EMQX订阅$SYS主题，捕获客户端上下线消息

acl.config文件定义了可订阅$SYS主题的权限。

{allow, {user, "dashboard"}, subscribe, ["$SYS/#"]}.

{allow, {ipaddr, "127.0.0.1"}, pubsub, ["$SYS/#", "#"]}.

%%%{deny, all, subscribe, ["$SYS/#", {eq, "#"}]}.

{allow, all}.

acl.config文件第三行默认是禁止客户端订阅$SYS/#主题。我们把它注释掉即可

订阅客户端上下线的主题

具体某个节点的某个clientId方式的主题是：

$SYS/brokers/(node)/clients/(clientId)/connected
$SYS/brokers/emqttd@172.16.6.161/clients/861694030142478/connected
$SYS/brokers/emqttd@172.16.6.161/clients/861694030142478/disconnected

通配方式的主题是：

$SYS/brokers/+/clients/+/connected
$SYS/brokers/+/clients/+/disconnected
$SYS/brokers/+/clients/#

出于安全起见，我们把acl.config文件修改为：

允许客户端订阅"$SYS/brokers/+/clients/#"主题,但是禁止订阅"$SYS/#"主题。

{allow, {user, "dashboard"}, subscribe, ["$SYS/#"]}.

{allow, {ipaddr, "127.0.0.1"}, pubsub, ["$SYS/#", "#"]}.

{allow, all, subscribe, ["$SYS/brokers/+/clients/#"]}.

{deny, all, subscribe, ["$SYS/#", {eq, "#"}]}.

{allow, all}.

接下来就可以接收客户上下线消息了