不用记 IP：用 mDNS 找到 ESP32，并准备发现失败的后路

idf.py --version
idf.py set-target esp32s3
idf.py menuconfig
idf.py build
idf.py -p REPLACE_WITH_UART_PORT flash monitor

// SPDX-License-Identifier: MIT
#pragma once
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
static inline bool discovery_hostname(char *out, size_t cap, const char *prefix, const uint8_t mac[6]) {
 if (!out || !cap || !prefix || !mac) return false;
 out[0]='\0'; size_t n=strlen(prefix);
 if (n<1 || n>56 || prefix[0]=='-' || prefix[n-1]=='-') return false;
 for(size_t i=0;i<n;i++) if (!((prefix[i]>='a'&&prefix[i]<='z') || (prefix[i]>='0'&&prefix[i]<='9') || prefix[i]=='-')) return false;
 int k=snprintf(out,cap,"%s-%02x%02x%02x",prefix,mac[3],mac[4],mac[5]);
 if(k<0 || (size_t)k>=cap) {out[0]='\0'; return false;} return true;
}

// SPDX-License-Identifier: MIT
#include <stdio.h>
#include <string.h>
#include "freertos/FreeRTOS.h"
#include "freertos/queue.h"
#include "esp_http_server.h"
#include "esp_log.h"
#include "esp_mac.h"
#include "esp_netif.h"
#include "mdns.h"
#include "sdkconfig.h"
#include "net.h"
#include "discovery_policy.h"
static const char *TAG="N06";
#ifdef CONFIG_DEMO_DISABLE_MDNS
static const bool disable_mdns=true;
#else
static const bool disable_mdns=false;
#endif
typedef struct {char host[64], ip[16]; bool online, registered;} snapshot_t;
static QueueHandle_t snapshots;
static const char page[]="<!doctype html><html lang=en><meta charset=utf-8><meta name=viewport content='width=device-width,initial-scale=1'><title>N06 discovery</title><h1>ESP32 discovery status</h1><p>This page is read-only. Registration does not prove client discovery.</p><pre id=s>Loading /status</pre><p>Fallback: use the current IP printed on the serial console.</p><script>async function tick(){try{let r=await fetch('/status',{cache:'no-store'});if(!r.ok)throw Error(r.status);document.getElementById('s').textContent=JSON.stringify(await r.json(),null,2)}catch(e){document.getElementById('s').textContent='Connection unavailable: '+e.message}}tick();setInterval(tick,2000)</script></html>";
static esp_err_t root_get(httpd_req_t *req) {httpd_resp_set_type(req,"text/html; charset=utf-8"); return httpd_resp_send(req,page,HTTPD_RESP_USE_STRLEN);}
static esp_err_t status_get(httpd_req_t *req) {
 snapshot_t s={0}; if(xQueuePeek(snapshots,&s,0)!=pdTRUE) return httpd_resp_send_err(req,HTTPD_500_INTERNAL_SERVER_ERROR,"snapshot not ready");
 char body[256]; int n=snprintf(body,sizeof body,"{\"configured_host\":\"%s.local\",\"ip\":\"%s\",\"online\":%s,\"service_registered\":%s,\"client_discovery\":\"not-measured\"}",s.host,s.ip,s.online?"true":"false",s.registered?"true":"false");
 if(n<0 || n>=(int)sizeof body) return httpd_resp_send_err(req,HTTPD_500_INTERNAL_SERVER_ERROR,"snapshot overflow");
 httpd_resp_set_type(req,"application/json"); httpd_resp_set_hdr(req,"Cache-Control","no-store"); return httpd_resp_send(req,body,n);
}
static bool start_http(httpd_handle_t *server) {
 httpd_config_t cfg=HTTPD_DEFAULT_CONFIG();cfg.server_port=80;
 esp_err_t err=httpd_start(server,&cfg);
 if(err==ESP_OK) {httpd_uri_t root={.uri="/",.method=HTTP_GET,.handler=root_get};httpd_uri_t status={.uri="/status",.method=HTTP_GET,.handler=status_get};err=httpd_register_uri_handler(*server,&root);if(err==ESP_OK)err=httpd_register_uri_handler(*server,&status);}
 if(err!=ESP_OK) {ESP_LOGE(TAG,"HTTP start=%s",esp_err_to_name(err));if(*server){httpd_stop(*server);*server=NULL;}return false;}return true;
}
static bool start_mdns(const char *host) {
 esp_err_t err=mdns_init(); bool initialized=err==ESP_OK;
 if(err==ESP_OK)err=mdns_hostname_set(host);
 if(err==ESP_OK)err=mdns_instance_name_set(host);
 mdns_txt_item_t txt[]={{"path","/"},{"board","s3-n8r8"},{"sdk","idf-5.5.5"}};
 if(err==ESP_OK)err=mdns_service_add(NULL,"_http","_tcp",80,txt,3);
 if(err!=ESP_OK){ESP_LOGW(TAG,"mDNS registration=%s; HTTP/IP retained",esp_err_to_name(err));if(initialized)mdns_free();return false;}
 ESP_LOGI(TAG,"service_registered=true configured_host=%s.local _http._tcp:80; client discovery NOT measured",host);return true;
}
void app_main(void) {
 snapshots=xQueueCreate(1,sizeof(snapshot_t));if(!snapshots){ESP_LOGE(TAG,"queue allocation failed");return;}
 uint8_t mac[6]; ESP_ERROR_CHECK(esp_read_mac(mac,ESP_MAC_WIFI_STA));
 snapshot_t s={.ip="0.0.0.0"}; bool name_ok=discovery_hostname(s.host,sizeof s.host,CONFIG_DEMO_HOST_PREFIX,mac);
 if(!name_ok){strcpy(s.host,"invalid-config");ESP_LOGE(TAG,"invalid prefix: mDNS disabled, HTTP/IP fallback retained");}
 xQueueOverwrite(snapshots,&s);net_start();httpd_handle_t server=NULL;unsigned retry=0;bool was_online=false;
 for(;;){
  s.online=net_online();
  if(s.online){
   esp_netif_ip_info_t ip;esp_netif_t *sta=esp_netif_get_handle_from_ifkey("WIFI_STA_DEF");
   if(sta && esp_netif_get_ip_info(sta,&ip)==ESP_OK){char current[16];snprintf(current,sizeof current,IPSTR,IP2STR(&ip.ip));if(!was_online||strcmp(s.ip,current)){strcpy(s.ip,current);ESP_LOGI(TAG,"IP fallback http://%s/ configured_name=%s.local",s.ip,s.host);}}
   if(!server)start_http(&server);
   // HTTP is established first and is independent of mDNS.
   if(server && name_ok && !disable_mdns && !s.registered && retry==0){s.registered=start_mdns(s.host);retry=10;}
  }
  if(retry)--retry;
  xQueueOverwrite(snapshots,&s);was_online=s.online;
  vTaskDelay(pdMS_TO_TICKS(1000));
 }
}

curl --max-time 5 http://REPLACE_WITH_CURRENT_IP/status
dns-sd -B _http._tcp local
dns-sd -L 'REPLACE_WITH_ACTUAL_INSTANCE_NAME' _http._tcp local
dns-sd -G v4 REPLACE_WITH_ACTUAL_HOST.local

cc -std=c11 -Wall -Wextra -Werror host_check.c -o /tmp/n06-host
/tmp/n06-host