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networking > protocols > ftp

FTP

This protocol, along with Telnet and SMTP/POP3, is one of the cornerstones of the Internet (beginning 10/4/1971). The security of the system is now rather dated (more secure systems are available) but it did provide a fundamental way to transfer files reliably from one machine to another. Once the FTP protocol dialogue is invoked, it has its own set of commands (in the same fashion as Telnet). Here is a command prompt with ftp invoked by typing it in and as can be seen, the commands of the protocol are shown after typing a ? for help.

C:\>ftp

ftp > ?

Commands may be abbreviated. Commands are:

Here are some example explanations of the Microsoft Windows FTP client provided through the Command Prompt window.

These were discovered by typing a ? mark in the FTP client and then the name of the command. Try it!

Notice the above list also contains filing system commands. Directories can be listed, created, navigated, and deleted. This means not only are the files accessible for download but the directory structure to receive uploaded files can also be controlled and altered by the client (with their permissions). This is how early World Wide Web pages were uploaded to be viewed around the world. Now web browsers like Internet Explorer can automatically invoke the FTP protocol to download files, and can also emulate a remote FTP site with Windows style dragging and dropping for organisation of files. In this case the browser is quietly invoking the FTP protocol above to interact with the remote site.

FTP sites are usually secure in that a user name and password is required to access them. However if an FTP site is used for general distribution of files and security is not an issue, then the FTP server can allow users to logon under the username "anonymous" (or an email address is sometimes requested for the password, so users can be notified of updates etc. Note, however, the warnings above for telling anyone your email address) and often no password is required. This feature was handy as it meant the system administrator was not forced to keep large lists of users and passwords up to date. It also meant that visitors remained just that, and could do no damage to the site. This system is ideal for distributing updates such as drivers for hardware etc. However, early system administrators were often unaware they had left their FTP site open for read and write- that is open to anyone to upload and download materials. People who wanted to distribute materials that were breaking copyright were using these sites to upload materials and then simply inform other people of the FTP URL i.e. where it was on the Internet. Systems Administrators were often unaware of their assistance to these law breakers until they noticed the traffic through their site has growing very quickly, or disk space had "disappeared" quickly, or even more embarrassing, they were informed that they were illegally distributing copyright materials. The moral of these stories are don't leave services open on servers that other people can access. If FTP is not being used then close the service down completely, or at least regularly check it is secure, and not being abused.

An FTP connection uses two TCP ports and this means the control sequences have their own port and the data is transferred on the second data port. This means pure binary can be transmitted on one port without interfering with the parsing of the instructions/commands on the "text port".

The early popularity of FTP allowed many files to be made available on many servers to the point where finding any one file became really difficult. The answer at this stage was to come up with a directory program that indexed many files into a centralised data structure. People who have used linked URLs or web search engines or file exchange programs such as Napster or Gnutella will probably be surprised that this concept was not immediately discovered. However the development at this time makes a big contribution, as it showed just being able to get the information was one thing, to find it was quite another. The system that was developed was called GOPHER. This became an indexed list of the locations for many different types of files. However the service did not index the contents of the files. This concept was to be really developed whne the following application, the Web Browser took off and indexing of sites with search engines such as AltaVista and Google.

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