){  struct ip_hdr *iphdr;  u16_t ip_id = 0;  snmp_inc_ipoutrequests();  if (dest != IP_HDRINCL) {    if (pbuf_header(p, IP_HLEN)) {      LWIP_DEBUGF(IP_DEBUG | 2, ("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;    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, htons(IP_DF));    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 {    iphdr = p->payload;    dest = &(iphdr->dest);  }#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  IP_STATS_INC(ip.xmit);  LWIP_DEBUGF(IP_DEBUG, ("ip_output_if: %c%c%u\n", netif->name[0], netif->name[1], netif->num));  ip_debug_print(p);  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. */err_tip_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 | 2, ("ip_output: No route to 0x%lx\n", dest->addr));    IP_STATS_INC(ip.rterr);    snmp_inc_ipoutdiscards();    return ERR_RTE;  }  return ip_output_if(p, src, dest, ttl, tos, proto, netif);}#if IP_DEBUGvoidip_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, ("|%2d |%2d |  0x%02x |     %5u     | (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, ("|    %5u      |%u%u%u|    %4u   | (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, ("|  %3u  |  %3u  |    0x%04x     | (ttl, proto, chksum)\n",                    IPH_TTL(iphdr),                    IPH_PROTO(iphdr),                    ntohs(IPH_CHKSUM(iphdr))));  LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));  LWIP_DEBUGF(IP_DEBUG, ("|  %3u  |  %3u  |  %3u  |  %3u  | (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, ("|  %3u  |  %3u  |  %3u  |  %3u  | (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 */