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DHCP

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2025/02/12 更新

本篇文章我们将详细讲解DHCP协议的基本信息、优势特点、工作原理、应用场景，并通过实战例程， 为大家讲解如何使用W5500动态获取IP信息，帮助读者更好地了解并运用这一协议。

有关初始化过程，请参考Network install章节，这里将不再详述。

DHCP协议简介

DHCP（Dynamic Host Configuration Protocol）即动态主机配置协议，是一个应用层协议。它主要 用于在 IP 网络中为客户端自动分配 IP 地址及其他相关网络配置参数，如子网掩码、默认网关、DNS 服务器地址等。这种动态分配的方式大大简化了网络管理员的工作，并且能够更有效地利用有限的 IP 地址资源。

DHCP协议特点

  • 便捷配置与管控：DHCP可自动分配IP地址、子网掩码、网关、DNS等网络参数，设备入网即 自动获取，用户无需手动操作。管理员能通过服务器集中管理IP分配，网络架构调整时，修改 服务器设置，客户端自动适配；静态IP则要逐台手动输入、调整，流程繁琐还易出错。
  • 灵活资源利用：DHCP动态分配IP，设备离线后地址回池再利用，契合公共场所临时大量接入 需求，提升地址利用率；还能按需灵活调配，为关键设备保留静态IP，其余动态分配。静态IP 固定占用，闲置浪费资源，灵活性差。
  • 高效维护与排障：DHCP自动分配，规避手动配置错误与IP地址冲突，服务器详细记录分配情 况。网络故障时，管理员依服务器日志锁定故障设备排查；静态IP手动配置易冲突，故障排查缺少有效记录，难度大、耗时久。
  • 适配移动与拓展：移动设备横跨网络时，DHCP让其自动获取IP配置，无需手动切换；网络规 模扩大、新增设备时，DHCP自动分配地址，助力快速扩容。静态IP需提前规划，易现地址不足、分配不合理问题，还增加设备移动操作难。

DHCP工作原理

DHCP工作原理如图所示：

从图示中我们可以直观明了地看出DHCP地工作原理，一般为四个阶段：

  1. 发现阶段：客户端接入网络时以广播形式发送DHCP Discover报文 （目的IP是255.255.255.255，源IP是0.0.0.0）寻找DHCP服务器，报文中含客户端MAC地址。若服务器和客户端不在同一子网，会通过中继代理（如路由器）转发。
  2. 提供阶段：DHCP服务器收到Discover报文后， 从IP地址池选一个未分配的IP地址，将其和子网掩码、默认网关、DNS服务器地址等信息封装进DHCP Offer报文，以广播或单播方式发给客户端，可能会有多个Offer报文。
  3. 请求阶段：客户端收到多个Offer后选择一个，以广播形式发送DHCP Request报文请求分配该IP地址等配置信息，且发送ARP请求检查IP地址唯一性。
  4. 确认阶段：服务器收到Request报文后，检查IP地址是否可用。若可用， 将以广播或单播的形式发送DHCP Ack报文，客户端收到后完成网络配置正常上网。若不可用，发送DHCP Nak报文，客户端收到后重新发起Discover流程。

DHCP协议报文

DHCP的报文格式如下：

DHCP 报文各字段的说明如下表所示：

Discover 报文实例：客户端通过 UDP 广播的方式发送 DHCP 发现报文，报文中包含了客户端 MAC 地址、主机名和请求的 IP 地址等信息 


|报文解析|
Message type: Boot Request (1) （op code为1，客户端请求报文）
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0xbf600589
Seconds elapsed: 0
Bootp flags: 0x0000 (Unicast)
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 0.0.0.0
Client MAC address: HP_b1:37:11 (64:4e:d7:b1:37:11)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Option: (61) Client identifier
Option: (50) Requested IP Address (192.168.1.115)
Option: (12) Host Name
Option: (60) Vendor class identifier
Option: (55) Parameter Request List
Option: (255) End
Padding: 0000000000000000

|报文原文|
01 01 06 00 bf 60 05 89 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 64 4e
d7 b1 37 11 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 63 82 53 63
35 01 01 3d 07 01 64 4e d7 b1 37 11 32 04 c0 a8 01 73 0c 05 46 41 45 5f 33 3c 08 4d 53 46
54 20 35 2e 30 37 0e 01 03 06 0f 1f 21 2b 2c 2e 2f 77 79 f9 fc ff 00 00 00 00 00 00 00 00

