MQTT 3.1.1 Clean Session连接断开后行为不符合预期

核心问题：测试场景不符合预期 #

你在测试中错误断开了发布者机器的以太网，但订阅者与Broker在同一机器，通过本地回环接口（127.0.0.1）通信，物理以太网断开不会影响本地连接——订阅者始终与Broker保持在线状态。因此发布者重连后发送的缓存消息会被Broker直接转发给订阅者，这与clean session特性无关。

原因拆解 #

1. 本地回环不受以太网断开影响：订阅者与Broker的连接基于127.0.0.1，属于本地通信，断开物理以太网不会中断该连接，Broker始终判定订阅者在线。
2. TCP缓存机制：发布者断连时发送的QoS0消息会被操作系统TCP栈缓存，待网络恢复后，TCP会自动将缓存消息发送给Broker。
3. clean session的作用边界：clean session仅在订阅者真正断开与Broker的连接且会话超时后生效——Broker会清除该订阅者的会话状态（包括订阅关系），此时收到的消息会被丢弃。但你的测试中订阅者从未断开，clean session未触发作用。

正确测试方法与解决方案 #

1. 模拟订阅者与Broker的真实断开 #

要验证clean session的效果，需让订阅者主动断开与Broker的连接，例如：

• 直接停止mosquitto_sub进程；
• 使用工具阻断本地连接，如执行tcpkill -i lo port 1883（需安装tcpkill）；
• 临时禁用回环接口（谨慎操作：执行sudo ifconfig lo down，测试后恢复sudo ifconfig lo up）。

2. 配置短Keepalive加速断开检测 #

为让Broker快速识别订阅者离线，给订阅者设置短Keepalive时间：

mosquitto_sub -q 0 -t test -k 5

Broker会在1.5倍Keepalive时间（即7.5秒）后判定订阅者离线，清除其会话（因clean session=true）。此时发布者发送的消息会被Broker直接丢弃，订阅者重连后不会收到断开期间的消息。

3. 理解QoS与消息流转逻辑 #

• QoS0消息是“至多一次”，发布者不等待Broker确认，TCP会负责缓存重传；若需避免断连时的消息缓存，需发布者主动检测网络状态（如通过Keepalive）并停止发送。
• 若使用QoS1/2，Broker会缓存消息直到订阅者上线，但clean session=true时，订阅者断开后Broker会清除会话，缓存的消息也会被丢弃。

原始测试信息 #

测试步骤

• 收发机器通过以太网连接，模拟断连场景
• 接收端启动Mosquitto：mosquitto -v -c /etc/mosquitto/mosquitto.conf
• 接收端启动mosquitto_sub：mosquitto_sub -q 0 -t test
• 发送端启动终端模式的mosquitto_pub：mosquitto_pub -q 0 -t test -d -h <ipAddress> -l
• 发送3条消息，订阅者正常接收
• 手动断开以太网电缆
• 再发送3条消息，订阅者未接收
• 重新连接电缆，预期订阅者不会收到最后3条消息，但实际仍收到。

补充信息

• 订阅者和发布者均使用QoS 0，尝试强制消息不保留，QoS1/2组合测试结果一致
• 根据Mosquitto文档，broker、发布者、订阅者默认启用clean session，无显式设置选项，仅可通过持久化标志禁用
• 曾尝试为接收端设置短keepalive（1s），但mosquitto_pub的-l和-k参数无法同时使用

日志

mosquitto_pub日志

Client mosq-q9b2Lbd1tARoQKJssX sending CONNECT
Client mosq-q9b2Lbd1tARoQKJssX received CONNACK (0)
one
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m1, 'test', ... (3 bytes))
two
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m2, 'test', ... (3 bytes))
three
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m3, 'test', ... (5 bytes))
#此处断开电缆并发送最后3条消息
4
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m4, 'test', ... (1 bytes))
five
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m5, 'test', ... (4 bytes))
six
Client mosq-q9b2Lbd1tARoQKJssX sending PUBLISH (d0, q0, r0, m6, 'test', ... (3 bytes))

mosquitto日志

1719010537: mosquitto version 1.4.15 (build date Tue, 18 Jun 2019 11:42:22 -0300) starting
1719010537: Config loaded from /etc/mosquitto/mosquitto.conf.
1719010537: Opening websockets listen socket on port 9001.
1719010537: Opening ipv4 listen socket on port 1883.
1719010537: Opening ipv6 listen socket on port 1883.
1719010537: New connection from <receiver> on port 1883.
1719010537: New client connected from <receiver> as mosq-Wze2t5NWmKTwMdQOzi (c1, k60).
1719010537: Sending CONNACK to mosq-Wze2t5NWmKTwMdQOzi (0, 0)
1719010573: Socket error on client mosq-Wze2t5NWmKTwMdQOzi, disconnecting.
1719010587: New connection from <sender> on port 1883.
1719010587: New client connected from <sender> as mosq-q9b2Lbd1tARoQKJssX (c1, k60).
1719010587: Sending CONNACK to mosq-q9b2Lbd1tARoQKJssX (0, 0)
1719010595: New connection from 127.0.0.1 on port 1883.
1719010595: New client connected from 127.0.0.1 as mosqsub|30731-MichaelJe (c1, k60).
1719010595: Sending CONNACK to mosqsub|30731-MichaelJe (0, 0)
1719010595: Received SUBSCRIBE from mosqsub|30731-MichaelJe
1719010595:     test (QoS 0)
1719010595: mosqsub|30731-MichaelJe 0 test
1719010595: Sending SUBACK to mosqsub|30731-MichaelJe
#前3条消息：
1719010599: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (3 bytes))
1719010599: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (3 bytes))
1719010601: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (3 bytes))
1719010601: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (3 bytes))
1719010602: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (5 bytes))
1719010602: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (5 bytes))

#此处断开电缆
#无操作
#重连后

1719010634: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (1 bytes))
1719010634: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (1 bytes))
1719010634: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (4 bytes))
1719010634: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (4 bytes))
1719010634: Received PUBLISH from mosq-q9b2Lbd1tARoQKJssX (d0, q0, r0, m0, 'test', ... (3 bytes))
1719010634: Sending PUBLISH to mosqsub|30731-MichaelJe (d0, q0, r0, m0, 'test', ... (3 bytes))

mosquitto_sub日志

one
two
three
#此处断开电缆
#无操作
#重连后
4
five
six

内容的提问来源于stack exchange，提问作者Barsaas