This was known as passive mode, or PASV, after the command used by the client to tell the server it is in passive mode.
这就是所谓的被动模式，或者叫做PASV，当客户端通知服务器它处于被动模式时才启用。
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The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command.