Offer报文实例：DHCP服务器收到Discover报文后，从IP地址池选一个未分配的IP地址，将其和子网掩码、默认网关等信息封装进Offer报文以广播或单播（这里为广播的方式）方式发给客户端 


|报文解析|
Message type: Boot Reply (2) （op code为2，服务器响应报文）
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0xbf600589
Seconds elapsed: 0
Bootp flags: 0x0000 (Unicast)
Client IP address: 0.0.0.0
Your (client) IP address: 192.168.1.115
Next server IP address: 0.0.0.0
Relay agent IP address: 0.0.0.0
Client MAC address: HP_b1:37:11 (64:4e:d7:b1:37:11)
Client hardware address padding: 00000000000000000000
Server host name not given
  Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Offer)
Option: (54) DHCP Server Identifier (192.168.1.1)
  Option: (51) IP Address Lease Time
  Option: (6) Domain Name Server
Option: (1) Subnet Mask (255.255.255.0)
Option: (3) Router
Option: (255) End

|报文原文|
02 01 06 00 bf 60 05 89 00 00 00 00 00 00 00 00 c0 a8 01 73 00 00 00 00 00 00 00 00 64 4e
d7 b1 37 11 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 63 82 53 63
35 01 02 36 04 c0 a8 01 01 33 04 00 00 1c 20 06 08 ca 60 86 21 ca 60 80 56 01 04 ff ff ff
00 03 04 c0 a8 01 01 ff

其他报文信息这里就不一一展示了，感兴趣的朋友可以自行抓取查看。

DHCP 应用场景

DHCP 的应用场景通常集中在需要动态分配 IP 地址的局域网环境中。例如，在大型的办公环境或者 学校中，由于有大量的网络设备需要连接到网络，手动为每个设备分配和管理 IP 地址会非常繁琐且 容易出错。使用 DHCP 可以集中管理 IP 地址的分配，提高网络管理员的工作效率，减少错误的发生， 且可以适应网络变化。

实现过程

接下来，我们一起来看看如何在 W5500 上实现 DHCP 动态获取网络地址信息。

步骤1：注册 DHCP 定时器中断到 1s 定时器中： 


/**
* @brief   Hardware Platform Timer Interrupt Callback Function
*/
void TIM2_IRQHandler(void)
{
    static uint32_t wiz_timer_1ms_count = 0;
    if (TIM_GetITStatus(TIM2, TIM_IT_Update) == SET)
    {
        wiz_timer_1ms_count++;
        if (wiz_timer_1ms_count >= 1000)
        {
            DHCP_time_handler();
            wiz_timer_1ms_count = 0;
        }
        TIM_ClearITPendingBit(TIM2, TIM_IT_Update);
    }
}

注册 DHCP 定时器中断主要为了 DHCP 超时处理。 在 dhcp.h 文件中，定义了 DHCP 超时时间、重试次数、端口号和主机名等内容： 


/* Retry to processing DHCP */
#define	MAX_DHCP_RETRY          2        ///< Maximum retry count
#define	DHCP_WAIT_TIME          10       ///< Wait Time 10s


/* UDP port numbers for DHCP */
#define DHCP_SERVER_PORT      	67	      ///< DHCP server port number
#define DHCP_CLIENT_PORT         68	      ///< DHCP client port number


#define MAGIC_COOKIE             0x63825363  ///< You should not modify it number.

#define DCHP_HOST_NAME           "WIZnet\0"

步骤2：启用DHCP动态获取：

首先需要将默认网络地址信息结构体中的模式改为DHCP模式 


/* network information */
wiz_NetInfo default_net_info = {
    .mac  = {0x00, 0x08, 0xdc, 0x12, 0x22, 0x12},
    .ip   = {192, 168, 1, 30},
    .gw   = {192, 168, 1, 1},
    .sn   = {255, 255, 255, 0},
    .dns  = {8, 8, 8, 8},
    .dhcp = NETINFO_DHCP
};

请注意:

使用 DHCP 动态获取 IP 时，必需将网络结构体配置中 dhcp 的值改为 NETINFO_DHCP，这样 才能运行 DHCP 模式。

步骤3：DHCP获取网络地址信息

首先是在初始化完硬件和W5500之后调用network_init进行网络地址信息配置 


network_init(ethernet_buf, &default_net_info);

这个函数需要将DHCP处理用到的缓存数组以及默认网络地址信息传入，函数具体内容如下： 


/**
* @brief   set network information
*
* First determine whether to use DHCP. If DHCP is used, first obtain the Internet Protocol Address through DHCP.
* When DHCP fails, use static IP to configure network information. If static IP is used, configure network information directly
*
* @param   sn: socketid
* @param   ethernet_buff:
* @param   net_info: network information struct
* @return  none
*/
void network_init(uint8_t *ethernet_buff, wiz_NetInfo *conf_info)
{
    int ret;
    wizchip_setnetinfo(conf_info); // Configuring Network Information
    if (conf_info->dhcp == NETINFO_DHCP)
    {
        ret = wiz_dhcp_process(0, ethernet_buff);
        if (ret == 0)
        {
            conf_info->dhcp = NETINFO_STATIC;
            wizchip_setnetinfo(conf_info);
        }
    }
    print_network_information();
}

