FTP (File Transfer Protocol) is a standard network protocol used for transferring computer files from a server to a client or vice versa over a TCP/IP-based network, such as the internet. It’s one of the oldest protocols still in use today, dating back to the early days of the internet.

Core Purpose of FTP:

The primary goal of FTP is to enable the sharing, uploading, and downloading of files between different systems in a standardized, reliable, and efficient way.

How FTP Works (The Dual-Channel Approach):

One of the defining characteristics of FTP is its use of two separate TCP connections for each transfer session:

1. Control Channel (Port 21 – TCP):

   • This channel is established first and remains open for the entire duration of the FTP session.

   • It’s used for sending commands from the client to the server (e.g., USER, PASS, LIST, GET, PUT, CWD – change working directory, DELE – delete).

   • It’s also used for sending responses (status codes and messages) from the server back to the client.

   • This channel handles the “conversation” about what files to transfer and how, but not the actual file data itself.

2. Data Channel (Often Port 20 or dynamic – TCP):

   • This channel is established only when actual file data or directory listings need to be transferred.

   • It’s a separate connection used solely for the binary transfer of the file content.

   • Once a file transfer is complete, this data channel is typically closed, and a new one is opened for the next transfer.

FTP Modes (How the Data Channel is Established):

The way the data channel is established is crucial and determines whether FTP operates in “active” or “passive” mode:

1. Active Mode FTP:

   • Client behavior: The client sends a PORT command (or EPRT for IPv6) to the server on the control channel, informing the server of an IP address and a dynamic port number (e.g., 20000) on the client’s side where it will “listen” for the incoming data connection.

   • Server behavior: The FTP server then initiates a new TCP connection from its data port (typically port 20) to the specified IP address and port on the client’s side.

   • Problem with Firewalls: Active mode often runs into issues with firewalls (especially on the client side) and NAT devices. Since the server initiates the data connection back to the client, the client’s firewall might block this incoming connection as unsolicited, even if the original request came from inside. This is why active mode is rarely used by clients behind most modern firewalls.

2. Passive Mode FTP (Preferred for Firewall Compatibility):

   • Client behavior: The client sends a PASV command (or EPSV for IPv6) to the server on the control channel, indicating that it wants to operate in passive mode.

   • Server behavior: The FTP server responds by providing its own IP address and a dynamic port number (e.g., 50000) on its side where it will “listen” for the client to connect for data transfer.

   • Client behavior (cont.): The client then initiates a new TCP connection from its own dynamic port to the IP address and dynamic port provided by the server.

   • Firewall Compatibility: This mode is much more firewall-friendly because the client always initiates both connections (control and data). Outbound connections from internal networks are typically allowed by firewalls, resolving the issues faced by active mode. This is the default mode for most modern FTP clients.

Authentication:

FTP servers usually require authentication. Users provide a username and password to log in.

• Anonymous FTP: Some FTP servers allow “anonymous” access, typically for distributing public files (e.g., software downloads). In this case, users often log in with the username “anonymous” and their email address as the password. Permissions are usually restricted to downloading only.

Advantages of FTP (Historical):

• Standardization: Widely recognized and supported across various operating systems and platforms.

• Simplicity: Relatively simple to understand and implement.

• Efficiency for Large Files: Designed to handle large file transfers efficiently.

• Resume Transfers: Can often resume interrupted transfers.

Major Disadvantage: Security

The biggest drawback of traditional FTP is its complete lack of security features:

• Plaintext Credentials: Usernames and passwords are sent over the control channel in unencrypted, clear text. Anyone sniffing network traffic can easily capture them.

• Plaintext Data Transfer: The actual file data is also transferred over the data channel in unencrypted, clear text. Sensitive information can be intercepted and read.

• Vulnerability to MITM Attacks: Due to the lack of encryption and server authentication, FTP is highly susceptible to Man-in-the-Middle attacks.

Modern Alternatives (Secure FTP Protocols):

Because of these severe security vulnerabilities, traditional FTP should never be used for transferring sensitive or confidential data over untrusted networks like the internet. It has largely been replaced by secure alternatives:

• SFTP (SSH File Transfer Protocol):

  • Not actually FTP. SFTP is a completely different protocol that runs over SSH (Secure Shell).

  • It uses only one connection (port 22 by default) for both commands and data.

  • All communication, including authentication credentials and data, is fully encrypted by SSH.

  • It provides strong authentication (password or SSH keys) and data integrity.

  • Recommended for secure file transfers.

• FTPS (FTP Secure / FTP over SSL/TLS):

  • An extension of traditional FTP. FTPS adds a layer of SSL/TLS encryption to the standard FTP protocol.

  • It still uses the two-channel model (control and data).

  • Encryption can be applied to both the control channel and/or the data channel.

  • Often uses port 21 for control and a range of ports for data, or sometimes port 990 for implicit FTPS.

  • While more secure than plain FTP, it can be more complex to configure with firewalls due to its dual-channel nature and dynamic data ports compared to SFTP.

In summary, while FTP was groundbreaking in its time and is still sometimes used in controlled, isolated environments or for public, non-sensitive data, its fundamental lack of security means that SFTP or FTPS should always be the preferred choice for any file transfers involving sensitive information over the internet.