What is a FTP Server
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Contents |
Overview
The file-transfer provided by FTP copies a complete file from one system to another (SERVER to CLIENT, CLIENT to SERVER). In this transfer a computer running BulletProof FTP Server (http://www.bpftpserver.com) is called the SERVER or HOST and a person connecting to the SERVER is called a CLIENT. A CLIENT is a piece of software which adheres to the Internet FTP standard (RFC-959), such as our specially designed BulletProof FTP Client (http://www.bpftp.com) or even your favorite web-browser (Firefox, Internet Explorer, etc).
Control/Data Connections
When setting up a SERVER, it is important to understand that there are two types of connections made, control-connections and data-connections.
Control Connections
The control-connection is established in the typical client-server fashion, the SERVER creates a listening socket on the well-known port for FTP (TCP/21) and waits for the CLIENT to make a connection. This default TCP/IP connection on port 21 (shorthand TCP/21), can be changed to any value you wish. Often times, this port is changed to help "hide" the FTP service from would-be hackers or malicious software (virus, worms, etc). This control-connection remains open during the entire time that the CLIENT communicates with the SERVER. This connection is used for commands from the CLIENT to the SERVER and for the SERVER's replies.
Data Connections
The data-connection is created each time a file or data is transferred between the CLIENT and SERVER. FTP uses this data-connection is used in three different ways:
- Sending a file from the CLIENT to the SERVER
- Sending a file from the SERVER to the CLIENT
- Sending a directory-listing from the SERVER to the CLIENT
How they are used
When a CLIENT (such as BulletProof FTP Client) requests a directory-listing from the SERVER (BulletProof FTP Server), the command (LIST) is sent across the control-connection and a data-connection is made to transfer the directory-listing. This avoids any potential limitations that might restrict the size of a directory-listing and makes it easier for the client to save the output and display it in an easy to use point-click interface. During this transfer, the control-connection remains idle while the data-connection is in use.
PORT vs PASV
The creation of the data-connection is dictated by the CLIENT upon issuing of an FTP command (get a file, put a file or obtain a directory-listing). Unfortunately, this is when FTP becomes difficult for most people:
- In the default mode, called port-mode (PORT-mode), the CLIENT issues the command and then begins to listen on a random TCP/IP port for the SERVER to connect and establish the data-connection. Although this mode is effective, it requires the technical ability for the person running the CLIENT to understand how to secure and properly open their network's Firewall/NAT to allow this incoming connection.
- Optionally, the CLIENT can request data-connections to operate in assive-mode (PASV-mode). Thankfully, this mode asks the SERVER to make the listening TCP/IP port for the CLIENT to connect and establish the data-connection.