在这个函数中，会先设置一遍网络地址到W5500中，然后判断模式是否为DHCP模式，如果为DHCP模式，则使用wiz_dhcp_process函数来执行DHCP进程， 在通过DHCP方式成功获取到网络地址后更新到W5500中，最后将网络地址信息打印出来。wiz_dhcp_process函数内容如下： 


/**
* @brief DHCP process
* @param sn :socket number
* @param buffer :socket buffer
*/
static uint8_t wiz_dhcp_process(uint8_t sn, uint8_t *buffer)
{
    wiz_NetInfo conf_info;
    uint8_t     dhcp_run_flag = 1;
    uint8_t     dhcp_ok_flag  = 0;
    /* Registration DHCP_time_handler to 1 second timer */
    DHCP_init(sn, buffer);
    printf("DHCP running\r\n");
    while (1)
    {
        switch (DHCP_run())  // Do the DHCP client
        {
        case DHCP_IP_LEASED: // DHCP Acquiring network information successfully

            if (dhcp_ok_flag == 0)
            {
                dhcp_ok_flag  = 1;
                dhcp_run_flag = 0;
            }
            break;

        case DHCP_FAILED:
            dhcp_run_flag = 0;
            break;
        }
        if (dhcp_run_flag == 0)
        {
            printf("DHCP %s!\r\n", dhcp_ok_flag ? "success" : "fail");
            DHCP_stop();

            if (dhcp_ok_flag)
            {
                getIPfromDHCP(conf_info.ip);
                getGWfromDHCP(conf_info.gw);
                getSNfromDHCP(conf_info.sn);
                getDNSfromDHCP(conf_info.dns);
                conf_info.dhcp = NETINFO_DHCP;
                getSHAR(conf_info.mac);
                wizchip_setnetinfo(&conf_info); // Update network information to network information obtained by DHCP
                return 1;
            }
            return 0;
        }
    }
}

在该函数体中，首先会调用DHCP_init 函数进行初始化DHCP配置： 


void DHCP_init(uint8_t s, uint8_t * buf)
{
    uint8_t zeroip[4] = {0,0,0,0};
    getSHAR(DHCP_CHADDR);
    if((DHCP_CHADDR[0] | DHCP_CHADDR[1]  | DHCP_CHADDR[2] | DHCP_CHADDR[3] | DHCP_CHADDR[4] | DHCP_CHADDR[5]) == 0x00)
    {
      // assigning temporary mac address, you should be set SHAR before call this function. 
      DHCP_CHADDR[0] = 0x00;
      DHCP_CHADDR[1] = 0x08;
      DHCP_CHADDR[2] = 0xdc;      
      DHCP_CHADDR[3] = 0x00;
      DHCP_CHADDR[4] = 0x00;
      DHCP_CHADDR[5] = 0x00; 
      setSHAR(DHCP_CHADDR);     
    }

  DHCP_SOCKET = s; // SOCK_DHCP
  pDHCPMSG = (RIP_MSG*)buf;
  DHCP_XID = 0x12345678;
  {
    DHCP_XID += DHCP_CHADDR[3];
    DHCP_XID += DHCP_CHADDR[4];
    DHCP_XID += DHCP_CHADDR[5];
    DHCP_XID += (DHCP_CHADDR[3] ^ DHCP_CHADDR[4] ^ DHCP_CHADDR[5]);
  }
  // WIZchip Netinfo Clear
  setSIPR(zeroip);
  setGAR(zeroip);

  reset_DHCP_timeout();
  dhcp_state = STATE_DHCP_INIT;
}

然后是在DHCP主循环中运行DHCP_run函数，它的主要作用是进行DHCP组包，发送发现、请求等报文，对服务器的提供、 响应等内容进行解析以及超时处理，这里只需要根据DHCP_run函数的返回值进行相应处理即可。DHCP_run函数内容如下： 


uint8_t DHCP_run(void)
{
  uint8_t  type;
  uint8_t  ret;

  if(dhcp_state == STATE_DHCP_STOP) return DHCP_STOPPED;

  if(getSn_SR(DHCP_SOCKET) != SOCK_UDP)
      socket(DHCP_SOCKET, Sn_MR_UDP, DHCP_CLIENT_PORT, 0x00);

  ret = DHCP_RUNNING;
  type = parseDHCPMSG();

