1.DHCP 简介

DHCP（Dynamic Host Configuration Protocol，动态主机配置协议）通常被应用在大型的局域网络环境中，主要作用是集中的管理、分配IP地址，使网络环境中的主机动态的获得IP地址、Gateway地址、DNS服务器地址等信息，并能够提升IP地址的使用率。

简单来说，DHCP就是一个不需要账号密码登录的、自动给内网机器分配IP地址等信息的协议。它遵循服务器-客户端模型，在WiFi架构中，AP（即热点，提供连网功能的设备）中运行DHCPS(即DHCP-Server)，而STA（即需要接入AP的设备）中运行DHCPC(即DHCP-Client)。通过如下图所示的连接过程，DHCPS将提供给DHCPC一个暂时租用的IP，以使得运行DHCPC的设备具备上网的能力。（注意，这个IP通常是以“租用”的方式给DHCPC使用，过段时间可能会回收再利用）

2.在ESP32中查看、修改DHCP的配置信息

DHCP的功能对于联网设备非常重要，ESP32的开发框架中提供了灵活的查看DHCP相关配置的接口，今天就来聊一聊如何使用这些接口。

不多说，上代码。（使用开发板为ESP32，SDK为ESP-IDF v3.1.7，开发模板为examples目录下的wifi/simple_wifi）

[code]/* Simple WiFi Example*/
#include <string.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/event_groups.h"
#include "esp_system.h"
#include "esp_wifi.h"
#include "esp_event_loop.h"
#include "esp_log.h"
#include "nvs_flash.h"

#include "lwip/err.h"
#include "lwip/sys.h"

/* The examples use simple WiFi configuration that you can set via
'make menuconfig'.

If you'd rather not, just change the below entries to strings with
the config you want - ie #define EXAMPLE_WIFI_SSID "mywifissid"
*/
#define EXAMPLE_ESP_WIFI_MODE_AP   CONFIG_ESP_WIFI_MODE_AP //TRUE:AP FALSE:STA
#define EXAMPLE_ESP_WIFI_SSID      CONFIG_ESP_WIFI_SSID
#define EXAMPLE_ESP_WIFI_PASS      CONFIG_ESP_WIFI_PASSWORD
#define EXAMPLE_MAX_STA_CONN       CONFIG_MAX_STA_CONN

/* FreeRTOS event group to signal when we are connected*/
static EventGroupHandle_t wifi_event_group;

/* The event group allows multiple bits for each event,
but we only care about one event - are we connected
to the AP with an IP? */
const int WIFI_CONNECTED_BIT = BIT0;

static const char *TAG = "simple wifi";

/* Use for DHCP */
tcpip_adapter_dhcp_status_t dhcp_get_status;
tcpip_adapter_option_id_t dhcp_op_id;
uint32_t op_len;
uint32_t op_val = -1;

static esp_err_t event_handler(void *ctx, system_event_t *event)
{
switch(event->event_id) {
case SYSTEM_EVENT_STA_START:
esp_wifi_connect();
break;
case SYSTEM_EVENT_STA_GOT_IP:
ESP_LOGI(TAG, "got ip:%s",
ip4addr_ntoa(&event->event_info.got_ip.ip_info.ip));
xEventGroupSetBits(wifi_event_group, WIFI_CONNECTED_BIT);
break;
case SYSTEM_EVENT_AP_STACONNECTED:
ESP_LOGI(TAG, "station:"MACSTR" join, AID=%d",
MAC2STR(event->event_info.sta_connected.mac),
event->event_info.sta_connected.aid);
/* Attention:you must call tcpip_adapter_dhcps_start(TCPIP_ADAPTER_IF_AP) to start dhcps when the dhcps
is stoped or it has not started yet,otherwise,you can not get the current option.Besides,you had better
call the sizeof() to get the current op_len,otherwise,you also can not get the currect option.*/
op_len = sizeof(dhcps_time_t);
/* get the option*/
tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_GET, TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME, &op_val, op_len);
ESP_LOGI(TAG, "dhcp_get_option = %d",
op_val);
break;
case SYSTEM_EVENT_AP_STADISCONNECTED:
ESP_LOGI(TAG, "station:"MACSTR"leave, AID=%d",
MAC2STR(event->event_info.sta_disconnected.mac),
event->event_info.sta_disconnected.aid);
break;
case SYSTEM_EVENT_STA_DISCONNECTED:
esp_wifi_connect();
xEventGroupClearBits(wifi_event_group, WIFI_CONNECTED_BIT);
break;
default:
break;
}
return ESP_OK;
}

