File Transfer Protocol

lastmod 2026-02-20created 2014-01-0416 edits

File Transfer Protocol (FTP) is a protocol for transferring files between a server and a client over the TCP/IP protocol. FTP belongs to the application layer of the TCP/IP protocol table, and although it has a long history, it is still frequently used on the Internet.

Connection types

Unlike HTTP, there are two types of connections:

• Command connection: First, a connection for user authentication and commands is made to the server's port 21, the control port, and the commands issued by the client are delivered through it.
• Data transfer connection: For the actual file transfer, a new connection is made when needed.
  • Active mode (photo mode): The server makes a data connection from its data port, port 20, to the point specified by the client. The port specified by the client is usually one numbered above 1023. It may not work well when the client is in an environment that uses a firewall, NAT (IP masquerading), etc.; in that case, use passive mode.
  • Passive mode: Lets the client connect to the server port specified by the server. In this case, both sides usually use ports above 1023.

In either mode, two connections are made. However, if the FTP port is changed and there is a firewall, depending on the firewall's Inspection mode settings, FTP communication may work normally while communication that uses the FTP-DATA port, such as the get or ls commands, may not.

Example

get

Download

mget

Download multiple files

put

Upload

mput

Upload multiple files

ls

List files on the connected SFTP

!ls

List files on the local server

!{명령어}

Run on the local server

Main FTP commands

Main FTP response codes

• 200: Command OK.
• 500: Syntax error, command not recognized.
• 501: Syntax error in parameters or arguments.
• 502: Command not implemented.
• 110: Restart marker reply.
• 119: Terminal not available, attempting mailbox function.
• 211: System status, or system help reply.
• 212: Directory status.
• 213: File status.
• 214: Help message.
• 120: Service ready in nnn minutes.
• 125: Data connection already open; transfer starting.
• 225: Data connection open; no transfer in progress.
• 425: Can't open data connection.
• 226: Closing data connection; requested file action successful.
• 227: Entering passive mode. h1, h2, h3, h4, p1, p2.
• 230: User logged in, proceed.
• 530: Not logged in.
• 331: User name ok, need password.
• 332: Need account for login.
• 532: Need account for storing files.
• 150: File status ok; about to open data connection.
• 152: Unknown user; mail will be forwarded by the administrator.
• 250: Requested file action ok, completed.
• 350: Requested file action pending further information.
• 450: Requested file action not taken: file unavailable.
• 550: Requested action not taken: file unavailable.
• 451: Requested action aborted: local error in processing.
• 551: Requested action aborted: page type unknown.
• 452: Requested action not taken: insufficient storage space in system.
• 552: Requested file action aborted: exceeded storage allocation.
• 553: Requested action not taken: no file name.

Projects

ftpclient-cpp

https://github.com/embeddedmz/ftpclient-cpp

C++ client for making FTP requests

FileZilla

FTP client

fineftp-server

https://github.com/continental/fineftp-server

C++ FTP Server Library for Windows, Linux & more

Network-attached storage (NAS)

• Network File System (NFS)
• Server Message Block (SMB)
• Common Internet File System (CIFS)
• Apple Filing Protocol (AFP)
• File Transfer Protocol (FTP)

See also

• Trivial File Transfer Protocol (TFTP)
• Copyparty - open source that turns almost any device into a file server

Favorite site

• How FTP works, its commands, and response codes
• Wikipedia (en) File Transfer Protocol
• Wikipedia (en) List of FTP server return codes ¹
• Connecting via SFTP and transferring files

References