HyQuest Solutions iRIS 270, Руководство пользователя

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Appendices
8.2 FTP Terminology
This chapter contains the following subsection:
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Active verses Passive mode
8.2.1 Active verses Passive mode
The following explanation is paraphrased from the web page https://slacksite.com/other/ftp.html
Introduction
One of the most commonly seen questions when dealing with firewalls and other Internet connectivity issues is the
difference between active and passive FTP and how best to support either or both of them.
FTP is a TCP based service exclusively. There is no UDP component to FTP. FTP is an unusual service in that it utilizes two
ports, a 'data' port and a 'command' port (also known as the control port). Traditionally these are port 21 for the
command port and port 20 for the data port. The confusion begins however, when we find that depending on the mode,
the data port is not always on port 20.
Active FTP
In active mode FTP the client (iRIS) connects from a random unprivileged port (N > 1023) to the FTP server's command
port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The
server will then connect back to the client's specified data port from its local data port, which is port 20.
From the server-side firewall's standpoint, to support active mode FTP the following communication channels need to be
opened:
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FTP server's port 21 from anywhere (Client initiates connection)
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FTP server's port 21 to ports > 1023 (Server responds to client's control port)
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FTP server's port 20 to ports > 1023 (Server initiates data connection to client's data port)
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FTP server's port 20 from ports > 1023 (Client sends ACKs to server's data port)
When drawn out, the connection appears as follows:
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