[ftp]解析PASV命令

sunorr

我看文档上说这样说的：

This command requests the server-DTP to "listen" on a data
            port (which is not its default data port) and to wait for a
            connection rather than initiate one upon receipt of a
            transfer command.  The response to this command includes the
            host and port address this server is listening on.

这可能是为了用浏览器访问而做的规定吧，可是我该怎么解析这个命令呢？

大概需要作什么工作呢？

sunorr

给大家添麻烦了，怪偶没仔细看文档

When data is to be transferred between two servers, A and B (refer
      to Figure 2), the user-PI, C, sets up control connections with
      both server-PI's.  One of the servers, say A, is then sent a PASV
      command telling him to "listen" on his data port rather than
      initiate a connection when he receives a transfer service command.
      When the user-PI receives an acknowledgment to the PASV command,
      which includes the identity of the host and port being listened
      on, the user-PI then sends A's port, a, to B in a PORT command; a
      reply is returned.  The user-PI may then send the corresponding
      service commands to A and B.  Server B initiates the connection
      and the transfer proceeds.  The command-reply sequence is listed
      below where the messages are vertically synchronous but
      horizontally asynchronous

dxcnjupt

PASV是为了穿透防火墙