void wifi_init_softap()
{
wifi_event_group = xEventGroupCreate();

tcpip_adapter_init();
ESP_ERROR_CHECK(esp_event_loop_init(event_handler, NULL));

wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
ESP_ERROR_CHECK(esp_wifi_init(&cfg));
wifi_config_t wifi_config = {
.ap = {
.ssid = EXAMPLE_ESP_WIFI_SSID,
.ssid_len = strlen(EXAMPLE_ESP_WIFI_SSID),
.password = EXAMPLE_ESP_WIFI_PASS,
.max_connection = EXAMPLE_MAX_STA_CONN,
.authmode = WIFI_AUTH_WPA_WPA2_PSK
},
};
if (strlen(EXAMPLE_ESP_WIFI_PASS) == 0) {
wifi_config.ap.authmode = WIFI_AUTH_OPEN;
}

ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_AP));
ESP_ERROR_CHECK(esp_wifi_set_config(ESP_IF_WIFI_AP, &wifi_config));
ESP_ERROR_CHECK(esp_wifi_start());

ESP_LOGI(TAG, "wifi_init_softap finished.SSID:%s password:%s",
EXAMPLE_ESP_WIFI_SSID, EXAMPLE_ESP_WIFI_PASS);
/*DHCP option,1.get_option 2.get_status.3.get_option.4.set_option.5.get_option*/
/*1.*/
op_len = sizeof(dhcps_time_t);
tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_GET, TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME, &op_val, op_len);
ESP_LOGI(TAG, "dhcp_get_option = %d",
op_val);
/*2.*/
ESP_ERROR_CHECK(tcpip_adapter_dhcps_get_status(TCPIP_ADAPTER_IF_AP, &dhcp_get_status));
ESP_LOGI(TAG, "dhcp_get_status = %d",
dhcp_get_status);
/*stop dhcps*/
tcpip_adapter_dhcps_stop(TCPIP_ADAPTER_IF_AP);
/*2.*/
ESP_ERROR_CHECK(tcpip_adapter_dhcps_get_status(TCPIP_ADAPTER_IF_AP, &dhcp_get_status));
ESP_LOGI(TAG, "dhcp_get_status = %d",
dhcp_get_status);

/*3.get the option*/
/* 3.1 Attention:you must call tcpip_adapter_dhcps_start(TCPIP_ADAPTER_IF_AP) to start dhcps when the dhcps
is stoped or it has not started yet,otherwise,you can not get the current option.Besides,you had better
call the sizeof() to get the current op_len,otherwise,you also can not get the currect option.*/
op_len = sizeof(dhcps_time_t);
tcpip_adapter_dhcps_start(TCPIP_ADAPTER_IF_AP);
/* 3.2 now ,you can get the option*/
tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_GET, TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME, &op_val, op_len);
ESP_LOGI(TAG, "dhcp_get_option = %d",
op_val);
/* 4.change the option,and set the option*/
/*4.1 change the option value*/
op_val = 110;
op_len = sizeof(dhcps_time_t);
/*4.2 Attention:you must ensure that dhcps has been stoped,and then you can set the option*/
tcpip_adapter_dhcps_stop(TCPIP_ADAPTER_IF_AP);
tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_SET, TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME, &op_val, op_len);
/*5.get the option after changing the option,please start the dhcps before get the option*/
tcpip_adapter_dhcps_start(TCPIP_ADAPTER_IF_AP);
tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_GET, TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME, &op_val, op_len);
ESP_LOGI(TAG, "dhcp_get_option = %d",
op_val);
/*you cann't try to get the Domain_name,because it's nothing*/
// char op_dhcp_name[64];
// op_len = sizeof(dhcps_offer_t);
// tcpip_adapter_dhcps_option(TCPIP_ADAPTER_OP_GET, TCPIP_ADAPTER_DOMAIN_NAME_SERVER, op_dhcp_name, op_len);
// ESP_LOGI(TAG, "dhcp_get_option = %s",op_dhcp_name);

