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FTP Control Connection Establishment, User Authentication and Anonymous FTP Access
(Page 1 of 3)
operational model describes the distinct
logical data and control channels that are established between an FTP
client (user) and an FTP server. Before the data connection can be used
to send actual files, the control connection must be established. A
specific process is followed to set up this connection and thereby create
the permanent FTP session between devices that can be used for transferring
As with other client/server protocols,
the FTP server assumes a passive role in the control connection process.
The server protocol interpreter (Server-PI) listens on the
TCP port reserved for FTP control connections:
port 21. The User-PI initiates the connection by opening a TCP connection
from the user device to the server on this port. It uses an ephemeral
port number as its source port in the
Once TCP has been set up, the control
connection between the devices is established, allowing commands to
be sent from the User-PI to the Server-PI, and reply codes to be sent
back in response. The first order of business after the channel is operating
is user authentication, which the FTP standard calls the login
sequence. There are two purposes for this process:
- Access Control: The authentication process
allows access to the server to be restricted to only authorized users.
It also lets the server control what types of access each user has.
- Resource Selection: By identifying the
user making the connection, the FTP server can make decisions about
what resources to make available to the user.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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