dhcp_option_trailer(dhcp);

    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE, ("dhcp_discover: realloc()ing\n"));
    pbuf_realloc(dhcp->p_out, sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN + dhcp->options_out_len);

    /* set receive callback function with netif as user data */
    udp_recv(dhcp->pcb, dhcp_recv, netif);
    udp_bind(dhcp->pcb, IP_ADDR_ANY, DHCP_CLIENT_PORT);
    udp_connect(dhcp->pcb, IP_ADDR_ANY, DHCP_SERVER_PORT);
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE, ("dhcp_discover: sendto(DISCOVER, IP_ADDR_BROADCAST, DHCP_SERVER_PORT)\n"));
    udp_sendto(dhcp->pcb, dhcp->p_out, IP_ADDR_BROADCAST, DHCP_SERVER_PORT);
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE, ("dhcp_discover: deleting()ing\n"));
    dhcp_delete_request(netif);
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_discover: SELECTING\n"));
    dhcp_set_state(dhcp, DHCP_SELECTING);
  } else {
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 2, ("dhcp_discover: could not allocate DHCP request\n"));
  }
  dhcp->tries++;
  msecs = dhcp->tries < 4 ? (dhcp->tries + 1) * 1000 : 10 * 1000;
  dhcp->request_timeout = (msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS;
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 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)
{
  struct dhcp *dhcp = netif->dhcp;
  struct ip_addr sn_mask, gw_addr;
  LWIP_ASSERT("dhcp_bind: netif != NULL", netif != NULL);
  LWIP_ASSERT("dhcp_bind: dhcp != NULL", dhcp != NULL);
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 3, ("dhcp_bind(netif=%p) %c%c%"U16_F"\n", (void*)netif, netif->name[0], netif->name[1], (u16_t)netif->num));

  /* temporary DHCP lease? */
  if (dhcp->offered_t1_renew != 0xffffffffUL) {
    /* set renewal period timer */
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE, ("dhcp_bind(): t1 renewal timer %"U32_F" secs\n", dhcp->offered_t1_renew));
    dhcp->t1_timeout = (dhcp->offered_t1_renew + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS;
    if (dhcp->t1_timeout == 0) dhcp->t1_timeout = 1;
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_bind(): set request timeout %"U32_F" msecs\n", dhcp->offered_t1_renew*1000));
  }
  /* set renewal period timer */
  if (dhcp->offered_t2_rebind != 0xffffffffUL) {
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE, ("dhcp_bind(): t2 rebind timer %"U32_F" secs\n", dhcp->offered_t2_rebind));
    dhcp->t2_timeout = (dhcp->offered_t2_rebind + DHCP_COARSE_TIMER_SECS / 2) / DHCP_COARSE_TIMER_SECS;
    if (dhcp->t2_timeout == 0) dhcp->t2_timeout = 1;
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_bind(): set request timeout %"U32_F" msecs\n", dhcp->offered_t2_rebind*1000));
  }
  /* copy offered network mask */
  ip_addr_set(&sn_mask, &dhcp->offered_sn_mask);

  /* subnet mask not given? */
  /* TODO: this is not a valid check. what if the network mask is 0? */
  if (sn_mask.addr == 0) {
    /* choose a safe subnet mask given the network class */
    u8_t first_octet = ip4_addr1(&sn_mask);
    if (first_octet <= 127) sn_mask.addr = htonl(0xff000000);
    else if (first_octet >= 192) sn_mask.addr = htonl(0xffffff00);
    else sn_mask.addr = htonl(0xffff0000);
  }

  ip_addr_set(&gw_addr, &dhcp->offered_gw_addr);
  /* gateway address not given? */
  if (gw_addr.addr == 0) {
    /* copy network address */
    gw_addr.addr = (dhcp->offered_ip_addr.addr & sn_mask.addr);
    /* use first host address on network as gateway */
    gw_addr.addr |= htonl(0x00000001);
  }

