In this example, the client has specified Active FTP by sending a PORT command. The server will return its response to the client over the data channel, so before FTP client can send the LIST command, the client has to specify whether to use Active or Passive. The client's next task is to request a directory listing, which is denoted by the LIST command. So far all of the client/server communication has taken place over the FTP command channel, and up to now the conversation is identical to Passive FTP, but that is about to change. After negotiating the FTP host, username, and password, the client retrieves the name of the current directory. In the beginning of this exchange, the client connects to the server from one of its ephemeral data ports to the server's port for the FTP command channel. Transferred 106 bytes in 0.008 seconds QUIT 221 Goodbye. drwxrwxrwx 1 owner group 0 Feb 15 19:26 aspnet_client -rwxrwxrwx 1 owner group 689 Jan 31 22:27 default.htm 226 Transfer complete. LIST 125 Data connection already open Transfer starting. USER robert 331 Password required for robert. 220 Microsoft FTP Service HOST 220 Host accepted. The following output shows the communication between an FTP client and FTP server using Active FTP to retrieve a simple directory listing, and I manually color-coded the output so that the client and server's responses would be a little easier to see: OPEN Resolving. Unfortunately, neither of those bullet points are the least bit significant to you if you don't understand what they actually mean, so I will describe them in detail below. That sums up the whole concept into two bullet points. When you are using Passive FTP, your FTP server determines how your data connections will work.When you are using Active FTP, your FTP client determines how your data connections will work.Here is the briefest description that I can think of to describe the different between Active and Passive FTP: With all of this in mind, I thought that I would put together a quick blog that explains Active and Passive FTP connections and why those matter when you are connecting to an FTP server over the Internet. I get a lot of questions about Active versus Passive FTP Connections, specifically when people are configuring their FTP firewall settings as described in my articles like Configuring FTP Firewall Settings in IIS and FTP Firewall Support, and I get related questions when people are trying to figure out why they can't use the command-line FTP.EXE utility that ships with Windows over the Internet.
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