text: /**
   * @file
   * Dynamic Host Configuration Protocol client
   *
   * @defgroup dhcp4 DHCPv4
   * @ingroup ip4
   * DHCP (IPv4) related functions
   * This is a DHCP client for the lwIP TCP/IP stack. It aims to conform
   * with RFC 2131 and RFC 2132.
   *
   * @todo:
   * - Support for interfaces other than Ethernet (SLIP, PPP, ...)
   *
   * Options:
   * @ref DHCP_COARSE_TIMER_SECS (recommended 60 which is a minute)
   * @ref DHCP_FINE_TIMER_MSECS (recommended 500 which equals TCP coarse timer)
   *
   * dhcp_start() starts a DHCP client instance which
   * configures the interface by obtaining an IP address lease and maintaining it.
   *
   * Use dhcp_release() to end the lease and use dhcp_stop()
   * to remove the DHCP client.
   *
   * @see LWIP_HOOK_DHCP_APPEND_OPTIONS
   * @see LWIP_HOOK_DHCP_PARSE_OPTION
   *
   * @see netifapi_dhcp4
   */
  
  /*
   * Copyright (c) 2001-2004 Leon Woestenberg <leon.woestenberg@gmx.net>
   * Copyright (c) 2001-2004 Axon Digital Design B.V., The Netherlands.
   * 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.
   * The Swedish Institute of Computer Science and Adam Dunkels
   * are specifically granted permission to redistribute this
   * source code.
   *
   * Author: Leon Woestenberg <leon.woestenberg@gmx.net>
   *
   */
  
  #include "lwip/opt.h"
  
  #if LWIP_IPV4 && LWIP_DHCP /* don't build if not configured for use in lwipopts.h */
  
  #include "lwip/stats.h"
  #include "lwip/mem.h"
  #include "lwip/udp.h"
  #include "lwip/ip_addr.h"
  #include "lwip/netif.h"
  #include "lwip/def.h"
  #include "lwip/dhcp.h"
  #include "lwip/autoip.h"
  #include "lwip/dns.h"
  #include "lwip/etharp.h"
  #include "lwip/prot/dhcp.h"
  #include "lwip/prot/iana.h"
  
  #include <string.h>
  
  #ifdef LWIP_HOOK_FILENAME
  #include LWIP_HOOK_FILENAME
  #endif
  #ifndef LWIP_HOOK_DHCP_APPEND_OPTIONS
  #define LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, state, msg, msg_type, options_len_ptr)
  #endif
  #ifndef LWIP_HOOK_DHCP_PARSE_OPTION
  #define LWIP_HOOK_DHCP_PARSE_OPTION(netif, dhcp, state, msg, msg_type, option, len, pbuf, offset) do { LWIP_UNUSED_ARG(msg); } while(0)
  #endif
  
  /** DHCP_CREATE_RAND_XID: if this is set to 1, the xid is created using
   * LWIP_RAND() (this overrides DHCP_GLOBAL_XID)
   */
  #ifndef DHCP_CREATE_RAND_XID
  #define DHCP_CREATE_RAND_XID        1
  #endif
  
  /** Default for DHCP_GLOBAL_XID is 0xABCD0000
   * This can be changed by defining DHCP_GLOBAL_XID and DHCP_GLOBAL_XID_HEADER, e.g.
   *  \#define DHCP_GLOBAL_XID_HEADER "stdlib.h"
   *  \#define DHCP_GLOBAL_XID rand()
   */
  #ifdef DHCP_GLOBAL_XID_HEADER
  #include DHCP_GLOBAL_XID_HEADER /* include optional starting XID generation prototypes */
  #endif
  
  /** DHCP_OPTION_MAX_MSG_SIZE is set to the MTU
   * MTU is checked to be big enough in dhcp_start */
  #define DHCP_MAX_MSG_LEN(netif)        (netif->mtu)
  #define DHCP_MAX_MSG_LEN_MIN_REQUIRED  576
  /** Minimum length for reply before packet is parsed */
  #define DHCP_MIN_REPLY_LEN             44
  
  #define REBOOT_TRIES                2
  
  #if LWIP_DNS && LWIP_DHCP_MAX_DNS_SERVERS
  #if DNS_MAX_SERVERS > LWIP_DHCP_MAX_DNS_SERVERS
  #define LWIP_DHCP_PROVIDE_DNS_SERVERS LWIP_DHCP_MAX_DNS_SERVERS
  #else
  #define LWIP_DHCP_PROVIDE_DNS_SERVERS DNS_MAX_SERVERS
  #endif
  #else
  #define LWIP_DHCP_PROVIDE_DNS_SERVERS 0
  #endif
  
  /** Option handling: options are parsed in dhcp_parse_reply
   * and saved in an array where other functions can load them from.
   * This might be moved into the struct dhcp (not necessarily since
   * lwIP is single-threaded and the array is only used while in recv
   * callback). */
  enum dhcp_option_idx {
    DHCP_OPTION_IDX_OVERLOAD = 0,
    DHCP_OPTION_IDX_MSG_TYPE,
    DHCP_OPTION_IDX_SERVER_ID,
    DHCP_OPTION_IDX_LEASE_TIME,
    DHCP_OPTION_IDX_T1,
    DHCP_OPTION_IDX_T2,
    DHCP_OPTION_IDX_SUBNET_MASK,
    DHCP_OPTION_IDX_ROUTER,
  #if LWIP_DHCP_PROVIDE_DNS_SERVERS
    DHCP_OPTION_IDX_DNS_SERVER,
    DHCP_OPTION_IDX_DNS_SERVER_LAST = DHCP_OPTION_IDX_DNS_SERVER + LWIP_DHCP_PROVIDE_DNS_SERVERS - 1,
  #endif /* LWIP_DHCP_PROVIDE_DNS_SERVERS */
  #if LWIP_DHCP_GET_NTP_SRV
    DHCP_OPTION_IDX_NTP_SERVER,
    DHCP_OPTION_IDX_NTP_SERVER_LAST = DHCP_OPTION_IDX_NTP_SERVER + LWIP_DHCP_MAX_NTP_SERVERS - 1,
  #endif /* LWIP_DHCP_GET_NTP_SRV */
    DHCP_OPTION_IDX_MAX
  };
  
  /** Holds the decoded option values, only valid while in dhcp_recv.
      @todo: move this into struct dhcp? */
  u32_t dhcp_rx_options_val[DHCP_OPTION_IDX_MAX];
  /** Holds a flag which option was received and is contained in dhcp_rx_options_val,
      only valid while in dhcp_recv.
      @todo: move this into struct dhcp? */
  u8_t  dhcp_rx_options_given[DHCP_OPTION_IDX_MAX];
  
  static u8_t dhcp_discover_request_options[] = {
    DHCP_OPTION_SUBNET_MASK,
    DHCP_OPTION_ROUTER,
    DHCP_OPTION_BROADCAST
  #if LWIP_DHCP_PROVIDE_DNS_SERVERS
    , DHCP_OPTION_DNS_SERVER
  #endif /* LWIP_DHCP_PROVIDE_DNS_SERVERS */
  #if LWIP_DHCP_GET_NTP_SRV
    , DHCP_OPTION_NTP
  #endif /* LWIP_DHCP_GET_NTP_SRV */
  };
  
  #ifdef DHCP_GLOBAL_XID
  static u32_t xid;
  static u8_t xid_initialised;
  #endif /* DHCP_GLOBAL_XID */
  
  #define dhcp_option_given(dhcp, idx)          (dhcp_rx_options_given[idx] != 0)
  #define dhcp_got_option(dhcp, idx)            (dhcp_rx_options_given[idx] = 1)
  #define dhcp_clear_option(dhcp, idx)          (dhcp_rx_options_given[idx] = 0)
  #define dhcp_clear_all_options(dhcp)          (memset(dhcp_rx_options_given, 0, sizeof(dhcp_rx_options_given)))
  #define dhcp_get_option_value(dhcp, idx)      (dhcp_rx_options_val[idx])
  #define dhcp_set_option_value(dhcp, idx, val) (dhcp_rx_options_val[idx] = (val))
  
  static struct udp_pcb *dhcp_pcb;
  static u8_t dhcp_pcb_refcount;
  
  /* DHCP client state machine functions */
  static err_t dhcp_discover(struct netif *netif);
  static err_t dhcp_select(struct netif *netif);
  static void dhcp_bind(struct netif *netif);
  #if DHCP_DOES_ARP_CHECK
  static err_t dhcp_decline(struct netif *netif);
  #endif /* DHCP_DOES_ARP_CHECK */
  static err_t dhcp_rebind(struct netif *netif);
  static err_t dhcp_reboot(struct netif *netif);
  static void dhcp_set_state(struct dhcp *dhcp, u8_t new_state);
  
  /* receive, unfold, parse and free incoming messages */
  static void dhcp_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port);
  
  /* set the DHCP timers */
  static void dhcp_timeout(struct netif *netif);
  static void dhcp_t1_timeout(struct netif *netif);
  static void dhcp_t2_timeout(struct netif *netif);
  
  /* build outgoing messages */
  /* create a DHCP message, fill in common headers */
  static struct pbuf *dhcp_create_msg(struct netif *netif, struct dhcp *dhcp, u8_t message_type, u16_t *options_out_len);
  /* add a DHCP option (type, then length in bytes) */
  static u16_t dhcp_option(u16_t options_out_len, u8_t *options, u8_t option_type, u8_t option_len);
  /* add option values */
  static u16_t dhcp_option_byte(u16_t options_out_len, u8_t *options, u8_t value);
  static u16_t dhcp_option_short(u16_t options_out_len, u8_t *options, u16_t value);
  static u16_t dhcp_option_long(u16_t options_out_len, u8_t *options, u32_t value);
  #if LWIP_NETIF_HOSTNAME
  static u16_t dhcp_option_hostname(u16_t options_out_len, u8_t *options, struct netif *netif);
  #endif /* LWIP_NETIF_HOSTNAME */
  /* always add the DHCP options trailer to end and pad */
  static void dhcp_option_trailer(u16_t options_out_len, u8_t *options, struct pbuf *p_out);
  
