How does FTP use TCP connections?

A persistent control connection on port 21 and a data connection per file transfer (port 20 in active mode)

A single connection on port 21 for both commands and files

UDP port 20 for commands and TCP port 21 for data

One new connection on port 80 per command

What happens in passive mode?

The server opens a random port and the client connects to it, which is firewall friendly

The server opens the data connection to the client's port

Data goes over the control connection

The client disconnects after login

What is the main security weakness of FTP, and a secure alternative?

Passwords and data are sent as plain text; use SFTP or FTPS

It has no authentication; use HTTP

It only allows anonymous login; use POP3

2074 Bhadra · 4 marks

Short note: FTP

Short answer

FTP uses two TCP connections: control on port 21 for commands and data on port 20 for transfers. It supports active and passive modes; secure alternatives are SFTP and FTPS.

Write a short note on FTP.

File Transfer Protocol (FTP) is an application layer protocol (RFC 959) used to transfer files between a client and a server over TCP. It is a client/server protocol with user authentication.

Two connections

Control connection: TCP port 21. It stays open for the whole session and carries commands (USER, PASS, LIST, RETR, STOR, CWD, QUIT) and replies (e.g. 220, 230, 226, 550). It is "out-of-band" control.
Data connection: TCP port 20 in active mode. A new data connection is opened for every file transfer and closed after it.

 FTP client                         FTP server
 [control] ---- TCP 21 commands --> [control]
 [data]    <--- TCP 20 files ------> [data]

Modes

Active mode: the server opens the data connection from port 20 to the client's port. Often blocked by client firewalls/NAT.
Passive mode (PASV): the server opens a random port and the client connects to it. Firewall friendly.

Features

Transfer type: ASCII or binary; transmission mode: stream.
FTP keeps state about the user (current directory, login).
Supports anonymous access (user "anonymous", e-mail as password).
Weakness: user name, password and data go as plain text. Secure alternatives are SFTP (over SSH) and FTPS (FTP over TLS).

Common clients: FileZilla, ftp command; common servers: vsftpd, ProFTPD.

✓ Answer checked by Claude Opus 5.5 (high effort)

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