16. FTP

Contents:
The FTP Protocol
Net::FTP
FTP Configuration with Net::Netrc

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.

16.1 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 be given access to 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 file system
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