  LWIP_DEBUGF(DHCP_DEBUG | DBG_STATE, ("dhcp_bind(): IP: 0x%08"X32_F"\n", dhcp->offered_ip_addr.addr));
  netif_set_ipaddr(netif, &dhcp->offered_ip_addr);
  LWIP_DEBUGF(DHCP_DEBUG | DBG_STATE, ("dhcp_bind(): SN: 0x%08"X32_F"\n", sn_mask.addr));
  netif_set_netmask(netif, &sn_mask);
  LWIP_DEBUGF(DHCP_DEBUG | DBG_STATE, ("dhcp_bind(): GW: 0x%08"X32_F"\n", gw_addr.addr));
  netif_set_gw(netif, &gw_addr);
  /* bring the interface up */
  netif_set_up(netif);
  /* netif is now bound to DHCP leased address */
  dhcp_set_state(dhcp, DHCP_BOUND);
}

/**
 * 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;
  err_t result;
  u16_t msecs;
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 3, ("dhcp_renew()\n"));
  dhcp_set_state(dhcp, DHCP_RENEWING);

  /* create and initialize the DHCP message header */
  result = dhcp_create_request(netif);
  if (result == ERR_OK) {

    dhcp_option(dhcp, DHCP_OPTION_MESSAGE_TYPE, DHCP_OPTION_MESSAGE_TYPE_LEN);
    dhcp_option_byte(dhcp, DHCP_REQUEST);

    dhcp_option(dhcp, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
    /* TODO: use netif->mtu in some way */
    dhcp_option_short(dhcp, 576);

#if 0
    dhcp_option(dhcp, DHCP_OPTION_REQUESTED_IP, 4);
    dhcp_option_long(dhcp, ntohl(dhcp->offered_ip_addr.addr));
#endif

#if 0
    dhcp_option(dhcp, DHCP_OPTION_SERVER_ID, 4);
    dhcp_option_long(dhcp, ntohl(dhcp->server_ip_addr.addr));
#endif
    /* append DHCP message trailer */
    dhcp_option_trailer(dhcp);

    pbuf_realloc(dhcp->p_out, sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN + dhcp->options_out_len);

    udp_bind(dhcp->pcb, IP_ADDR_ANY, DHCP_CLIENT_PORT);
    udp_connect(dhcp->pcb, &dhcp->server_ip_addr, DHCP_SERVER_PORT);
    udp_send(dhcp->pcb, dhcp->p_out);
    dhcp_delete_request(netif);

    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_renew: RENEWING\n"));
  } else {
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 2, ("dhcp_renew: could not allocate DHCP request\n"));
  }
  dhcp->tries++;
  /* back-off on retries, but to a maximum of 20 seconds */
  msecs = dhcp->tries < 10 ? dhcp->tries * 2000 : 20 * 1000;
  dhcp->request_timeout = (msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS;
   LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 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;
  err_t result;
  u16_t msecs;
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_rebind()\n"));
  dhcp_set_state(dhcp, DHCP_REBINDING);

  /* create and initialize the DHCP message header */
  result = dhcp_create_request(netif);
  if (result == ERR_OK)
  {

    dhcp_option(dhcp, DHCP_OPTION_MESSAGE_TYPE, DHCP_OPTION_MESSAGE_TYPE_LEN);
    dhcp_option_byte(dhcp, DHCP_REQUEST);

    dhcp_option(dhcp, DHCP_OPTION_MAX_MSG_SIZE, DHCP_OPTION_MAX_MSG_SIZE_LEN);
    dhcp_option_short(dhcp, 576);

#if 0
    dhcp_option(dhcp, DHCP_OPTION_REQUESTED_IP, 4);
    dhcp_option_long(dhcp, ntohl(dhcp->offered_ip_addr.addr));

    dhcp_option(dhcp, DHCP_OPTION_SERVER_ID, 4);
    dhcp_option_long(dhcp, ntohl(dhcp->server_ip_addr.addr));
#endif

    dhcp_option_trailer(dhcp);

    pbuf_realloc(dhcp->p_out, sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN + dhcp->options_out_len);

