FTP (File Transfer Protocol) – Detailed Explanation
What is FTP?
Definition: FTP (File Transfer Protocol) is a standard network protocol used to transfer files between a client and a server over a TCP/IP network.
Purpose: Enables users to upload, download, and manage files on remote systems.
Example: John wants to upload his website files from his PC to a web server. He uses an FTP client (like FileZilla) to connect and transfer files.
FTP Modes
Active Mode
- The client opens a random port and tells the server where to connect.
- The server initiates the data connection back to the client.
- Issue: Often blocked by firewalls since the server tries to connect to the client.
Passive Mode
- The server opens a random port and tells the client which port to connect to.
- The client initiates all connections.
- Advantage: Works better behind client-side firewalls and NAT devices.
Tip: Use passive mode if you are behind a firewall or NAT.
FTP Operation
Control Connection: Used for sending FTP commands and responses, always on TCP port 21.
Data Connection: Used for transferring files, on TCP port 20 (active mode) or a negotiated port (passive mode).
Example: John opens FileZilla and connects to ftp.example.com (control connection on port 21), then requests to upload myfile.txt. A data connection is established to transfer the file.
FTP Ports
| Port | Purpose | Mode |
|---|---|---|
| 21 | Control connection (commands and responses) | Active/Passive |
| 20 | Data connection (file transfers) | Active |
| Random (1024+) | Data connection (file transfers) | Passive |
FTP Commands and Responses
| Command | Description | Example Usage |
|---|---|---|
| USER | Specify username | USER john |
| PASS | Provide password | PASS password123 |
| LIST | List directory contents | LIST |
| RETR | Retrieve (download) file | RETR report.pdf |
| STOR | Store (upload) file | STOR data.csv |
Server responses use numeric codes (e.g., 200 OK, 550 Permission Denied) to indicate status.
Authentication and Security
Anonymous FTP: Allows users to log in without a password, typically for public downloads.
Authenticated FTP: Requires username and password for access.
Security Limitation: Standard FTP transmits credentials and data in plaintext, making it insecure for sensitive files.
Secure Variants of FTP
- FTPS: FTP over SSL/TLS. Adds encryption but uses the same commands and ports.
- SFTP (SSH File Transfer Protocol): Runs over SSH (port 22). Different protocol, encrypts both commands and data.
Tip: Use FTPS or SFTP when transferring sensitive data to protect credentials and files.
Configuring FTP Servers and Clients
Server Setup
- Install FTP server software (e.g., vsftpd, ProFTPD, Windows IIS FTP).
- Set user accounts and directory permissions.
- Configure firewall rules to allow ports 21 and the passive port range.
Client Setup
- Use FTP client software (FileZilla, WinSCP, command-line ftp).
- Enter server address, username, and password.
- Select active or passive mode based on network setup.
Firewall and NAT Considerations
- Active Mode Issues: Server initiates data connection to client; often blocked by firewalls/NAT.
- Passive Mode Issues: Server must have passive ports open and properly configured.
Recommendation: Prefer passive mode, especially behind NAT or client firewalls.
Common Use Cases and Applications
- Website management: Uploading and managing website files.
- Large file transfers between systems.
- Public file distribution via anonymous FTP.
Troubleshooting FTP Issues
- Check firewall rules for ports 21, 20, and passive range.
- Confirm server availability and credentials.
- Switch between active and passive modes if transfers fail.
- Ensure correct directory permissions.
Exam Tips & Key Points
- FTP uses TCP port 21 for control; port 20 for active data or random ports for passive data.
- Active mode: server connects back to client; passive mode: client initiates all connections.
- Standard FTP is not encrypted; use FTPS or SFTP for secure transfers.
- Common FTP commands: USER, PASS, LIST, RETR, STOR.
- Firewall/NAT often block FTP; know how to select the correct mode.
Example Scenario
John manages a website and uses FileZilla to connect to ftp.mywebsite.com with his username and password. Since his office firewall blocks incoming connections, he sets FileZilla to passive mode to upload files successfully.