  /** Ensure DHCP PCB is allocated and bound */
  static err_t
  dhcp_inc_pcb_refcount(void)
  {
    if (dhcp_pcb_refcount == 0) {
      LWIP_ASSERT("dhcp_inc_pcb_refcount(): memory leak", dhcp_pcb == NULL);
  
      /* allocate UDP PCB */
      dhcp_pcb = udp_new();
  
      if (dhcp_pcb == NULL) {
        return ERR_MEM;
      }
  
      ip_set_option(dhcp_pcb, SOF_BROADCAST);
  
      /* set up local and remote port for the pcb -> listen on all interfaces on all src/dest IPs */
      udp_bind(dhcp_pcb, IP4_ADDR_ANY, LWIP_IANA_PORT_DHCP_CLIENT);
      udp_connect(dhcp_pcb, IP4_ADDR_ANY, LWIP_IANA_PORT_DHCP_SERVER);
      udp_recv(dhcp_pcb, dhcp_recv, NULL);
    }
  
    dhcp_pcb_refcount++;
  
    return ERR_OK;
  }
  
  /** Free DHCP PCB if the last netif stops using it */
  static void
  dhcp_dec_pcb_refcount(void)
  {
    LWIP_ASSERT("dhcp_pcb_refcount(): refcount error", (dhcp_pcb_refcount > 0));
    dhcp_pcb_refcount--;
  
    if (dhcp_pcb_refcount == 0) {
      udp_remove(dhcp_pcb);
      dhcp_pcb = NULL;
    }
  }
  
  /**
   * Back-off the DHCP client (because of a received NAK response).
   *
   * Back-off the DHCP client because of a received NAK. Receiving a
   * NAK means the client asked for something non-sensible, for
   * example when it tries to renew a lease obtained on another network.
   *
   * We clear any existing set IP address and restart DHCP negotiation
   * afresh (as per RFC2131 3.2.3).
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_handle_nak(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_handle_nak(netif=%p) %c%c%"U16_F"\n",
                (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
    /* Change to a defined state - set this before assigning the address
       to ensure the callback can use dhcp_supplied_address() */
    dhcp_set_state(dhcp, DHCP_STATE_BACKING_OFF);
    /* remove IP address from interface (must no longer be used, as per RFC2131) */
    netif_set_addr(netif, IP4_ADDR_ANY4, IP4_ADDR_ANY4, IP4_ADDR_ANY4);
    /* We can immediately restart discovery */
    dhcp_discover(netif);
  }
  
  #if DHCP_DOES_ARP_CHECK
  /**
   * Checks if the offered IP address is already in use.
   *
   * It does so by sending an ARP request for the offered address and
   * entering CHECKING state. If no ARP reply is received within a small
   * interval, the address is assumed to be free for use by us.
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_check(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result;
    u16_t msecs;
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_check(netif=%p) %c%c\n", (void *)netif, (s16_t)netif->name[0],
                (s16_t)netif->name[1]));
    dhcp_set_state(dhcp, DHCP_STATE_CHECKING);
    /* create an ARP query for the offered IP address, expecting that no host
       responds, as the IP address should not be in use. */
    result = etharp_query(netif, &dhcp->offered_ip_addr, NULL);
    if (result != ERR_OK) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("dhcp_check: could not perform ARP query\n"));
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    msecs = 500;
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_check(): set request timeout %"U16_F" msecs\n", msecs));
  }
  #endif /* DHCP_DOES_ARP_CHECK */
  
  /**
   * Remember the configuration offered by a DHCP server.
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_handle_offer(struct netif *netif, struct dhcp_msg *msg_in)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_handle_offer(netif=%p) %c%c%"U16_F"\n",
                (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
    /* obtain the server address */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_SERVER_ID)) {
      dhcp->request_timeout = 0; /* stop timer */
  
      ip_addr_set_ip4_u32(&dhcp->server_ip_addr, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_SERVER_ID)));
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_STATE, ("dhcp_handle_offer(): server 0x%08"X32_F"\n",
                  ip4_addr_get_u32(ip_2_ip4(&dhcp->server_ip_addr))));
      /* remember offered address */
      ip4_addr_copy(dhcp->offered_ip_addr, msg_in->yiaddr);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_STATE, ("dhcp_handle_offer(): offer for 0x%08"X32_F"\n",
                  ip4_addr_get_u32(&dhcp->offered_ip_addr)));
  
      dhcp_select(netif);
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
                  ("dhcp_handle_offer(netif=%p) did not get server ID!\n", (void *)netif));
    }
  }
  
  /**
   * Select a DHCP server offer out of all offers.
   *
   * Simply select the first offer received.
   *
   * @param netif the netif under DHCP control
   * @return lwIP specific error (see error.h)
   */
  static err_t
  dhcp_select(struct netif *netif)
  {
    struct dhcp *dhcp;
    err_t result;
    u16_t msecs;
    u8_t i;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_ERROR("dhcp_select: netif != NULL", (netif != NULL), return ERR_ARG;);
    dhcp = netif_dhcp_data(netif);
    LWIP_ERROR("dhcp_select: dhcp != NULL", (dhcp != NULL), return ERR_VAL;);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_select(netif=%p) %c%c%"U16_F"\n", (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
    dhcp_set_state(dhcp, DHCP_STATE_REQUESTING);
  
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_REQUEST, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN(netif));
  
      /* MUST request the offered IP address */
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_REQUESTED_IP, 4);
      options_out_len = dhcp_option_long(options_out_len, msg_out->options, lwip_ntohl(ip4_addr_get_u32(&dhcp->offered_ip_addr)));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_SERVER_ID, 4);
      options_out_len = dhcp_option_long(options_out_len, msg_out->options, lwip_ntohl(ip4_addr_get_u32(ip_2_ip4(&dhcp->server_ip_addr))));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_PARAMETER_REQUEST_LIST, LWIP_ARRAYSIZE(dhcp_discover_request_options));
      for (i = 0; i < LWIP_ARRAYSIZE(dhcp_discover_request_options); i++) {
        options_out_len = dhcp_option_byte(options_out_len, msg_out->options, dhcp_discover_request_options[i]);
      }
  
  #if LWIP_NETIF_HOSTNAME
      options_out_len = dhcp_option_hostname(options_out_len, msg_out->options, netif);
  #endif /* LWIP_NETIF_HOSTNAME */
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_REQUESTING, msg_out, DHCP_REQUEST, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      /* send broadcast to any DHCP server */
      result = udp_sendto_if_src(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif, IP4_ADDR_ANY);
      pbuf_free(p_out);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_select: REQUESTING\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("dhcp_select: could not allocate DHCP request\n"));
      result = ERR_MEM;
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    msecs = (u16_t)((dhcp->tries < 6 ? 1 << dhcp->tries : 60) * 1000);
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_STATE, ("dhcp_select(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  
  /**
   * The DHCP timer that checks for lease renewal/rebind timeouts.
   * Must be called once a minute (see @ref DHCP_COARSE_TIMER_SECS).
   */
  void
  dhcp_coarse_tmr(void)
  {
    struct netif *netif;
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_coarse_tmr()\n"));
    /* iterate through all network interfaces */
    NETIF_FOREACH(netif) {
      /* only act on DHCP configured interfaces */
      struct dhcp *dhcp = netif_dhcp_data(netif);
      if ((dhcp != NULL) && (dhcp->state != DHCP_STATE_OFF)) {
        /* compare lease time to expire timeout */
        if (dhcp->t0_timeout && (++dhcp->lease_used == dhcp->t0_timeout)) {
          LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_coarse_tmr(): t0 timeout\n"));
          /* this clients' lease time has expired */
          dhcp_release_and_stop(netif);
          dhcp_start(netif);
          /* timer is active (non zero), and triggers (zeroes) now? */
        } else if (dhcp->t2_rebind_time && (dhcp->t2_rebind_time-- == 1)) {
          LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_coarse_tmr(): t2 timeout\n"));
          /* this clients' rebind timeout triggered */
          dhcp_t2_timeout(netif);
          /* timer is active (non zero), and triggers (zeroes) now */
        } else if (dhcp->t1_renew_time && (dhcp->t1_renew_time-- == 1)) {
          LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_coarse_tmr(): t1 timeout\n"));
          /* this clients' renewal timeout triggered */
          dhcp_t1_timeout(netif);
        }
      }
    }
  }
  
  /**
   * DHCP transaction timeout handling (this function must be called every 500ms,
   * see @ref DHCP_FINE_TIMER_MSECS).
   *
   * A DHCP server is expected to respond within a short period of time.
   * This timer checks whether an outstanding DHCP request is timed out.
   */
  void
  dhcp_fine_tmr(void)
  {
    struct netif *netif;
    /* loop through netif's */
    NETIF_FOREACH(netif) {
      struct dhcp *dhcp = netif_dhcp_data(netif);
      /* only act on DHCP configured interfaces */
      if (dhcp != NULL) {
        /* timer is active (non zero), and is about to trigger now */
        if (dhcp->request_timeout > 1) {
          dhcp->request_timeout--;
        } else if (dhcp->request_timeout == 1) {
          dhcp->request_timeout--;
          /* { dhcp->request_timeout == 0 } */
          LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_fine_tmr(): request timeout\n"));
          /* this client's request timeout triggered */
          dhcp_timeout(netif);
        }
      }
    }
  }
  