  switch ( dhcp_state ) {
      case STATE_DHCP_INIT     :
          DHCP_allocated_ip[0] = 0;
          DHCP_allocated_ip[1] = 0;
          DHCP_allocated_ip[2] = 0;
          DHCP_allocated_ip[3] = 0;
        send_DHCP_DISCOVER();
        dhcp_state = STATE_DHCP_DISCOVER;
        break;
    case STATE_DHCP_DISCOVER :
      if (type == DHCP_OFFER){
#ifdef _DHCP_DEBUG_
        printf("> Receive DHCP_OFFER\r\n");
#endif
            DHCP_allocated_ip[0] = pDHCPMSG->yiaddr[0];
            DHCP_allocated_ip[1] = pDHCPMSG->yiaddr[1];
            DHCP_allocated_ip[2] = pDHCPMSG->yiaddr[2];
            DHCP_allocated_ip[3] = pDHCPMSG->yiaddr[3];

        send_DHCP_REQUEST();
        dhcp_state = STATE_DHCP_REQUEST;
      } else ret = check_DHCP_timeout();
          break;

    case STATE_DHCP_REQUEST :
      if (type == DHCP_ACK) {

#ifdef _DHCP_DEBUG_
        printf("> Receive DHCP_ACK\r\n");
#endif
        if (check_DHCP_leasedIP()) {
          // Network info assignment from DHCP
          dhcp_ip_assign();
          reset_DHCP_timeout();

          dhcp_state = STATE_DHCP_LEASED;
        } else {
          // IP address conflict occurred
          reset_DHCP_timeout();
          dhcp_ip_conflict();
            dhcp_state = STATE_DHCP_INIT;
        }
      } else if (type == DHCP_NAK) {

#ifdef _DHCP_DEBUG_
        printf("> Receive DHCP_NACK\r\n");
#endif

        reset_DHCP_timeout();

        dhcp_state = STATE_DHCP_DISCOVER;
      } else ret = check_DHCP_timeout();
    break;

    case STATE_DHCP_LEASED :
        ret = DHCP_IP_LEASED;
      if ((dhcp_lease_time != INFINITE_LEASETIME) && ((dhcp_lease_time/2) < dhcp_tick_1s)) {
        
#ifdef _DHCP_DEBUG_
          printf("> Maintains the IP address \r\n");
#endif

        type = 0;
        OLD_allocated_ip[0] = DHCP_allocated_ip[0];
        OLD_allocated_ip[1] = DHCP_allocated_ip[1];
        OLD_allocated_ip[2] = DHCP_allocated_ip[2];
        OLD_allocated_ip[3] = DHCP_allocated_ip[3];
        
        DHCP_XID++;

        send_DHCP_REQUEST();

        reset_DHCP_timeout();

        dhcp_state = STATE_DHCP_REREQUEST;
      }
    break;

    case STATE_DHCP_REREQUEST :
        ret = DHCP_IP_LEASED;
      if (type == DHCP_ACK) {
        dhcp_retry_count = 0;
        if (OLD_allocated_ip[0] != DHCP_allocated_ip[0] || 
            OLD_allocated_ip[1] != DHCP_allocated_ip[1] ||
            OLD_allocated_ip[2] != DHCP_allocated_ip[2] ||
            OLD_allocated_ip[3] != DHCP_allocated_ip[3]) 
        {
          ret = DHCP_IP_CHANGED;
          dhcp_ip_update();
                #ifdef _DHCP_DEBUG_
                  printf(">IP changed.\r\n");
                #endif
          
        }
          #ifdef _DHCP_DEBUG_
            else printf(">IP is continued.\r\n");
          #endif            				
        reset_DHCP_timeout();
        dhcp_state = STATE_DHCP_LEASED;
      } else if (type == DHCP_NAK) {

#ifdef _DHCP_DEBUG_
        printf("> Receive DHCP_NACK, Failed to maintain ip\r\n");
#endif

        reset_DHCP_timeout();

        dhcp_state = STATE_DHCP_DISCOVER;
      } else ret = check_DHCP_timeout();
        break;
    default :
        break;
  }

  return ret;
}

运行结果

请注意:

测试实例需要PC端和W5500处于同一网段。

烧录例程运行后，首先打印了PHY链路检测的结果以及DHCP日志信息， 然后打印了网络地址信息，这里可以看到配置方式为DHCP，IP地址为192.168.1.137，最后是PING提示消息。 接着在PC端打开CMD，PING W5500的IP地址，可以正常PING通。

总结

本文介绍 DHCP 协议，包括其在 IP 网络自动分配参数的功能、便捷配置等特点、工作原理、报文格式和应用场景。通过 W5500 实战例程展示动态获取网络地址信息过程，含注册定时器中断、启用模式和获取信息等步骤，烧录后可完成检测与信息打印，PC 端能 PING 通设备。 下一篇文章将讲解如何在 W5500 芯片上实现 TCP 客户端模式，解析 TCP 客户端连接服务器进行回环测试的核心原理及应用，同时通过实战例程讲解具体实现步骤与要点，敬请期待！

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