- 抓取报文。前三次是tcp建立连，所以能检查端口
https://www.ibm.com/docs/en/i/7.3?topic=ssw_ibm_i_73/cl/telnet.htm
防火墙
https://netbeez.net/blog/telnet-to-test-connectivity-to-tcp/ 这文章写得好。
If the service is running with no firewalls in-between blocking incoming connections, the telnet command will return the following prompt:
As you can see, the telnet command returns the resolved IP address associated to the provided hostname, and give notice of the escape characters, ‘
这种ok.
有两种：1.明确拒绝。reject 2. 直接丢掉drop . telnet表现是不一样的。
可以使用In this case, further troubleshooting is required, such as verifying with a traceroute from the client side where the connection is interrupted or getting in touch with the administrator of the remote host to ask whether the remote service is running and allowed by any firewall in between.