  /**
   * A DHCP negotiation transaction, or ARP request, has timed out.
   *
   * The timer that was started with the DHCP or ARP request has
   * timed out, indicating no response was received in time.
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_timeout(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_timeout()\n"));
    /* back-off period has passed, or server selection timed out */
    if ((dhcp->state == DHCP_STATE_BACKING_OFF) || (dhcp->state == DHCP_STATE_SELECTING)) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_timeout(): restarting discovery\n"));
      dhcp_discover(netif);
      /* receiving the requested lease timed out */
    } else if (dhcp->state == DHCP_STATE_REQUESTING) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_timeout(): REQUESTING, DHCP request timed out\n"));
      if (dhcp->tries <= 5) {
        dhcp_select(netif);
      } else {
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_timeout(): REQUESTING, releasing, restarting\n"));
        dhcp_release_and_stop(netif);
        dhcp_start(netif);
      }
  #if DHCP_DOES_ARP_CHECK
      /* received no ARP reply for the offered address (which is good) */
    } else if (dhcp->state == DHCP_STATE_CHECKING) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_timeout(): CHECKING, ARP request timed out\n"));
      if (dhcp->tries <= 1) {
        dhcp_check(netif);
        /* no ARP replies on the offered address,
           looks like the IP address is indeed free */
      } else {
        /* bind the interface to the offered address */
        dhcp_bind(netif);
      }
  #endif /* DHCP_DOES_ARP_CHECK */
    } else if (dhcp->state == DHCP_STATE_REBOOTING) {
      if (dhcp->tries < REBOOT_TRIES) {
        dhcp_reboot(netif);
      } else {
        dhcp_discover(netif);
      }
    }
  }
  
  /**
   * The renewal period has timed out.
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_t1_timeout(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_STATE, ("dhcp_t1_timeout()\n"));
    if ((dhcp->state == DHCP_STATE_REQUESTING) || (dhcp->state == DHCP_STATE_BOUND) ||
        (dhcp->state == DHCP_STATE_RENEWING)) {
      /* just retry to renew - note that the rebind timer (t2) will
       * eventually time-out if renew tries fail. */
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE,
                  ("dhcp_t1_timeout(): must renew\n"));
      /* This slightly different to RFC2131: DHCPREQUEST will be sent from state
         DHCP_STATE_RENEWING, not DHCP_STATE_BOUND */
      dhcp_renew(netif);
      /* Calculate next timeout */
      if (((dhcp->t2_timeout - dhcp->lease_used) / 2) >= ((60 + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS)) {
        dhcp->t1_renew_time = (u16_t)((dhcp->t2_timeout - dhcp->lease_used) / 2);
      }
    }
  }
  
  /**
   * The rebind period has timed out.
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_t2_timeout(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_t2_timeout()\n"));
    if ((dhcp->state == DHCP_STATE_REQUESTING) || (dhcp->state == DHCP_STATE_BOUND) ||
        (dhcp->state == DHCP_STATE_RENEWING) || (dhcp->state == DHCP_STATE_REBINDING)) {
      /* just retry to rebind */
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE,
                  ("dhcp_t2_timeout(): must rebind\n"));
      /* This slightly different to RFC2131: DHCPREQUEST will be sent from state
         DHCP_STATE_REBINDING, not DHCP_STATE_BOUND */
      dhcp_rebind(netif);
      /* Calculate next timeout */
      if (((dhcp->t0_timeout - dhcp->lease_used) / 2) >= ((60 + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS)) {
        dhcp->t2_rebind_time = (u16_t)((dhcp->t0_timeout - dhcp->lease_used) / 2);
      }
    }
  }
  
  /**
   * Handle a DHCP ACK packet
   *
   * @param netif the netif under DHCP control
   */
  static void
  dhcp_handle_ack(struct netif *netif, struct dhcp_msg *msg_in)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
  #if LWIP_DHCP_PROVIDE_DNS_SERVERS || LWIP_DHCP_GET_NTP_SRV
    u8_t n;
  #endif /* LWIP_DHCP_PROVIDE_DNS_SERVERS || LWIP_DHCP_GET_NTP_SRV */
  #if LWIP_DHCP_GET_NTP_SRV
    ip4_addr_t ntp_server_addrs[LWIP_DHCP_MAX_NTP_SERVERS];
  #endif
  
    /* clear options we might not get from the ACK */
    ip4_addr_set_zero(&dhcp->offered_sn_mask);
    ip4_addr_set_zero(&dhcp->offered_gw_addr);
  #if LWIP_DHCP_BOOTP_FILE
    ip4_addr_set_zero(&dhcp->offered_si_addr);
  #endif /* LWIP_DHCP_BOOTP_FILE */
  
    /* lease time given? */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_LEASE_TIME)) {
      /* remember offered lease time */
      dhcp->offered_t0_lease = dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_LEASE_TIME);
    }
    /* renewal period given? */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_T1)) {
      /* remember given renewal period */
      dhcp->offered_t1_renew = dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_T1);
    } else {
      /* calculate safe periods for renewal */
      dhcp->offered_t1_renew = dhcp->offered_t0_lease / 2;
    }
  
    /* renewal period given? */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_T2)) {
      /* remember given rebind period */
      dhcp->offered_t2_rebind = dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_T2);
    } else {
      /* calculate safe periods for rebinding (offered_t0_lease * 0.875 -> 87.5%)*/
      dhcp->offered_t2_rebind = (dhcp->offered_t0_lease * 7U) / 8U;
    }
  
    /* (y)our internet address */
    ip4_addr_copy(dhcp->offered_ip_addr, msg_in->yiaddr);
  
  #if LWIP_DHCP_BOOTP_FILE
    /* copy boot server address,
       boot file name copied in dhcp_parse_reply if not overloaded */
    ip4_addr_copy(dhcp->offered_si_addr, msg_in->siaddr);
  #endif /* LWIP_DHCP_BOOTP_FILE */
  
    /* subnet mask given? */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_SUBNET_MASK)) {
      /* remember given subnet mask */
      ip4_addr_set_u32(&dhcp->offered_sn_mask, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_SUBNET_MASK)));
      dhcp->subnet_mask_given = 1;
    } else {
      dhcp->subnet_mask_given = 0;
    }
  
    /* gateway router */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_ROUTER)) {
      ip4_addr_set_u32(&dhcp->offered_gw_addr, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_ROUTER)));
    }
  
  #if LWIP_DHCP_GET_NTP_SRV
    /* NTP servers */
    for (n = 0; (n < LWIP_DHCP_MAX_NTP_SERVERS) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_NTP_SERVER + n); n++) {
      ip4_addr_set_u32(&ntp_server_addrs[n], lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_NTP_SERVER + n)));
    }
    dhcp_set_ntp_servers(n, ntp_server_addrs);
  #endif /* LWIP_DHCP_GET_NTP_SRV */
  
  #if LWIP_DHCP_PROVIDE_DNS_SERVERS
    /* DNS servers */
    for (n = 0; (n < LWIP_DHCP_PROVIDE_DNS_SERVERS) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n); n++) {
      ip_addr_t dns_addr;
      ip_addr_set_ip4_u32_val(dns_addr, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n)));
      dns_setserver(n, &dns_addr);
    }
  #endif /* LWIP_DHCP_PROVIDE_DNS_SERVERS */
  }
  
  /**
   * @ingroup dhcp4
   * Set a statically allocated struct dhcp to work with.
   * Using this prevents dhcp_start to allocate it using mem_malloc.
   *
   * @param netif the netif for which to set the struct dhcp
   * @param dhcp (uninitialised) dhcp struct allocated by the application
   */
  void
  dhcp_set_struct(struct netif *netif, struct dhcp *dhcp)
  {
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_ASSERT("netif != NULL", netif != NULL);
    LWIP_ASSERT("dhcp != NULL", dhcp != NULL);
    LWIP_ASSERT("netif already has a struct dhcp set", netif_dhcp_data(netif) == NULL);
  
    /* clear data structure */
    memset(dhcp, 0, sizeof(struct dhcp));
    /* dhcp_set_state(&dhcp, DHCP_STATE_OFF); */
    netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_DHCP, dhcp);
  }
  
  /**
   * @ingroup dhcp4
   * Removes a struct dhcp from a netif.
   *
   * ATTENTION: Only use this when not using dhcp_set_struct() to allocate the
   *            struct dhcp since the memory is passed back to the heap.
   *
   * @param netif the netif from which to remove the struct dhcp
   */
  void dhcp_cleanup(struct netif *netif)
  {
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_ASSERT("netif != NULL", netif != NULL);
  
    if (netif_dhcp_data(netif) != NULL) {
      mem_free(netif_dhcp_data(netif));
      netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_DHCP, NULL);
    }
  }
  
  /**
   * @ingroup dhcp4
   * Start DHCP negotiation for a network interface.
   *
   * If no DHCP client instance was attached to this interface,
   * a new client is created first. If a DHCP client instance
   * was already present, it restarts negotiation.
   *
   * @param netif The lwIP network interface
   * @return lwIP error code
   * - ERR_OK - No error
   * - ERR_MEM - Out of memory
   */
  err_t
  dhcp_start(struct netif *netif)
  {
    struct dhcp *dhcp;
    err_t result;
  
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_ERROR("netif != NULL", (netif != NULL), return ERR_ARG;);
    LWIP_ERROR("netif is not up, old style port?", netif_is_up(netif), return ERR_ARG;);
    dhcp = netif_dhcp_data(netif);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_start(netif=%p) %c%c%"U16_F"\n", (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
  
    /* check MTU of the netif */
    if (netif->mtu < DHCP_MAX_MSG_LEN_MIN_REQUIRED) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_start(): Cannot use this netif with DHCP: MTU is too small\n"));
      return ERR_MEM;
    }
  
