What Are FTP Client Connections?
While it may be difficult to believe today, when the Internet first began is was nothing more than a means for scientists, librarians and other technical professionals to share information. With no common language to communicate between computers, it did not take long to realize the need for standardizing the way computers "talked" with one another. From this need was born the concept of protocols, essentially universal ways of communicating between computer systems. The File Transfer Protocol (FTP) methodology was an early response to this need.
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What FTP Is
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Transferring information from one computer to another, or one server to another, is one of the basic functions of Internet technology. However, the transferring of large files across the Internet often requires more security than simply attaching documents to an email. The size of a file may limit the ability to use less secure means as well. The answer is FTP, since making a client connection to an FTP server requires accessing a secure site (the FTP server). Once accessed, a user (or "client") can upload or download files securely to or from the FTP site. These can then be accessed by other users (clients) with the same permissions. The result is the ability to securely share files and information, without the size and security limitations of email.
How FTP Works
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Initiating a file transfer using file transfer protocol begins with a client computer. This could be your desktop computer at home or in the office. The "client" then contacts a computer server (FTP in this case) by typing in the right address, just like accessing a webpage, and enters the appropriate user name and password. Now, the client is able to view, upload (send files) or download (save files sitting on the FTP server) data securely. If there are limitations to what can be seen or actions taken on an FTP server, they are likely due to the site administrator limiting access for security reasons.
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Active FTP connections
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Transferring files using active FTP begins with the computer client actively initiating a connection; in other words inputting the user name and password, to securely open a line, or port, with the FTP server. Once the connection has been made, the client waits while the FTP server makes the final connection. This process was fine until the advent of firewalls. As security became a bigger concern, firewalls that blocked unauthorized intrusion were necessities. The problem arises when trying to make an active FTP connection with firewalls because the firewall perceives the attempt as a potential security threat, and will likely not allow it to take place. Administrators can sometimes set the firewall to allow FTP server connections.
Passive FTP connections
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Avoiding firewall issues in the sending and retrieving of files using FTP is managed by many using passive FTP connections. Unlike active connections that require the client to initiate contact before the FTP server will "acknowledge" them, passive connections are accomplished by keeping the FTP server lines (ports) open and waiting for a computer client to make contact. This process of initiating FTP client connections avoids the troublesome nature of firewalls when trying to share information among clients and FTP servers.
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