WHAT IS AN FTP?

Ftp means file transfer protocol, the File Transfer Protocol is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data connections between the client and the server.

File Transfer Protocol is a set of protocols that the networked computers use to talk over the internet. … Using an FTP client we can upload, download, delete, move, rename and copy the file on a server. If you send your file through FTP then your files mainly perform upload or download from the FTP serve

Types of FTP

There are three types of FTP connections possible (Cerberus FTP Server supports all three):

FTP

Plain, unencrypted FTP that defaults over port 21. Most web browsers support basic FTP.

FTPS

Implicit SSL/TLS encrypted FTP that works just like HTTPS. Security is enabled with SSL as soon as the connection starts. The default FTPS port is 990. This protocol was the first version of encrypted FTP available, and while considered deprecated, is still widely used. None of the major web browsers support FTPS.

FTPES

Explicit FTP over SSL/TLS. This starts out as plain FTP over port 21, but through special FTP commands is upgraded to TLS/SSL encryption. This upgrade usually occurs before the user credentials are sent over the connection. FTPES is a somewhat newer form of encrypted FTP (although still over a decade old), and is considered the preferred way to establish encrypted connections because it can be more firewall friendly. None of the major web browsers support FTPES.

These three protocols should not be confused with the SFTP protocol. SFTP is an entirely different file transfer protocol that runs over SSH2.

Features of FTP

The basic features of FTP are:

1. Data representation

• FTP handles three types of data representations-ASCII (7 bit), EBCDIC (8-bit) and 8-binary data.

• The ASCII file is the default format for transferring text files

• Each character is encoded using 7-bit ASCII. The sender transforms the file from its own representation into ASCII characters and the receiver transforms the ASCII character to its own representation.

• The image file is the default format for transferring binary files. The file is sent as continuous streams of bits without any interpretation or encoding.

2. File organization and Data structures

• FTP supports both unstructured and structured file.

• An unstructured file contains string of bytes and is enl-marked by EOF (End of file). The data structure that corresponds to such a file is called file structure.

• A structured file contains a list of records and each record is delimited by EDR (End of Record). The data structure of such file is called record structure i.e. file is divided into records.

 Another structured file contains pages, with each page having a page number and a page header. The pages can be stored and accessed randomly or sequentially. The corresponding data structure is called page structure i.e. file is divided into pages.

3. Transmission modes

• FTP can transfer a file by using one of the following three modes:

Stream mode

• It is the default mode.

• File is transmitted as continuous stream of bytes to TCP.

• TCP is responsible for chopping data into segments of appropriate size.

• If data is simply a stream of bytes (file structure), no end-of-file is needed. EOF in this case is the closing of the data connection by the sender.

• If data is divided into records (record structure), each record has a I-byte EOR (End-of-Record) character and the end of the file has a I-byte EOF (End-of-file) character.

Block mode

• Data is delivered from FTP to TCP in blocks.

• Each block is preceded by 3 bytes header.

• The first byte is called the block descriptor.

• The second and third byte defines the size of the block in bytes.

Compressed mode

• Data is usually compressed if the file to be transmitted is very big.

• The compression method normally used in Run-length encoding.

• In a text file, usually spaces (blanks) are removed.

• In a binary file, null characters are compressed.

4. Error control

• Since TCP is used for data transfer no additional error recovery mechanism is required.

5. Access control

• File access protection is done using login procedure with login name and password.

FTP operation

• FTP uses client/server model for communication. 

• Two TCP connections are used for file transfer.

• On one connection control signals (commands and responses) are exchanged and the other connection is used for actual data transfer. These two connections are called control connection and data connection respectively.

Control Connection

The Control connection has following features:

1. It is used to transfer control signals (commands and responses) between the client and server.

2. This connection is used by the control process of client and server. The control process is called Protocol Interpreter (PI).

3. The TCP connection for control signal uses well-known FTP server port 21.

4. This control connection remains connected during the entire interactive FTP session.

5. The control connection uses very simple rules of communication. We need to transfer only a line of command or a line of response at a time.