    /* no DHCP client attached yet? */
    if (dhcp == NULL) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_start(): mallocing new DHCP client\n"));
      dhcp = (struct dhcp *)mem_malloc(sizeof(struct dhcp));
      if (dhcp == NULL) {
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_start(): could not allocate dhcp\n"));
        return ERR_MEM;
      }
  
      /* store this dhcp client in the netif */
      netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_DHCP, dhcp);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_start(): allocated dhcp"));
      /* already has DHCP client attached */
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_start(): restarting DHCP configuration\n"));
  
      if (dhcp->pcb_allocated != 0) {
        dhcp_dec_pcb_refcount(); /* free DHCP PCB if not needed any more */
      }
      /* dhcp is cleared below, no need to reset flag*/
    }
  
    /* clear data structure */
    memset(dhcp, 0, sizeof(struct dhcp));
    /* dhcp_set_state(&dhcp, DHCP_STATE_OFF); */
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_start(): starting DHCP configuration\n"));
  
    if (dhcp_inc_pcb_refcount() != ERR_OK) { /* ensure DHCP PCB is allocated */
      return ERR_MEM;
    }
    dhcp->pcb_allocated = 1;
  
    if (!netif_is_link_up(netif)) {
      /* set state INIT and wait for dhcp_network_changed() to call dhcp_discover() */
      dhcp_set_state(dhcp, DHCP_STATE_INIT);
      return ERR_OK;
    }
  
    /* (re)start the DHCP negotiation */
    result = dhcp_discover(netif);
    if (result != ERR_OK) {
      /* free resources allocated above */
      dhcp_release_and_stop(netif);
      return ERR_MEM;
    }
    return result;
  }
  
  /**
   * @ingroup dhcp4
   * Inform a DHCP server of our manual configuration.
   *
   * This informs DHCP servers of our fixed IP address configuration
   * by sending an INFORM message. It does not involve DHCP address
   * configuration, it is just here to be nice to the network.
   *
   * @param netif The lwIP network interface
   */
  void
  dhcp_inform(struct netif *netif)
  {
    struct dhcp dhcp;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_ERROR("netif != NULL", (netif != NULL), return;);
  
    if (dhcp_inc_pcb_refcount() != ERR_OK) { /* ensure DHCP PCB is allocated */
      return;
    }
  
    memset(&dhcp, 0, sizeof(struct dhcp));
    dhcp_set_state(&dhcp, DHCP_STATE_INFORMING);
  
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, &dhcp, DHCP_INFORM, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN(netif));
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, &dhcp, DHCP_STATE_INFORMING, msg_out, DHCP_INFORM, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_inform: INFORMING\n"));
  
      udp_sendto_if(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif);
  
      pbuf_free(p_out);
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_inform: could not allocate DHCP request\n"));
    }
  
    dhcp_dec_pcb_refcount(); /* delete DHCP PCB if not needed any more */
  }
  
  /** Handle a possible change in the network configuration.
   *
   * This enters the REBOOTING state to verify that the currently bound
   * address is still valid.
   */
  void
  dhcp_network_changed(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
  
    if (!dhcp) {
      return;
    }
    switch (dhcp->state) {
      case DHCP_STATE_REBINDING:
      case DHCP_STATE_RENEWING:
      case DHCP_STATE_BOUND:
      case DHCP_STATE_REBOOTING:
        dhcp->tries = 0;
        dhcp_reboot(netif);
        break;
      case DHCP_STATE_OFF:
        /* stay off */
        break;
      default:
        LWIP_ASSERT("invalid dhcp->state", dhcp->state <= DHCP_STATE_BACKING_OFF);
        /* INIT/REQUESTING/CHECKING/BACKING_OFF restart with new 'rid' because the
           state changes, SELECTING: continue with current 'rid' as we stay in the
           same state */
  #if LWIP_DHCP_AUTOIP_COOP
        if (dhcp->autoip_coop_state == DHCP_AUTOIP_COOP_STATE_ON) {
          autoip_stop(netif);
          dhcp->autoip_coop_state = DHCP_AUTOIP_COOP_STATE_OFF;
        }
  #endif /* LWIP_DHCP_AUTOIP_COOP */
        /* ensure we start with short timeouts, even if already discovering */
        dhcp->tries = 0;
        dhcp_discover(netif);
        break;
    }
  }
  
  #if DHCP_DOES_ARP_CHECK
  /**
   * Match an ARP reply with the offered IP address:
   * check whether the offered IP address is not in use using ARP
   *
   * @param netif the network interface on which the reply was received
   * @param addr The IP address we received a reply from
   */
  void
  dhcp_arp_reply(struct netif *netif, const ip4_addr_t *addr)
  {
    struct dhcp *dhcp;
  
    LWIP_ERROR("netif != NULL", (netif != NULL), return;);
    dhcp = netif_dhcp_data(netif);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_arp_reply()\n"));
    /* is a DHCP client doing an ARP check? */
    if ((dhcp != NULL) && (dhcp->state == DHCP_STATE_CHECKING)) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_arp_reply(): CHECKING, arp reply for 0x%08"X32_F"\n",
                  ip4_addr_get_u32(addr)));
      /* did a host respond with the address we
         were offered by the DHCP server? */
      if (ip4_addr_cmp(addr, &dhcp->offered_ip_addr)) {
        /* we will not accept the offered address */
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE | LWIP_DBG_LEVEL_WARNING,
                    ("dhcp_arp_reply(): arp reply matched with offered address, declining\n"));
        dhcp_decline(netif);
      }
    }
  }
  
  /**
   * Decline an offered lease.
   *
   * Tell the DHCP server we do not accept the offered address.
   * One reason to decline the lease is when we find out the address
   * is already in use by another host (through ARP).
   *
   * @param netif the netif under DHCP control
   */
  static err_t
  dhcp_decline(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result;
    u16_t msecs;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_decline()\n"));
    dhcp_set_state(dhcp, DHCP_STATE_BACKING_OFF);
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_DECLINE, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_REQUESTED_IP, 4);
      options_out_len = dhcp_option_long(options_out_len, msg_out->options, lwip_ntohl(ip4_addr_get_u32(&dhcp->offered_ip_addr)));
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_BACKING_OFF, msg_out, DHCP_DECLINE, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      /* per section 4.4.4, broadcast DECLINE messages */
      result = udp_sendto_if_src(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif, IP4_ADDR_ANY);
      pbuf_free(p_out);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_decline: BACKING OFF\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
                  ("dhcp_decline: could not allocate DHCP request\n"));
      result = ERR_MEM;
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    msecs = 10 * 1000;
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_decline(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  #endif /* DHCP_DOES_ARP_CHECK */
  
  
  /**
   * Start the DHCP process, discover a DHCP server.
   *
   * @param netif the netif under DHCP control
   */
  static err_t
  dhcp_discover(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result = ERR_OK;
    u16_t msecs;
    u8_t i;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_discover()\n"));
  
    ip4_addr_set_any(&dhcp->offered_ip_addr);
    dhcp_set_state(dhcp, DHCP_STATE_SELECTING);
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_DISCOVER, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_discover: making request\n"));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN(netif));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_PARAMETER_REQUEST_LIST, LWIP_ARRAYSIZE(dhcp_discover_request_options));
      for (i = 0; i < LWIP_ARRAYSIZE(dhcp_discover_request_options); i++) {
        options_out_len = dhcp_option_byte(options_out_len, msg_out->options, dhcp_discover_request_options[i]);
      }
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_SELECTING, msg_out, DHCP_DISCOVER, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_discover: sendto(DISCOVER, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER)\n"));
      udp_sendto_if_src(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif, IP4_ADDR_ANY);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_discover: deleting()ing\n"));
      pbuf_free(p_out);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_discover: SELECTING\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_discover: could not allocate DHCP request\n"));
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
  #if LWIP_DHCP_AUTOIP_COOP
    if (dhcp->tries >= LWIP_DHCP_AUTOIP_COOP_TRIES && dhcp->autoip_coop_state == DHCP_AUTOIP_COOP_STATE_OFF) {
      dhcp->autoip_coop_state = DHCP_AUTOIP_COOP_STATE_ON;
      autoip_start(netif);
    }
  #endif /* LWIP_DHCP_AUTOIP_COOP */
    msecs = (u16_t)((dhcp->tries < 6 ? 1 << dhcp->tries : 60) * 1000);
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_discover(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  
  
  /**
   * Bind the interface to the offered IP address.
   *
   * @param netif network interface to bind to the offered address
   */
  static void
  dhcp_bind(struct netif *netif)
  {
    u32_t timeout;
    struct dhcp *dhcp;
    ip4_addr_t sn_mask, gw_addr;
    LWIP_ERROR("dhcp_bind: netif != NULL", (netif != NULL), return;);
    dhcp = netif_dhcp_data(netif);
    LWIP_ERROR("dhcp_bind: dhcp != NULL", (dhcp != NULL), return;);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_bind(netif=%p) %c%c%"U16_F"\n", (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
  
    /* reset time used of lease */
    dhcp->lease_used = 0;
  
    if (dhcp->offered_t0_lease != 0xffffffffUL) {
      /* set renewal period timer */
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_bind(): t0 renewal timer %"U32_F" secs\n", dhcp->offered_t0_lease));
      timeout = (dhcp->offered_t0_lease + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS;
      if (timeout > 0xffff) {
        timeout = 0xffff;
      }
      dhcp->t0_timeout = (u16_t)timeout;
      if (dhcp->t0_timeout == 0) {
        dhcp->t0_timeout = 1;
      }
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_bind(): set request timeout %"U32_F" msecs\n", dhcp->offered_t0_lease * 1000));
    }
  
