FTP

Dual-Channel Architecture

FTP (File Transfer Protocol) is one of the oldest internet application protocols, designed to transfer files between a client and server. It uniquely uses two separate TCPTransmission Control Protocol. A reliable, connection-oriented transport protocol that guarantees ordered, error-checked delivery of data through three-way handshakes, acknowledgments, and retransmission. The foundation of HTTP, SSH, and most internet services. connections: a control channel (port 21) for commands and responses, and a data channel for actual file transfers. The data channel port changes with each transfer and is negotiated through the control channel, which historically caused problems with FirewallA network security device or software that monitors and filters incoming and outgoing traffic based on predefined rules. Firewalls can block traffic by IP address, port number, protocol, or application-layer content. and NATNetwork Address Translation. A method of remapping private IP addresses to a single public IP address (and vice versa) at a router, allowing multiple devices to share one public IP. A key technique for mitigating IPv4 address exhaustion. traversal.

Active vs. Passive Mode

In active mode, the server initiates the data connection back to the client on a port the client specifies — this fails when the client is behind NATNetwork Address Translation. A method of remapping private IP addresses to a single public IP address (and vice versa) at a router, allowing multiple devices to share one public IP. A key technique for mitigating IPv4 address exhaustion. because inbound connections cannot reach it. Passive mode solves this: the server opens a high-numbered port and tells the client to connect to it, so the client always initiates both connections. Modern FTP clients default to passive mode for compatibility with NAT and firewalls.

Security Considerations

FTP transmits credentials and file data in plain text with no EncryptionThe process of converting plaintext data into ciphertext using a cryptographic algorithm and key, making it unreadable without the corresponding decryption key. The foundation of secure communication on the internet., making it trivially vulnerable to Man-in-the-Middle AttackAn attack where an adversary secretly intercepts and potentially alters communication between two parties who believe they are communicating directly. HTTPS and certificate pinning are primary defenses against this attack. interception on any network segment. FTPS (FTP over SSL) adds SSL/TLSSecure Sockets Layer / Transport Layer Security. Cryptographic protocols that provide encrypted, authenticated communication over a network. SSL is deprecated; modern implementations use TLS 1.2 or TLS 1.3. to the control and data channels. SFTP, despite the similar name, is a completely different protocol built on SSHSecure Shell. A cryptographic protocol for secure remote login, command execution, and file transfer over an unsecured network. SSH (port 22) replaced insecure protocols like Telnet and rlogin. that provides encrypted file transfer over a single connection. For any new deployment, SFTP or FTPS should be used instead of plain FTP. Use Open Port Checker to verify which file transfer ports are open on a server.