6. The two control processes (client & server) or PI communicates using NVT syntax.

7. The PIs are responsible for translating the local code or syntax. (e.g. DOS or UNIX) into NVT syntax and vice-vers

Data Connection

The Data connection has following features:

1. Data connection is used for actual data transfer.

2. This connection is established between the Data Transfer Process (DTP) of client and server

3. The server port used for data connection is Port 20.

4. The data connection is opened and then closed for each file transferred. It opens each time commands that involve transferring files are used, and it closes when the file is transferred.

5. File transfer occurs over the data connection under the control of the commands sent over the control connection.

6. During the file transfer, the client must define the type of file to be transferred, the structure of data and the transmission mode.

Thus file transfer in FTP means one of three things:

1. A file is to be copied from the server to the client. This is called retrieving a file. It is done with help of RETR command.

2. A file is to be copied from the client to the server. This is called storing a file. It is done with STOR command.

3. A list of directory or file names is to be sent from the server to the client. This is done with LLST command.    

Anonymous FTP

• To Use FTP, a user needs an account (user name) and a password on the Remote server.

• Some sites have a set of files available for public access; to enable anonymous FTP.

• To access these files, a user does not need to have an account or password. Instead, the user can use Anonymous as the user name and guest as the password.

FTP Servers

Similar to the Web servers, the Internet also has the installations of FTP servers. Many organizations use FTP servers to handle the distribution of files. When a user links to download something, the link actually redirects to FTP, instead of HTTP. Some files in the FTP servers may be accessible to the general public, while others are accessible only by the user. To separate the general public from the more private users, FTP servers are divided into two parts:

Anonymous server

Non-anonymous server

1. Anonymous Server: Anonymous server is the most common use of FTP, the Internet file transfer protocol. FTP sites that allow anonymous FTP do not require a password for access. You only have to log in as anonymous and enter your e-mail address as password (for their records).

2. Non-anonymous Server: If you use a non-anonymous server, then you will log in as yourself and give your password.

FTP AUTHENTICATION: FTP authentication is very easy. To authenticate the users against an existing FTP server, edit ezproxy.usr and then write a line like this: : :ftp =ftpserv.mylib.org

After writing this line, replace “ftpserv.mylib.org” with the appropriate host name for the

FTP server. Changing this line with host name takes effect immediately, without the need to restart EZproxy. When EZproxy receives authentication requests, it will connect the user to the FTP server, and if the FTP server shows login success, then the remote user will be authenticated and allowed to proceed.

FTP CLIENTS

FTP runs on a client /server model. For using FTP you will need client software to run on your PC.

To begin an FTP session, you run the FTP client software and contact the FTP server from where you want to download files from. You can use FTP client software available in Windows 2000. The FTP client talks to an FTP server. FTP uses TCP to handle communications and creates a session between the two hosts.

The FTP daemon runs on the FTP server. This daemon handles all FTP transactions. When an FTP client contacts a server, the daemon will ask for an account number (or user name) and password. Many FTP sites allow anonymous FTP.

When you log on to the FTP server, a connection called a command link is opened between your computer and the server. This link is used for sending commands to the server from your computer, and sending messages and information back from the server to your computer.

When you want to change directories on the FTP server, your client software sends an instruction to the FTP daemon, using the command link. The daemon changes directories and then sends back a listing of files on that directory via the command link. On your client software, you will see a listing of files in the new server directory. When you want to download a file, the request is issued over the command Jink.

When you issue a command to download a file, a second connection is opened up, called the data connection or data link. This connection can be opened up in one of the following two modes:

ASCII Mode

Binary Mode

Line Mode

1. ASCII Mode: The ASCII mode is used for sending text files and alters things such as line feed and carriage returns.

2. Binary Mode: The binary mode is used for sending binary files and lets files through untouched.

3. Line Mode: Line mode FTP is used to send text files line by line or it is similar to ASCII mode but with additional features. The file is downloaded from the server to your computer via the data connection. After the file is [downloaded, the data link closes automatically. After this, the command link stays open. You can then change directories or download more files.] After you have finished the whole job, you log off command link closes. Then, you are no longer selected to the FTP server.

