FTP (Perl in a Nutshell, 2nd Edition)

The File Transfer Protocol (FTP) is a popular means of transferring files between computers. FTP communication follows the client/server model: the client initiates a conversation by sending commands, and the server responds with messages and status codes, as well as by sending or receiving files. This chapter discusses two FTP-related modules included in the libnet distribution: Net::FTP, which provides a number of wrapper functions for implementing the client side of FTP, and Net::Netrc, which provides an interface for getting information from a .netrc file.

The FTP protocol permits two-way file transactions, in which files can be sent to or taken from an FTP server. These transactions involve the local filesystem (on the client side) and the remote filesystem (on the server side). When a file is transferred between the local and remote systems, its filename on the destination system is the same as on the source system, unless you specify a new filename.

The FTP protocol also lists the types of files that can be transferred. These types define (among many other things) how end-of-line characters are handled for different types of files.

18.1. The FTP Protocol

When a server accepts FTP requests, it opens a port (generally port 21) for incoming connections and authenticates clients based on account or anonymous privileges. A user may log in with a legitimate account on that machine, provide her own password, and access any file she normally has access to under the Unix shell. Many servers also allow "anonymous" FTP, in which users log in with the name "anonymous" and use their email address as the password. They are then granted restricted access to a limited portion of the filesystem.

The FTP commands defined in RFC 959 are listed in the following table:

Command	Meaning
ABOR	Abort previous FTP command.
ACCT	Specify the user's account.
ALLO	Tell server to allocate additional storage for new file.
APPE	Tell server to append to an existing file.
CDUP	Change to parent directory.
CWD	Change directory.
DELE	Delete a file.
HELP	Get help.
LIST	List files for display.
MKD	Make a directory.
MODE	Specify the data transfer mode.
NLST	List files for additional processing.
NOOP	No-op.
PASS	Specify the user's password.
PASV	Tell server to go into "passive" mode.
PORT	Specify data port for connection.
PWD	Print working directory.
QUIT	Close connection.
REIN	Reinitialize connection.
REST	Restart a file transfer.
RETR	Retrieve a file.
RMD	Remove a directory.
RNFR	Specify pathname of file to be renamed.
RNTO	Specify new name of file being renamed.
SITE	Provide additional site-specific services.
SMNT	Mount a different filesystem.
STAT	Get status.
STOR	Tell server to accept a file for storage.
STOU	Tell server to create unique name for new file.
STRU	Specify the file structure.
SYST	Tell server to declare its operating system.
TYPE	Specify the data representation type.
USER	Specify the username.

Exactly which FTP commands are available depends on the server; some servers implement a subset, or possibly a superset, of the commands defined in the RFC. Net::FTP, which we'll discuss in this chapter, provides methods that implement all the commands except the following:

ALLO

HELP

MODE

REIN

SITE

SMNT

STAT

STRU

SYST

Chapter 18. FTP

Contents:

18.1. The FTP Protocol