Networking

File Transfer Protocol

Application layer protocol for reliably transferring files between hosts using separate control and data connections

File Transfer Protocol (FTP)

  • Application layer protocol (Layer 7) designed for transferring files between hosts over TCP/IP networks
  • Uses two separate TCP connections - one for commands (control) and one for data transfer
  • Operates on a client-server model where FTP client initiates connections to FTP server
  • Built for reliability over speed (uses TCP, not UDP) - mission-critical file transfers need guaranteed delivery

Connection Architecture

  • Control Connection (Port 21): Persistent connection for commands, authentication, and responses
  • Data Connection (Port 20 or ephemeral): Temporary connection established for each file transfer
  • Control connection remains open throughout entire FTP session
  • Data connection opens/closes for each transfer operation

FTP Modes

Active Mode:

  • Client opens random high port and sends PORT command with this port number
  • Server initiates data connection FROM port 20 TO client’s specified port
  • Problem: Client firewalls often block incoming connections from server

Passive Mode (PASV):

  • Client sends PASV command requesting passive mode
  • Server opens random high port and sends port number to client
  • Client initiates data connection TO server’s specified port
  • Firewall-friendly because client initiates both connections

Authentication Methods

Method Security Level Use Case
Anonymous FTP None Public file repositories
Username/Password Basic Internal networks
FTPS Encrypted Secure file transfers
SFTP Encrypted + Key-based Modern secure alternative

Transfer Modes

ASCII Mode:

  • Text files with automatic line ending conversion
  • Use for .txt, .html, .cfg files

Binary Mode:

  • Raw byte transfer with no modification
  • Critical for executable files, images, compressed files

Notes

  • FTP sends passwords in plaintext - never use over untrusted networks without encryption
  • SFTP (port 22) is preferred over FTPS for new implementations
  • Troubleshooting tip: If active mode fails, try passive mode - 90% of FTP issues are firewall-related