    /* set remote IP association to any DHCP server */
    udp_bind(dhcp->pcb, IP_ADDR_ANY, DHCP_CLIENT_PORT);
    udp_connect(dhcp->pcb, IP_ADDR_ANY, DHCP_SERVER_PORT);
    /* broadcast to server */
    udp_sendto(dhcp->pcb, dhcp->p_out, IP_ADDR_BROADCAST, DHCP_SERVER_PORT);
    dhcp_delete_request(netif);
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_rebind: REBINDING\n"));
  } else {
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 2, ("dhcp_rebind: could not allocate DHCP request\n"));
  }
  dhcp->tries++;
  msecs = dhcp->tries < 10 ? dhcp->tries * 1000 : 10 * 1000;
  dhcp->request_timeout = (msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS;
   LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_rebind(): set request timeout %"U16_F" msecs\n", msecs));
  return result;
}

/**
 * Release a DHCP lease.
 *
 * @param netif network interface which must release its lease
 */
err_t dhcp_release(struct netif *netif)
{
  struct dhcp *dhcp = netif->dhcp;
  err_t result;
  u16_t msecs;
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 3, ("dhcp_release()\n"));

  /* idle DHCP client */
  dhcp_set_state(dhcp, DHCP_OFF);
  /* clean old DHCP offer */
  dhcp->server_ip_addr.addr = 0;
  dhcp->offered_ip_addr.addr = dhcp->offered_sn_mask.addr = 0;
  dhcp->offered_gw_addr.addr = dhcp->offered_bc_addr.addr = 0;
  dhcp->offered_t0_lease = dhcp->offered_t1_renew = dhcp->offered_t2_rebind = 0;
  dhcp->dns_count = 0;
  
  /* create and initialize the DHCP message header */
  result = dhcp_create_request(netif);
  if (result == ERR_OK) {
    dhcp_option(dhcp, DHCP_OPTION_MESSAGE_TYPE, DHCP_OPTION_MESSAGE_TYPE_LEN);
    dhcp_option_byte(dhcp, DHCP_RELEASE);

    dhcp_option_trailer(dhcp);

    pbuf_realloc(dhcp->p_out, sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN + dhcp->options_out_len);

    udp_bind(dhcp->pcb, IP_ADDR_ANY, DHCP_CLIENT_PORT);
    udp_connect(dhcp->pcb, &dhcp->server_ip_addr, DHCP_SERVER_PORT);
    udp_send(dhcp->pcb, dhcp->p_out);
    dhcp_delete_request(netif);
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_release: RELEASED, DHCP_OFF\n"));
  } else {
    LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 2, ("dhcp_release: could not allocate DHCP request\n"));
  }
  dhcp->tries++;
  msecs = dhcp->tries < 10 ? dhcp->tries * 1000 : 10 * 1000;
  dhcp->request_timeout = (msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS;
  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | DBG_STATE, ("dhcp_release(): set request timeout %"U16_F" msecs\n", msecs));
  /* bring the interface down */
  netif_set_down(netif);
  /* remove IP address from interface */
  netif_set_ipaddr(netif, IP_ADDR_ANY);
  netif_set_gw(netif, IP_ADDR_ANY);
  netif_set_netmask(netif, IP_ADDR_ANY);
  
  /* TODO: netif_down(netif); */
  return result;
}
/**
 * Remove the DHCP client from the interface.
 *
 * @param netif The network interface to stop DHCP on
 */
void dhcp_stop(struct netif *netif)
{
  struct dhcp *dhcp = netif->dhcp;
  LWIP_ASSERT("dhcp_stop: netif != NULL", netif != NULL);

  LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 3, ("dhcp_stop()\n"));
  /* netif is DHCP configured? */
  if (dhcp != NULL)
  {
    if (dhcp->pcb != NULL)
    {
      udp_remove(dhcp->pcb);
      dhcp->pcb = NULL;
    }
    if (dhcp->p != NULL)
    {
      pbuf_free(dhcp->p);
      dhcp->p = NULL;
    }
    /* free unfolded reply */
    dhcp_free_reply(dhcp);
    mem_free((void *)dhcp);
    netif->dhcp = NULL;
  }
}