Connecting with FTP

Typical FTP session starts as client, when you the FTP program and connect to an FTP site, spending on the program you have, you will ? enter your login information before connect or you will be prompted to do it after you connect. If you are using a Web browser to connect to anonymous site, the browser will prompt you to connect to an FTP site; you need to know the IP of an account on the FTP server. In other Is, you need to login with a user name and password so that the server knows who you are. y accessible FTP sites expect you to use a account called anonymous. Unless you spec-, Internet Explorer assumes that you warn to con-to FTP sites as an anonymous user, and it supplies the server with the user name anonymous and a password consisting of your own e-mail address.

To connect to an FTP site do this:

1. Enter the URL of the FTP site in the Address box and click go. For example, to connect to FTP site rtfm.mit.edu/publ enters the URL as follows ftp://rtfm.mit.edu/pub/

2. The browser opens the specified sites.

3. Display the directory that contains the file you want to download.

4. Right click the file and choose copy to folder.

5. Once you have saved the file in computer’s drive, you can open it using the appropriate software, depending upon the type of file. If it is a Doc file, you can use Microsoft Word to open this file.

FTP Common commands

The basic command is:

FTP hostname: This command written at the DOS prompt and it opens an interactive FTP session. When you want to run FTP in a script, the available switches are as follows: ftp [-v] [-ell [-i] [-n] [-g] [-s:filename] [-a] [-w:windowsize] [computer] summarizes the parameters that you can enter the command.

Most of the time, you use the FTP client in interactive mode. You should be aware of several commands when you are in this mode. Table lists some of the most common valid commands for the Microsoft FTP clients.

Many of the FTP commands are similar to DOS commands. For example, rmdir is the same as rd in DOS. mkdir, mdir are both used to make directory on the remote host. The other command is help which has the same function as in DOS. The type command types the ASCII characters of the file.

WHAT IS ANONYMOUS FTP

Anonymous File Transfer Protocol (FTP) enables remote users to use the FTP server without an assigned user ID and password. Anonymous FTP enables unprotected access (no password required) to selected information about a remote system. The remote site determines what information is made available for general access.

Such information is considered to be publicly accessible and can be read by anyone. It is the responsibility of the person who owns the information and the system to assure that only appropriate information is made available.

To access this information, a user logs on to the hosts using the user ID ANONYMOUS. The user ANONYMOUS has limited access rights to the files on the FTP server and has some operating restrictions. Typically, the following operations are only operations allowed.

• Logging on using FTP
• Listing the contents of a limited set of directories
• Retrieving files from these directories.

Typically, anonymous users are not allowed to transfer files to the FTP server. Some systems do provide an incoming directory for anonymous users to send data to. Traditionally, the special anonymous user account accepts a string as a password, although it is common to use either the password guest or one’s e-mail address. Some archive sites explicitly ask for the user’s e-mail address and do not allow logon with the guest password. Providing an e-mail address is a courtesy that allows the archive site operators to get some idea of who is using their services.

Anonymous FTP: Benefits and Disadvantages

FTP (File Transfer Protocol) servers normally require authentication. A user must login, providing a username and password, and the session will last only as long as the user stays logged in.

Anonymous FTP provides users with the ability to access an FTP server without providing password authentication. At first glance, this may sound insane, but it does have some practical uses.

Despite the name, it is not completely anonymous, and you can still track users who access your server through their IP addresses and the information that you require when they login (Some anonymous FTP services require the user to provide an email address).

The advantage of anonymous FTP is that you can provide fast access to public archives without having to tie up web server processes with multiple extended connections, especially when distributing large files. Because of this advantage, many public institutions like universities and libraries will distribute downloadable material through anonymous FTP. Some of them, like ibiblio.org, also distribute Linux distributions and other free and open source software.

The disadvantage of anonymous FTP is that you have little control over who accesses your FTP server or how often they do it. If you have particularly popular file downloads, it can place quite a load on the server. For this reason, many organization with limited resources have chosen alternatives like BitTorrent to distribute large files.

Because anonymous FTP allows users to access the server without authentication, you will need to take extra security steps to prevent exploits. Be sure to disable the SITE EXEC command, chroot the public directory, and disable any write privileges. If you have no pressing need to offer anonymous FTP services, it is better to disable it completely.

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