When a new TCP connection comes in from a client by means of a packet with the SYN bit set, the server creates an entry for the half-open connection and responds with a SYN-ACK packet.
当从客户机发来新的TCP连接时，数据包设置了SY n位，服务器就为这个半开的连接创建一个条目，并用一个SYN - ACK数据包进行响应。
In response, the scanned host returns a packet with the SYN and ack flags set if the port is open.
如果该端口是打开的，则被扫描主机返回设置了syn和ack标志的包进行响应。
The server answers with a packet where both the SYN and ACK flags are set.
服务器以设置了 SYN 和 ACK 标志的包应答。
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