    /* temporary DHCP lease? */
    if (dhcp->offered_t1_renew != 0xffffffffUL) {
      /* set renewal period timer */
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_bind(): t1 renewal timer %"U32_F" secs\n", dhcp->offered_t1_renew));
      timeout = (dhcp->offered_t1_renew + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS;
      if (timeout > 0xffff) {
        timeout = 0xffff;
      }
      dhcp->t1_timeout = (u16_t)timeout;
      if (dhcp->t1_timeout == 0) {
        dhcp->t1_timeout = 1;
      }
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_bind(): set request timeout %"U32_F" msecs\n", dhcp->offered_t1_renew * 1000));
      dhcp->t1_renew_time = dhcp->t1_timeout;
    }
    /* set renewal period timer */
    if (dhcp->offered_t2_rebind != 0xffffffffUL) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_bind(): t2 rebind timer %"U32_F" secs\n", dhcp->offered_t2_rebind));
      timeout = (dhcp->offered_t2_rebind + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS;
      if (timeout > 0xffff) {
        timeout = 0xffff;
      }
      dhcp->t2_timeout = (u16_t)timeout;
      if (dhcp->t2_timeout == 0) {
        dhcp->t2_timeout = 1;
      }
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_bind(): set request timeout %"U32_F" msecs\n", dhcp->offered_t2_rebind * 1000));
      dhcp->t2_rebind_time = dhcp->t2_timeout;
    }
  
    /* If we have sub 1 minute lease, t2 and t1 will kick in at the same time. */
    if ((dhcp->t1_timeout >= dhcp->t2_timeout) && (dhcp->t2_timeout > 0)) {
      dhcp->t1_timeout = 0;
    }
  
    if (dhcp->subnet_mask_given) {
      /* copy offered network mask */
      ip4_addr_copy(sn_mask, dhcp->offered_sn_mask);
    } else {
      /* subnet mask not given, choose a safe subnet mask given the network class */
      u8_t first_octet = ip4_addr1(&dhcp->offered_ip_addr);
      if (first_octet <= 127) {
        ip4_addr_set_u32(&sn_mask, PP_HTONL(0xff000000UL));
      } else if (first_octet >= 192) {
        ip4_addr_set_u32(&sn_mask, PP_HTONL(0xffffff00UL));
      } else {
        ip4_addr_set_u32(&sn_mask, PP_HTONL(0xffff0000UL));
      }
    }
  
    ip4_addr_copy(gw_addr, dhcp->offered_gw_addr);
    /* gateway address not given? */
    if (ip4_addr_isany_val(gw_addr)) {
      /* copy network address */
      ip4_addr_get_network(&gw_addr, &dhcp->offered_ip_addr, &sn_mask);
      /* use first host address on network as gateway */
      ip4_addr_set_u32(&gw_addr, ip4_addr_get_u32(&gw_addr) | PP_HTONL(0x00000001UL));
    }
  
  #if LWIP_DHCP_AUTOIP_COOP
    if (dhcp->autoip_coop_state == DHCP_AUTOIP_COOP_STATE_ON) {
      autoip_stop(netif);
      dhcp->autoip_coop_state = DHCP_AUTOIP_COOP_STATE_OFF;
    }
  #endif /* LWIP_DHCP_AUTOIP_COOP */
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_STATE, ("dhcp_bind(): IP: 0x%08"X32_F" SN: 0x%08"X32_F" GW: 0x%08"X32_F"\n",
                ip4_addr_get_u32(&dhcp->offered_ip_addr), ip4_addr_get_u32(&sn_mask), ip4_addr_get_u32(&gw_addr)));
    /* netif is now bound to DHCP leased address - set this before assigning the address
       to ensure the callback can use dhcp_supplied_address() */
    dhcp_set_state(dhcp, DHCP_STATE_BOUND);
  
    netif_set_addr(netif, &dhcp->offered_ip_addr, &sn_mask, &gw_addr);
    /* interface is used by routing now that an address is set */
  }
  
  /**
   * @ingroup dhcp4
   * Renew an existing DHCP lease at the involved DHCP server.
   *
   * @param netif network interface which must renew its lease
   */
  err_t
  dhcp_renew(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result;
    u16_t msecs;
    u8_t i;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_renew()\n"));
    dhcp_set_state(dhcp, DHCP_STATE_RENEWING);
  
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_REQUEST, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN(netif));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_PARAMETER_REQUEST_LIST, LWIP_ARRAYSIZE(dhcp_discover_request_options));
      for (i = 0; i < LWIP_ARRAYSIZE(dhcp_discover_request_options); i++) {
        options_out_len = dhcp_option_byte(options_out_len, msg_out->options, dhcp_discover_request_options[i]);
      }
  
  #if LWIP_NETIF_HOSTNAME
      options_out_len = dhcp_option_hostname(options_out_len, msg_out->options, netif);
  #endif /* LWIP_NETIF_HOSTNAME */
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_RENEWING, msg_out, DHCP_REQUEST, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      result = udp_sendto_if(dhcp_pcb, p_out, &dhcp->server_ip_addr, LWIP_IANA_PORT_DHCP_SERVER, netif);
      pbuf_free(p_out);
  
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_renew: RENEWING\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_renew: could not allocate DHCP request\n"));
      result = ERR_MEM;
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    /* back-off on retries, but to a maximum of 20 seconds */
    msecs = (u16_t)(dhcp->tries < 10 ? dhcp->tries * 2000 : 20 * 1000);
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_renew(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  
  /**
   * Rebind with a DHCP server for an existing DHCP lease.
   *
   * @param netif network interface which must rebind with a DHCP server
   */
  static err_t
  dhcp_rebind(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result;
    u16_t msecs;
    u8_t i;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_rebind()\n"));
    dhcp_set_state(dhcp, DHCP_STATE_REBINDING);
  
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_REQUEST, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN(netif));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_PARAMETER_REQUEST_LIST, LWIP_ARRAYSIZE(dhcp_discover_request_options));
      for (i = 0; i < LWIP_ARRAYSIZE(dhcp_discover_request_options); i++) {
        options_out_len = dhcp_option_byte(options_out_len, msg_out->options, dhcp_discover_request_options[i]);
      }
  
  #if LWIP_NETIF_HOSTNAME
      options_out_len = dhcp_option_hostname(options_out_len, msg_out->options, netif);
  #endif /* LWIP_NETIF_HOSTNAME */
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_REBINDING, msg_out, DHCP_DISCOVER, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      /* broadcast to server */
      result = udp_sendto_if(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif);
      pbuf_free(p_out);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_rebind: REBINDING\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_rebind: could not allocate DHCP request\n"));
      result = ERR_MEM;
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    msecs = (u16_t)(dhcp->tries < 10 ? dhcp->tries * 1000 : 10 * 1000);
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_rebind(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  
  /**
   * Enter REBOOTING state to verify an existing lease
   *
   * @param netif network interface which must reboot
   */
  static err_t
  dhcp_reboot(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    err_t result;
    u16_t msecs;
    u8_t i;
    struct pbuf *p_out;
    u16_t options_out_len;
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_reboot()\n"));
    dhcp_set_state(dhcp, DHCP_STATE_REBOOTING);
  
    /* create and initialize the DHCP message header */
    p_out = dhcp_create_msg(netif, dhcp, DHCP_REQUEST, &options_out_len);
    if (p_out != NULL) {
      struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
      options_out_len = dhcp_option_short(options_out_len, msg_out->options, DHCP_MAX_MSG_LEN_MIN_REQUIRED);
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_REQUESTED_IP, 4);
      options_out_len = dhcp_option_long(options_out_len, msg_out->options, lwip_ntohl(ip4_addr_get_u32(&dhcp->offered_ip_addr)));
  
      options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_PARAMETER_REQUEST_LIST, LWIP_ARRAYSIZE(dhcp_discover_request_options));
      for (i = 0; i < LWIP_ARRAYSIZE(dhcp_discover_request_options); i++) {
        options_out_len = dhcp_option_byte(options_out_len, msg_out->options, dhcp_discover_request_options[i]);
      }
  
  #if LWIP_NETIF_HOSTNAME
      options_out_len = dhcp_option_hostname(options_out_len, msg_out->options, netif);
  #endif /* LWIP_NETIF_HOSTNAME */
  
      LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, DHCP_STATE_REBOOTING, msg_out, DHCP_REQUEST, &options_out_len);
      dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
      /* broadcast to server */
      result = udp_sendto_if(dhcp_pcb, p_out, IP_ADDR_BROADCAST, LWIP_IANA_PORT_DHCP_SERVER, netif);
      pbuf_free(p_out);
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_reboot: REBOOTING\n"));
    } else {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_reboot: could not allocate DHCP request\n"));
      result = ERR_MEM;
    }
    if (dhcp->tries < 255) {
      dhcp->tries++;
    }
    msecs = (u16_t)(dhcp->tries < 10 ? dhcp->tries * 1000 : 10 * 1000);
    dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_reboot(): set request timeout %"U16_F" msecs\n", msecs));
    return result;
  }
  
  /**
   * @ingroup dhcp4
   * Release a DHCP lease and stop DHCP statemachine (and AUTOIP if LWIP_DHCP_AUTOIP_COOP).
   *
   * @param netif network interface
   */
  void
  dhcp_release_and_stop(struct netif *netif)
  {
    struct dhcp *dhcp = netif_dhcp_data(netif);
    ip_addr_t server_ip_addr;
  
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_release_and_stop()\n"));
    if (dhcp == NULL) {
      return;
    }
  
    /* already off? -> nothing to do */
    if (dhcp->state == DHCP_STATE_OFF) {
      return;
    }
  
    ip_addr_copy(server_ip_addr, dhcp->server_ip_addr);
  
