$ telnet localhost 4000 Trying ::1... telnet: connect to address ::1: Connection refused Trying 127.0.0.1... telnet: connect to address 127.0.0.1: Connection refused telnet: Unable to connect to remote host
当端口打开后，返回：
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$ telnet localhost 4000 Trying ::1... Connected to localhost. Escape character is '^]'.
此时命令未退出！
根据提示 Escape character is '^]'.，可知：退出字符为 '^]'（CTRL+]）；
此时输入其它字符不能使其退出，CTRL+C 都不行！
输入 CTRL+] 后会进入命令模式：
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^] telnet>
此时再执行 quit 才会真正退出：
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telnet> quit Connection closed.
其中，Escape character 可以自定义，使用参数-e：
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# 使用p字符 $ telnet -e p localhost 4000 Telnet escape character is 'p'. Trying ::1... Connected to localhost. Escape character is 'p'. p telnet> quit Connection closed.