/*
 * Set the DHCP state of a DHCP client.
 *
 * If the state changed, reset the number of tries.
 *
 * TODO: we might also want to reset the timeout here?
 */
static void dhcp_set_state(struct dhcp *dhcp, u8_t new_state)
{
  if (new_state != dhcp->state)
  {
    dhcp->state = new_state;
    dhcp->tries = 0;
  }
}

/*
 * Concatenate an option type and length field to the outgoing
 * DHCP message.
 *
 */
static void dhcp_option(struct dhcp *dhcp, u8_t option_type, u8_t option_len)
{
  LWIP_ASSERT("dhcp_option_short: dhcp->options_out_len + 2 + option_len <= DHCP_OPTIONS_LEN", dhcp->options_out_len + 2 + option_len <= DHCP_OPTIONS_LEN);
  dhcp->msg_out->options[dhcp->options_out_len++] = option_type;
  dhcp->msg_out->options[dhcp->options_out_len++] = option_len;
}
/*
 * Concatenate a single byte to the outgoing DHCP message.
 *
 */
static void dhcp_option_byte(struct dhcp *dhcp, u8_t value)
{
  LWIP_ASSERT("dhcp_option_short: dhcp->options_out_len < DHCP_OPTIONS_LEN", dhcp->options_out_len < DHCP_OPTIONS_LEN);
  dhcp->msg_out->options[dhcp->options_out_len++] = value;
}
static void dhcp_option_short(struct dhcp *dhcp, u16_t value)
{
  LWIP_ASSERT("dhcp_option_short: dhcp->options_out_len + 2 <= DHCP_OPTIONS_LEN", dhcp->options_out_len + 2 <= DHCP_OPTIONS_LEN);
  dhcp->msg_out->options[dhcp->options_out_len++] = (value & 0xff00U) >> 8;
  dhcp->msg_out->options[dhcp->options_out_len++] =  value & 0x00ffU;
}
static void dhcp_option_long(struct dhcp *dhcp, u32_t value)
{
  LWIP_ASSERT("dhcp_option_long: dhcp->options_out_len + 4 <= DHCP_OPTIONS_LEN", dhcp->options_out_len + 4 <= DHCP_OPTIONS_LEN);
  dhcp->msg_out->options[dhcp->options_out_len++] = (value & 0xff000000UL) >> 24;
  dhcp->msg_out->options[dhcp->options_out_len++] = (value & 0x00ff0000UL) >> 16;
  dhcp->msg_out->options[dhcp->options_out_len++] = (value & 0x0000ff00UL) >> 8;
  dhcp->msg_out->options[dhcp->options_out_len++] = (value & 0x000000ffUL);
}

/**
 * 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 conitguous piece of memory, and
 * use that further on.
 *
 */
static err_t dhcp_unfold_reply(struct dhcp *dhcp)
{
  struct pbuf *p = dhcp->p;
  u8_t *ptr;
  u16_t i;
  u16_t j = 0;
  LWIP_ASSERT("dhcp->p != NULL", dhcp->p != NULL);
  /* free any left-overs from previous unfolds */
  dhcp_free_reply(dhcp);
  /* options present? */
  if (dhcp->p->tot_len > (sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN))
  {
    dhcp->options_in_len = dhcp->p->tot_len - (sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN);
    dhcp->options_in = mem_malloc(dhcp->options_in_len);
    if (dhcp->options_in == NULL)
    {
      LWIP_DEBUGF(DHCP_DEBUG | DBG_TRACE | 2, ("dhcp_unfold_reply(): could not allocate dhcp->options\n"));
      return ERR_MEM;
    }
  }
  dhcp->msg_in = mem_malloc(sizeof(struct dhcp_msg) - DHCP_OPTIONS_LEN);
  if (dhcp->msg_in == NULL)
  {