    /* clean old DHCP offer */
    ip_addr_set_zero_ip4(&dhcp->server_ip_addr);
    ip4_addr_set_zero(&dhcp->offered_ip_addr);
    ip4_addr_set_zero(&dhcp->offered_sn_mask);
    ip4_addr_set_zero(&dhcp->offered_gw_addr);
  #if LWIP_DHCP_BOOTP_FILE
    ip4_addr_set_zero(&dhcp->offered_si_addr);
  #endif /* LWIP_DHCP_BOOTP_FILE */
    dhcp->offered_t0_lease = dhcp->offered_t1_renew = dhcp->offered_t2_rebind = 0;
    dhcp->t1_renew_time = dhcp->t2_rebind_time = dhcp->lease_used = dhcp->t0_timeout = 0;
  
    /* send release message when current IP was assigned via DHCP */
    if (dhcp_supplied_address(netif)) {
      /* create and initialize the DHCP message header */
      struct pbuf *p_out;
      u16_t options_out_len;
      p_out = dhcp_create_msg(netif, dhcp, DHCP_RELEASE, &options_out_len);
      if (p_out != NULL) {
        struct dhcp_msg *msg_out = (struct dhcp_msg *)p_out->payload;
        options_out_len = dhcp_option(options_out_len, msg_out->options, DHCP_OPTION_SERVER_ID, 4);
        options_out_len = dhcp_option_long(options_out_len, msg_out->options, lwip_ntohl(ip4_addr_get_u32(ip_2_ip4(&server_ip_addr))));
  
        LWIP_HOOK_DHCP_APPEND_OPTIONS(netif, dhcp, dhcp->state, msg_out, DHCP_RELEASE, &options_out_len);
        dhcp_option_trailer(options_out_len, msg_out->options, p_out);
  
        udp_sendto_if(dhcp_pcb, p_out, &server_ip_addr, LWIP_IANA_PORT_DHCP_SERVER, netif);
        pbuf_free(p_out);
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_release: RELEASED, DHCP_STATE_OFF\n"));
      } else {
        /* sending release failed, but that's not a problem since the correct behaviour of dhcp does not rely on release */
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("dhcp_release: could not allocate DHCP request\n"));
      }
    }
  
    /* remove IP address from interface (prevents routing from selecting this interface) */
    //netif_set_addr(netif, IP4_ADDR_ANY4, IP4_ADDR_ANY4, IP4_ADDR_ANY4);
  
  #if LWIP_DHCP_AUTOIP_COOP
    if (dhcp->autoip_coop_state == DHCP_AUTOIP_COOP_STATE_ON) {
      autoip_stop(netif);
      dhcp->autoip_coop_state = DHCP_AUTOIP_COOP_STATE_OFF;
    }
  #endif /* LWIP_DHCP_AUTOIP_COOP */
  
    dhcp_set_state(dhcp, DHCP_STATE_OFF);
  
    if (dhcp->pcb_allocated != 0) {
      dhcp_dec_pcb_refcount(); /* free DHCP PCB if not needed any more */
      dhcp->pcb_allocated = 0;
    }
  }
  
  /**
   * @ingroup dhcp4
   * This function calls dhcp_release_and_stop() internally.
   * @deprecated Use dhcp_release_and_stop() instead.
   */
  err_t
  dhcp_release(struct netif *netif)
  {
    dhcp_release_and_stop(netif);
    return ERR_OK;
  }
  
  /**
   * @ingroup dhcp4
   * This function calls dhcp_release_and_stop() internally.
   * @deprecated Use dhcp_release_and_stop() instead.
   */
  void
  dhcp_stop(struct netif *netif)
  {
    dhcp_release_and_stop(netif);
  }
  
  /*
   * Set the DHCP state of a DHCP client.
   *
   * If the state changed, reset the number of tries.
   */
  static void
  dhcp_set_state(struct dhcp *dhcp, u8_t new_state)
  {
    if (new_state != dhcp->state) {
      dhcp->state = new_state;
      dhcp->tries = 0;
      dhcp->request_timeout = 0;
    }
  }
  
  /*
   * Concatenate an option type and length field to the outgoing
   * DHCP message.
   *
   */
  static u16_t
  dhcp_option(u16_t options_out_len, u8_t *options, u8_t option_type, u8_t option_len)
  {
    LWIP_ASSERT("dhcp_option: options_out_len + 2 + option_len <= DHCP_OPTIONS_LEN", options_out_len + 2U + option_len <= DHCP_OPTIONS_LEN);
    options[options_out_len++] = option_type;
    options[options_out_len++] = option_len;
    return options_out_len;
  }
  /*
   * Concatenate a single byte to the outgoing DHCP message.
   *
   */
  static u16_t
  dhcp_option_byte(u16_t options_out_len, u8_t *options, u8_t value)
  {
    LWIP_ASSERT("dhcp_option_byte: options_out_len < DHCP_OPTIONS_LEN", options_out_len < DHCP_OPTIONS_LEN);
    options[options_out_len++] = value;
    return options_out_len;
  }
  
  static u16_t
  dhcp_option_short(u16_t options_out_len, u8_t *options, u16_t value)
  {
    LWIP_ASSERT("dhcp_option_short: options_out_len + 2 <= DHCP_OPTIONS_LEN", options_out_len + 2U <= DHCP_OPTIONS_LEN);
    options[options_out_len++] = (u8_t)((value & 0xff00U) >> 8);
    options[options_out_len++] = (u8_t) (value & 0x00ffU);
    return options_out_len;
  }
  
  static u16_t
  dhcp_option_long(u16_t options_out_len, u8_t *options, u32_t value)
  {
    LWIP_ASSERT("dhcp_option_long: options_out_len + 4 <= DHCP_OPTIONS_LEN", options_out_len + 4U <= DHCP_OPTIONS_LEN);
    options[options_out_len++] = (u8_t)((value & 0xff000000UL) >> 24);
    options[options_out_len++] = (u8_t)((value & 0x00ff0000UL) >> 16);
    options[options_out_len++] = (u8_t)((value & 0x0000ff00UL) >> 8);
    options[options_out_len++] = (u8_t)((value & 0x000000ffUL));
    return options_out_len;
  }
  
  #if LWIP_NETIF_HOSTNAME
  static u16_t
  dhcp_option_hostname(u16_t options_out_len, u8_t *options, struct netif *netif)
  {
    if (netif->hostname != NULL) {
      size_t namelen = strlen(netif->hostname);
      if (namelen > 0) {
        size_t len;
        const char *p = netif->hostname;
        /* Shrink len to available bytes (need 2 bytes for OPTION_HOSTNAME
           and 1 byte for trailer) */
        size_t available = DHCP_OPTIONS_LEN - options_out_len - 3;
        LWIP_ASSERT("DHCP: hostname is too long!", namelen <= available);
        len = LWIP_MIN(namelen, available);
        LWIP_ASSERT("DHCP: hostname is too long!", len <= 0xFF);
        options_out_len = dhcp_option(options_out_len, options, DHCP_OPTION_HOSTNAME, (u8_t)len);
        while (len--) {
          options_out_len = dhcp_option_byte(options_out_len, options, *p++);
        }
      }
    }
    return options_out_len;
  }
  #endif /* LWIP_NETIF_HOSTNAME */
  
  /**
   * Extract the DHCP message and the DHCP options.
   *
   * Extract the DHCP message and the DHCP options, each into a contiguous
   * piece of memory. As a DHCP message is variable sized by its options,
   * and also allows overriding some fields for options, the easy approach
   * is to first unfold the options into a contiguous piece of memory, and
   * use that further on.
   *
   */
  static err_t
  dhcp_parse_reply(struct pbuf *p, struct dhcp *dhcp)
  {
    u8_t *options;
    u16_t offset;
    u16_t offset_max;
    u16_t options_idx;
    u16_t options_idx_max;
    struct pbuf *q;
    int parse_file_as_options = 0;
    int parse_sname_as_options = 0;
    struct dhcp_msg *msg_in;
  #if LWIP_DHCP_BOOTP_FILE
    int file_overloaded = 0;
  #endif
  
    LWIP_UNUSED_ARG(dhcp);
  
    /* clear received options */
    dhcp_clear_all_options(dhcp);
    /* check that beginning of dhcp_msg (up to and including chaddr) is in first pbuf */
    if (p->len < DHCP_SNAME_OFS) {
      return ERR_BUF;
    }
    msg_in = (struct dhcp_msg *)p->payload;
  #if LWIP_DHCP_BOOTP_FILE
    /* clear boot file name */
    dhcp->boot_file_name[0] = 0;
  #endif /* LWIP_DHCP_BOOTP_FILE */
  
