Netcat 可以创建IPv4或IPv6的TCP 或UDP的connection，之后将在标准输入的到的data通过该connection发送到对端。-In the simplest usage, "nc6 host port" creates a TCP connection to the given port on the given target host (using either IPv4 or IPv6 as appropriate). Your standard input is then sent to the host, and anything that comes back across the connection is sent to your standard output. This continues indefinitely, until the network side of the connection shuts down. Note that this behavior is different from most applications which shut everything down and exit after an end-of-file on standard input.