EMQ 学习---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$SYS主题的权限。node {allow, {user, "dashboard"}, subscribe, ["$SYS/#"]}. {allow, {ipaddr, "127.0.0.1"}, pubsub, ["$SYS/#", "#"]}. %%%{deny, all, subscribe, ["$SYS/#", {eq, "#"}]}. {all

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