    /* parse options */
  
    /* start with options field */
    options_idx = DHCP_OPTIONS_OFS;
    /* parse options to the end of the received packet */
    options_idx_max = p->tot_len;
  again:
    q = p;
    while ((q != NULL) && (options_idx >= q->len)) {
      options_idx = (u16_t)(options_idx - q->len);
      options_idx_max = (u16_t)(options_idx_max - q->len);
      q = q->next;
    }
    if (q == NULL) {
      return ERR_BUF;
    }
    offset = options_idx;
    offset_max = options_idx_max;
    options = (u8_t *)q->payload;
    /* at least 1 byte to read and no end marker, then at least 3 bytes to read? */
    while ((q != NULL) && (offset < offset_max) && (options[offset] != DHCP_OPTION_END)) {
      u8_t op = options[offset];
      u8_t len;
      u8_t decode_len = 0;
      int decode_idx = -1;
      u16_t val_offset = (u16_t)(offset + 2);
      if (val_offset < offset) {
        /* overflow */
        return ERR_BUF;
      }
      /* len byte might be in the next pbuf */
      if ((offset + 1) < q->len) {
        len = options[offset + 1];
      } else {
        len = (q->next != NULL ? ((u8_t *)q->next->payload)[0] : 0);
      }
      /* LWIP_DEBUGF(DHCP_DEBUG, ("msg_offset=%"U16_F", q->len=%"U16_F, msg_offset, q->len)); */
      decode_len = len;
      switch (op) {
        /* case(DHCP_OPTION_END): handled above */
        case (DHCP_OPTION_PAD):
          /* special option: no len encoded */
          decode_len = len = 0;
          /* will be increased below */
          break;
        case (DHCP_OPTION_SUBNET_MASK):
          LWIP_ERROR("len == 4", len == 4, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_SUBNET_MASK;
          break;
        case (DHCP_OPTION_ROUTER):
          decode_len = 4; /* only copy the first given router */
          LWIP_ERROR("len >= decode_len", len >= decode_len, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_ROUTER;
          break;
  #if LWIP_DHCP_PROVIDE_DNS_SERVERS
        case (DHCP_OPTION_DNS_SERVER):
          /* special case: there might be more than one server */
          LWIP_ERROR("len %% 4 == 0", len % 4 == 0, return ERR_VAL;);
          /* limit number of DNS servers */
          decode_len = LWIP_MIN(len, 4 * DNS_MAX_SERVERS);
          LWIP_ERROR("len >= decode_len", len >= decode_len, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_DNS_SERVER;
          break;
  #endif /* LWIP_DHCP_PROVIDE_DNS_SERVERS */
        case (DHCP_OPTION_LEASE_TIME):
          LWIP_ERROR("len == 4", len == 4, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_LEASE_TIME;
          break;
  #if LWIP_DHCP_GET_NTP_SRV
        case (DHCP_OPTION_NTP):
          /* special case: there might be more than one server */
          LWIP_ERROR("len %% 4 == 0", len % 4 == 0, return ERR_VAL;);
          /* limit number of NTP servers */
          decode_len = LWIP_MIN(len, 4 * LWIP_DHCP_MAX_NTP_SERVERS);
          LWIP_ERROR("len >= decode_len", len >= decode_len, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_NTP_SERVER;
          break;
  #endif /* LWIP_DHCP_GET_NTP_SRV*/
        case (DHCP_OPTION_OVERLOAD):
          LWIP_ERROR("len == 1", len == 1, return ERR_VAL;);
          /* decode overload only in options, not in file/sname: invalid packet */
          LWIP_ERROR("overload in file/sname", options_idx == DHCP_OPTIONS_OFS, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_OVERLOAD;
          break;
        case (DHCP_OPTION_MESSAGE_TYPE):
          LWIP_ERROR("len == 1", len == 1, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_MSG_TYPE;
          break;
        case (DHCP_OPTION_SERVER_ID):
          LWIP_ERROR("len == 4", len == 4, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_SERVER_ID;
          break;
        case (DHCP_OPTION_T1):
          LWIP_ERROR("len == 4", len == 4, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_T1;
          break;
        case (DHCP_OPTION_T2):
          LWIP_ERROR("len == 4", len == 4, return ERR_VAL;);
          decode_idx = DHCP_OPTION_IDX_T2;
          break;
        default:
          decode_len = 0;
          LWIP_DEBUGF(DHCP_DEBUG, ("skipping option %"U16_F" in options\n", (u16_t)op));
          LWIP_HOOK_DHCP_PARSE_OPTION(ip_current_netif(), dhcp, dhcp->state, msg_in,
                                      dhcp_option_given(dhcp, DHCP_OPTION_IDX_MSG_TYPE) ? (u8_t)dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_MSG_TYPE) : 0,
                                      op, len, q, val_offset);
          break;
      }
      if (op == DHCP_OPTION_PAD) {
        offset++;
      } else {
        if (offset + len + 2 > 0xFFFF) {
          /* overflow */
          return ERR_BUF;
        }
        offset = (u16_t)(offset + len + 2);
        if (decode_len > 0) {
          u32_t value = 0;
          u16_t copy_len;
  decode_next:
          LWIP_ASSERT("check decode_idx", decode_idx >= 0 && decode_idx < DHCP_OPTION_IDX_MAX);
          if (!dhcp_option_given(dhcp, decode_idx)) {
            copy_len = LWIP_MIN(decode_len, 4);
            if (pbuf_copy_partial(q, &value, copy_len, val_offset) != copy_len) {
              return ERR_BUF;
            }
            if (decode_len > 4) {
              /* decode more than one u32_t */
              u16_t next_val_offset;
              LWIP_ERROR("decode_len %% 4 == 0", decode_len % 4 == 0, return ERR_VAL;);
              dhcp_got_option(dhcp, decode_idx);
              dhcp_set_option_value(dhcp, decode_idx, lwip_htonl(value));
              decode_len = (u8_t)(decode_len - 4);
              next_val_offset = (u16_t)(val_offset + 4);
              if (next_val_offset < val_offset) {
                /* overflow */
                return ERR_BUF;
              }
              val_offset = next_val_offset;
              decode_idx++;
              goto decode_next;
            } else if (decode_len == 4) {
              value = lwip_ntohl(value);
            } else {
              LWIP_ERROR("invalid decode_len", decode_len == 1, return ERR_VAL;);
              value = ((u8_t *)&value)[0];
            }
            dhcp_got_option(dhcp, decode_idx);
            dhcp_set_option_value(dhcp, decode_idx, value);
          }
        }
      }
      if (offset >= q->len) {
        offset = (u16_t)(offset - q->len);
        offset_max = (u16_t)(offset_max - q->len);
        if (offset < offset_max) {
          q = q->next;
          LWIP_ERROR("next pbuf was null", q != NULL, return ERR_VAL;);
          options = (u8_t *)q->payload;
        } else {
          /* We've run out of bytes, probably no end marker. Don't proceed. */
          return ERR_BUF;
        }
      }
    }
    /* is this an overloaded message? */
    if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_OVERLOAD)) {
      u32_t overload = dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_OVERLOAD);
      dhcp_clear_option(dhcp, DHCP_OPTION_IDX_OVERLOAD);
      if (overload == DHCP_OVERLOAD_FILE) {
        parse_file_as_options = 1;
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("overloaded file field\n"));
      } else if (overload == DHCP_OVERLOAD_SNAME) {
        parse_sname_as_options = 1;
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("overloaded sname field\n"));
      } else if (overload == DHCP_OVERLOAD_SNAME_FILE) {
        parse_sname_as_options = 1;
        parse_file_as_options = 1;
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("overloaded sname and file field\n"));
      } else {
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("invalid overload option: %d\n", (int)overload));
      }
    }
    if (parse_file_as_options) {
      /* if both are overloaded, parse file first and then sname (RFC 2131 ch. 4.1) */
      parse_file_as_options = 0;
      options_idx = DHCP_FILE_OFS;
      options_idx_max = DHCP_FILE_OFS + DHCP_FILE_LEN;
  #if LWIP_DHCP_BOOTP_FILE
      file_overloaded = 1;
  #endif
      goto again;
    } else if (parse_sname_as_options) {
      parse_sname_as_options = 0;
      options_idx = DHCP_SNAME_OFS;
      options_idx_max = DHCP_SNAME_OFS + DHCP_SNAME_LEN;
      goto again;
    }
  #if LWIP_DHCP_BOOTP_FILE
    if (!file_overloaded) {
      /* only do this for ACK messages */
      if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_MSG_TYPE) &&
        (dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_MSG_TYPE) == DHCP_ACK))
      /* copy bootp file name, don't care for sname (server hostname) */
      if (pbuf_copy_partial(p, dhcp->boot_file_name, DHCP_FILE_LEN-1, DHCP_FILE_OFS) != (DHCP_FILE_LEN-1)) {
        return ERR_BUF;
      }
      /* make sure the string is really NULL-terminated */
      dhcp->boot_file_name[DHCP_FILE_LEN-1] = 0;
    }
  #endif /* LWIP_DHCP_BOOTP_FILE */ 
    return ERR_OK;
  }
  
  /**
   * If an incoming DHCP message is in response to us, then trigger the state machine
   */
  static void
  dhcp_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port)
  {
    struct netif *netif = ip_current_input_netif();
    struct dhcp *dhcp = netif_dhcp_data(netif);
    struct dhcp_msg *reply_msg = (struct dhcp_msg *)p->payload;
    u8_t msg_type;
    u8_t i;
    struct dhcp_msg *msg_in;
  
    LWIP_UNUSED_ARG(arg);
  
    /* Caught DHCP message from netif that does not have DHCP enabled? -> not interested */
    if ((dhcp == NULL) || (dhcp->pcb_allocated == 0)) {
      goto free_pbuf_and_return;
    }
  
