auto reconnect stopped after a few times

[tdk2nyt]

mosquitto version: 2.0.15
system: Linux kernel 3.4.40

Hi,

I use libmosquitto as MQTT client. One day, I found MQTT connection lost, I checked log that recorded a few reconnection events, but it stopped to reconnect with Unable to connect (Lookup error.)
In log, there is a strange scenario: the app try to subscribe topic, this is invoked in on_connect callback, subscribe fails with code 4(MOSQ_ERR_NO_CONN)

I'm using these APIs:
MQTT_PROTOCOL_V311
mosquitto_connect_bind_async
mosquitto_loop_start
mosquitto_connect_v5_callback_set
mosquitto_subscribe_v5_callback_set
mosquitto_message_v5_callback_set
mosquitto_disconnect_v5_callback_set
mosquitto_publish_v5_callback_set

Any suggestion is appreciated, thanks!