Short answer

ESP32 MQTT client never stays connected. What usually breaks?

  • MQTT needs a working Station IP and a reachable broker. If DNS or the port is blocked, the client retries forever with little in the log.
  • Keep-alive, client ID clashes, and a broker that drops idle sockets look like ‘random disconnects’.
  • TLS time is wrong without SNTP (or an RTC). Certificates fail first; the MQTT library then looks guilty.
  • For a product, topics, last will, and back-off belong in firmware design — the official TCP example only proves the pipe.

中文

短答

ESP32 MQTT 总连不上或掉线，常见是什么？

  • 先要有 Station IP，broker 还要达得到。DNS 或端口被拦时，客户端会一直重试，日志里信息很少。
  • keepalive、client ID 冲突、broker 踢空闲连接，看起来都像随机掉线。
  • 没有 SNTP（或 RTC）时 TLS 时间是错的。先是证书失败，再被怪到 MQTT 库。
  • 产品里主题、遗愿和退避要写进固件设计。官方 TCP 例程只证明这条管道能通。