}

void wifi_init_sta()
{
wifi_event_group = xEventGroupCreate();

tcpip_adapter_init();
ESP_ERROR_CHECK(esp_event_loop_init(event_handler, NULL) );

wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
ESP_ERROR_CHECK(esp_wifi_init(&cfg));
wifi_config_t wifi_config = {
.sta = {
.ssid = EXAMPLE_ESP_WIFI_SSID,
.password = EXAMPLE_ESP_WIFI_PASS
},
};

ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA) );
ESP_ERROR_CHECK(esp_wifi_set_config(ESP_IF_WIFI_STA, &wifi_config) );
ESP_ERROR_CHECK(esp_wifi_start() );

ESP_LOGI(TAG, "wifi_init_sta finished.");
ESP_LOGI(TAG, "connect to ap SSID:%s password:%s",
EXAMPLE_ESP_WIFI_SSID, EXAMPLE_ESP_WIFI_PASS);
}

void app_main()
{
//Initialize NVS
esp_err_t ret = nvs_flash_init();
if (ret == ESP_ERR_NVS_NO_FREE_PAGES || ret == ESP_ERR_NVS_NEW_VERSION_FOUND) {
ESP_ERROR_CHECK(nvs_flash_erase());
ret = nvs_flash_init();
}
ESP_ERROR_CHECK(ret);

#if EXAMPLE_ESP_WIFI_MODE_AP
ESP_LOGI(TAG, "ESP_WIFI_MODE_AP");
wifi_init_softap();
#else
ESP_LOGI(TAG, "ESP_WIFI_MODE_STA");
wifi_init_sta();
#endif /*EXAMPLE_ESP_WIFI_MODE_AP*/

}

代码实现上，主要是在wifi_init_softap()中添加一系列操作DHCPS的代码，这些代码我添加了注释，可以帮助理解。特别提醒，ESP32应工作在AP模式下（因为，如你所见，我仅在AP的初始化代码wifi_init_softap()中添加了对DHCP的操作），因此在编译时请通过menuconfig将ESP32开发板配置为AP工作模式。

需要提醒的是：

在读取DHCPS的option（即配置选项）时，必须在DHCPS启动状态下读取，并且传入的option_len(即选项长度）要获取正确，再传入，否则会读取失败。

在设置DHCPS的option时，必须在关闭DHCPS的状态下，进行设置，同样的，必须传入正确的option_len(即选项长度）参数，否则设置失败。

设置成功后应当先使用tcpip_adapter_dhcps_start(TCPIP_ADAPTER_IF_AP);重新启动DHCPS，才能进行查询。

3.实验结果与分析

WiFi初始化完成后，将自动启动DHCP，实验中获取的option（即配置参数）为 TCPIP_ADAPTER_IP_ADDRESS_LEASE_TIME，即DHCPS分配给DHCPC的IP的租用时间（当然，你也可以获取DHCP的其他配置参数，原理是一样的，这里是举个例子）。

上面的打印信息依次是：

1.原来的租用时间为120min（是的，我查了原文件，初始化配置为120分钟的有效租用时间）

2.DHCPS当前状态为1，即启动状态（可以查看源文件：1代表的宏是Start状态）

3.DHCPS当前状态为2，即停止状态（因为我前面调用了Stop（)函数）

4.再次查看修改前的option为120min

5.查看修改后的option为110min，即完成修改。

（最后，ESP8266提供了上述操作DHCP的接口，但函数的名字和传的参数类型与ESP32有所不同，因此，若是使用ESP8266作为实验平台，请注意调整对应的函数名和传入参数。原理与注意事项是一模一样的）