    LWIP_ASSERT("invalid server address type", IP_IS_V4(addr));
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_recv(pbuf = %p) from DHCP server %"U16_F".%"U16_F".%"U16_F".%"U16_F" port %"U16_F"\n", (void *)p,
                ip4_addr1_16(ip_2_ip4(addr)), ip4_addr2_16(ip_2_ip4(addr)), ip4_addr3_16(ip_2_ip4(addr)), ip4_addr4_16(ip_2_ip4(addr)), port));
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("pbuf->len = %"U16_F"\n", p->len));
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("pbuf->tot_len = %"U16_F"\n", p->tot_len));
    /* prevent warnings about unused arguments */
    LWIP_UNUSED_ARG(pcb);
    LWIP_UNUSED_ARG(addr);
    LWIP_UNUSED_ARG(port);
  
    if (p->len < DHCP_MIN_REPLY_LEN) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("DHCP reply message or pbuf too short\n"));
      goto free_pbuf_and_return;
    }
  
    if (reply_msg->op != DHCP_BOOTREPLY) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("not a DHCP reply message, but type %"U16_F"\n", (u16_t)reply_msg->op));
      goto free_pbuf_and_return;
    }
    /* iterate through hardware address and match against DHCP message */
    for (i = 0; i < netif->hwaddr_len && i < LWIP_MIN(DHCP_CHADDR_LEN, NETIF_MAX_HWADDR_LEN); i++) {
      if (netif->hwaddr[i] != reply_msg->chaddr[i]) {
        LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING,
                    ("netif->hwaddr[%"U16_F"]==%02"X16_F" != reply_msg->chaddr[%"U16_F"]==%02"X16_F"\n",
                     (u16_t)i, (u16_t)netif->hwaddr[i], (u16_t)i, (u16_t)reply_msg->chaddr[i]));
        goto free_pbuf_and_return;
      }
    }
    /* match transaction ID against what we expected */
    if (lwip_ntohl(reply_msg->xid) != dhcp->xid) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING,
                  ("transaction id mismatch reply_msg->xid(%"X32_F")!=dhcp->xid(%"X32_F")\n", lwip_ntohl(reply_msg->xid), dhcp->xid));
      goto free_pbuf_and_return;
    }
    /* option fields could be unfold? */
    if (dhcp_parse_reply(p, dhcp) != ERR_OK) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
                  ("problem unfolding DHCP message - too short on memory?\n"));
      goto free_pbuf_and_return;
    }
  
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("searching DHCP_OPTION_MESSAGE_TYPE\n"));
    /* obtain pointer to DHCP message type */
    if (!dhcp_option_given(dhcp, DHCP_OPTION_IDX_MSG_TYPE)) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("DHCP_OPTION_MESSAGE_TYPE option not found\n"));
      goto free_pbuf_and_return;
    }
  
    msg_in = (struct dhcp_msg *)p->payload;
    /* read DHCP message type */
    msg_type = (u8_t)dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_MSG_TYPE);
    /* message type is DHCP ACK? */
    if (msg_type == DHCP_ACK) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("DHCP_ACK received\n"));
      /* in requesting state? */
      if (dhcp->state == DHCP_STATE_REQUESTING) {
        dhcp_handle_ack(netif, msg_in);
  #if DHCP_DOES_ARP_CHECK
        if ((netif->flags & NETIF_FLAG_ETHARP) != 0) {
          /* check if the acknowledged lease address is already in use */
          dhcp_check(netif);
        } else {
          /* bind interface to the acknowledged lease address */
          dhcp_bind(netif);
        }
  #else
        /* bind interface to the acknowledged lease address */
        dhcp_bind(netif);
  #endif
      }
      /* already bound to the given lease address? */
      else if ((dhcp->state == DHCP_STATE_REBOOTING) || (dhcp->state == DHCP_STATE_REBINDING) ||
               (dhcp->state == DHCP_STATE_RENEWING)) {
        dhcp_handle_ack(netif, msg_in);
        dhcp_bind(netif);
      }
    }
    /* received a DHCP_NAK in appropriate state? */
    else if ((msg_type == DHCP_NAK) &&
             ((dhcp->state == DHCP_STATE_REBOOTING) || (dhcp->state == DHCP_STATE_REQUESTING) ||
              (dhcp->state == DHCP_STATE_REBINDING) || (dhcp->state == DHCP_STATE_RENEWING  ))) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("DHCP_NAK received\n"));
      dhcp_handle_nak(netif);
    }
    /* received a DHCP_OFFER in DHCP_STATE_SELECTING state? */
    else if ((msg_type == DHCP_OFFER) && (dhcp->state == DHCP_STATE_SELECTING)) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("DHCP_OFFER received in DHCP_STATE_SELECTING state\n"));
      /* remember offered lease */
      dhcp_handle_offer(netif, msg_in);
    }
  
  free_pbuf_and_return:
    pbuf_free(p);
  }
  
  /**
   * Create a DHCP request, fill in common headers
   *
   * @param netif the netif under DHCP control
   * @param dhcp dhcp control struct
   * @param message_type message type of the request
   */
  static struct pbuf *
  dhcp_create_msg(struct netif *netif, struct dhcp *dhcp, u8_t message_type, u16_t *options_out_len)
  {
    u16_t i;
    struct pbuf *p_out;
    struct dhcp_msg *msg_out;
    u16_t options_out_len_loc;
  
  #ifndef DHCP_GLOBAL_XID
    /** default global transaction identifier starting value (easy to match
     *  with a packet analyser). We simply increment for each new request.
     *  Predefine DHCP_GLOBAL_XID to a better value or a function call to generate one
     *  at runtime, any supporting function prototypes can be defined in DHCP_GLOBAL_XID_HEADER */
  #if DHCP_CREATE_RAND_XID && defined(LWIP_RAND)
    static u32_t xid;
  #else /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
    static u32_t xid = 0xABCD0000;
  #endif /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
  #else
    if (!xid_initialised) {
      xid = DHCP_GLOBAL_XID;
      xid_initialised = !xid_initialised;
    }
  #endif
    LWIP_ERROR("dhcp_create_msg: netif != NULL", (netif != NULL), return NULL;);
    LWIP_ERROR("dhcp_create_msg: dhcp != NULL", (dhcp != NULL), return NULL;);
    p_out = pbuf_alloc(PBUF_TRANSPORT, sizeof(struct dhcp_msg), PBUF_RAM);
    if (p_out == NULL) {
      LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
                  ("dhcp_create_msg(): could not allocate pbuf\n"));
      return NULL;
    }
    LWIP_ASSERT("dhcp_create_msg: check that first pbuf can hold struct dhcp_msg",
                (p_out->len >= sizeof(struct dhcp_msg)));
  
    /* DHCP_REQUEST should reuse 'xid' from DHCPOFFER */
    if ((message_type != DHCP_REQUEST) || (dhcp->state == DHCP_STATE_REBOOTING)) {
      /* reuse transaction identifier in retransmissions */
      if (dhcp->tries == 0) {
  #if DHCP_CREATE_RAND_XID && defined(LWIP_RAND)
        xid = LWIP_RAND();
  #else /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
        xid++;
  #endif /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
      }
      dhcp->xid = xid;
    }
    LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE,
                ("transaction id xid(%"X32_F")\n", xid));
  
    msg_out = (struct dhcp_msg *)p_out->payload;
    memset(msg_out, 0, sizeof(struct dhcp_msg));
  
    msg_out->op = DHCP_BOOTREQUEST;
    /* @todo: make link layer independent */
    msg_out->htype = LWIP_IANA_HWTYPE_ETHERNET;
    msg_out->hlen = netif->hwaddr_len;
    msg_out->xid = lwip_htonl(dhcp->xid);
    /* we don't need the broadcast flag since we can receive unicast traffic
       before being fully configured! */
    /* set ciaddr to netif->ip_addr based on message_type and state */
    if ((message_type == DHCP_INFORM) || (message_type == DHCP_DECLINE) || (message_type == DHCP_RELEASE) ||
        ((message_type == DHCP_REQUEST) && /* DHCP_STATE_BOUND not used for sending! */
         ((dhcp->state == DHCP_STATE_RENEWING) || dhcp->state == DHCP_STATE_REBINDING))) {
      ip4_addr_copy(msg_out->ciaddr, *netif_ip4_addr(netif));
    }
    for (i = 0; i < LWIP_MIN(DHCP_CHADDR_LEN, NETIF_MAX_HWADDR_LEN); i++) {
      /* copy netif hardware address (padded with zeroes through memset already) */
      msg_out->chaddr[i] = netif->hwaddr[i];
    }
    msg_out->cookie = PP_HTONL(DHCP_MAGIC_COOKIE);
    /* Add option MESSAGE_TYPE */
    options_out_len_loc = dhcp_option(0, msg_out->options, DHCP_OPTION_MESSAGE_TYPE, DHCP_OPTION_MESSAGE_TYPE_LEN);
    options_out_len_loc = dhcp_option_byte(options_out_len_loc, msg_out->options, message_type);
    if (options_out_len) {
      *options_out_len = options_out_len_loc;
    }
    return p_out;
  }
  
  /**
   * Add a DHCP message trailer
   *
   * Adds the END option to the DHCP message, and if
   * necessary, up to three padding bytes.
   */
  static void
  dhcp_option_trailer(u16_t options_out_len, u8_t *options, struct pbuf *p_out)
  {
    options[options_out_len++] = DHCP_OPTION_END;
    /* packet is too small, or not 4 byte aligned? */
    while (((options_out_len < DHCP_MIN_OPTIONS_LEN) || (options_out_len & 3)) &&
           (options_out_len < DHCP_OPTIONS_LEN)) {
      /* add a fill/padding byte */
      options[options_out_len++] = 0;
    }
    /* shrink the pbuf to the actual content length */
    pbuf_realloc(p_out, (u16_t)(sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN + options_out_len));
  }
  
  /** check if DHCP supplied netif->ip_addr
   *
   * @param netif the netif to check
   * @return 1 if DHCP supplied netif->ip_addr (states BOUND or RENEWING),
   *         0 otherwise
   */
  u8_t
  dhcp_supplied_address(const struct netif *netif)
  {
    if ((netif != NULL) && (netif_dhcp_data(netif) != NULL)) {
      struct dhcp *dhcp = netif_dhcp_data(netif);
      return (dhcp->state == DHCP_STATE_BOUND) || (dhcp->state == DHCP_STATE_RENEWING) ||
             (dhcp->state == DHCP_STATE_REBINDING);
    }
    return 0;
  }
  
  #endif /* LWIP_IPV4 && LWIP_DHCP */
  