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Background:
After reading the old FTP tutorial, I decided rather then fixing
it up, a complete rewrite was necessary. In this new FTP tutorial,
instead of following in the footsteps of the old tutorial, I would
actually like to talk about the File Transfer Protocol. This tutorial
is aimed at the intermediate newbie. One that knows the basics of
the internet: what a socket is, what TCP/IP is, etc.
Down To Work:
The File Transfer Protocol (FTP), as its name states, is a set of
rules that dictates how files should be transfered over the TCP/IP
protocol. A basic FTP connection consists of a client and a server.
The client gets a file by opening a connection to the server. Usually,
the server is run on port 21, however, the system administrator can
change this if he or she wishes.
Once the client has connected to the server, the server will send
a “greeting” to the client. A greeting usually looks something like
the following:
| 220 SpiderMan's FTP server. Please login! |
The number before the text is known as a “completion code”. The
server greeting will be one of two codes: 220 if the server accepts
the connection or 421 if the server rejects the connection.
After the server has sent its greeting, the client sends a request.
A request is made up of a verb and, for some verbs, a parameter. Common
verbs are:
| Verbs |
Description |
| CWD |
Change the current directory on the server. |
| PWD |
Print the current directory on the server. |
| CDUP |
Moves up to the parent directory. |
| LIST |
List the contents of a directory. |
| MKD |
Creates a directory on the server. |
| RMD |
Removes a directory from the server. |
| DELE |
Removes a file from the server. |
| USER |
Sends the username for the login. |
| PASS |
Sends the password for the login. |
| ABOR |
Abort the transfer. |
| QUIT |
Closes the connection with the server. |
| STAT |
Gets the current status of the server. |
| TYPE |
Toggles the binary flag on the server. |
| PORT |
Asks the server to connect to the client. |
| PASV |
Requests a data connection on a new port. |
| RETR |
Requests the server to send a file. |
| STOR |
Sends a file from the client to the server |
| APPE |
Same as STOR, except data is appended. |
| REST |
Start a download at a certain position. |
| SYST |
Gets the OS information of the server. |
| HELP |
Get help on a verb. |
| NOOP |
No operation. |
After sending a request, the client should wait for the server
to respond before the client sends another request; however, the
client can send an ABOR, STAT, or QUIT request without waiting for
the server to respond. Typically, after the server has sent the
greeting, the client will respond with a USER request in which the
client sends the username to log into the FTP server with.
After the client has sent the server a request, the server replies
with a “response”. A response consists of a completion code followed
by one, or more, lines. Generally, if the second digit of the completion
code is a 0, it is a syntax error message; if the second digit of
the completion code is a 2, it is a hello or goodbye message.
Once the client is logged in, he or she would then get the file he
or she wishes to retrieve. A typical session would look similar to
this:
220 SpiderMan's FTP server. Please login!
USER SpiderMan
331 Username okay. Send password!
PASS password
230 Password accepted, user logged in.
LIST
150 Opening ASCII mode data connection for /bin/ls
226 Transfer complete
TYPE I
200 Type set to I
PASV
227 Entering passive mode (206,84,161,87,28,46)
RETR datafile.zip
150 Opening BINARY mode data connection for datafile.zip
226 Transfer complete |
When downloading a file, the client should specify whether he or
she wants the file sent in ASCII mode (each line ended with a CR/LF)
or binary mode. To set the mode of transfer, the client sends the
TYPE request. TYPE I would set the binary mode. By default, binary
mode is turned off. It is important that the server send the file
in the correct mode to avoid file corruption.
I should elaborate on the PASV and PORT request since they are very
important to FTP. You may be wondering just what the PASV request
is used for and what the odd looking response from the server is supposed
to mean. When the client sends a PASV request, the server opens up
a temporary socket and sends a reply to the client which informs the
client of port to connect to. The reply would look like this:
| PASV 227 Entering passive mode (206,84,161,87,28,46) |
The numbers in the parentheses play an important role. The numbers
separated by the first four commas make up the IP address to connect
to. In this example, the IP would be 206.84.161.87. The remaining
numbers specify which port to connect to. Usually, this is done
by multiplying the first number by 256 and adding the second number.
In this example, the port number would be 7214 (28*256+46= 7214).
Now that the server has replied to the PASV request, two channels
are open: the first (the original one) is the “communication channel”
where the requests are sent and the second is the “data channel”
where the data is transfered.
The PORT request is similar to the PASV request, however, when the
client sends a PASV request, the server opens another socket and the
client connects to it. When a client sends a PORT request, the server
connects to the client—usually on port 20.
With the advent of IPv6, you may be wondering how FTP will be carried
out since the servers are replying to the PASV requests with an IPv4
IP. One solution, proposed by D. Bernstein, would be to have IPv6
servers send a nonexistent IP address and have IPv6 clients ignore
the IP and skip straight to the port number. This way, older clients
using IPv4 will connect to a fake IP and give up trying to connect
to the server.
Now that you know how FTP works, I'll show you how to log into a server,
look around, and then get a file. Before I begin, I should tell you
about anonymous FTP servers. An anonymous FTP server is just like
a normal server, however, you can login using the username “anonymous”
and an e-mail as the password. In this example, I'll use a fake e-mail
of guest@guest.com. Most anonymous FTP servers only read up to the
@ sign, so if you're lazy you can just type in “guest@”. A lot of
sites offer anonymous FTP because it is an easy way to let the public
get files without assigning each person a login. On with the example!
First, I connect to the server and send my username and password.
220 SpiderMan's FTP server. Please login!
USER anonymous
331 Anonymous login okay, send e-mail as password.
PASS guest@guest.com
230 Password accepted, logged in as anonymous. |
Now that I'm in, I'll change to the “files” directory and download
a file.
CWD ./files
250 CWD command successful.
TYPE I
200 Type set to I
PASV
227 Entering passive mode (210,52,165,168,15,26)
RETR code.zip
150 Opening BINARY mode data connection for code.zip
226 Transfer complete |
I've got the file I want, now I'll move up to the parent directory,
move to another directory, and download another file.
CWD ..
250 Okay
CWD ./jokes
250 CWD command successful.
TYPE A
200 Type set to A
PASV
227 Entering passive mode (210,52,165,168,15,26)
RETR jokes.txt
150 Opening ASCII mode data connection for jokes.txt
226 Transfer complete |
I got the two files I wanted, so now I'll logout.
QUIT
221 Goodbye, please come back! |
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