Lab 11: File Transfer Protocol
Please read completely before you begin!
This lab introduces the FTP protocol and command set.
Review
The File Transfer Protocol or FTP (RFC 959) is used to move files from one machine to another. Like
email protocols, the protocol provides a simple command set that is used by FTP software. But in several
ways, FTP can be more complex.
Here are a few FTP commands
Command
USER user’s name
PASS password
SYST
STAT
HELP
QUIT
Description
Log into host
Supply user’s password
Get a description of the remote system
Find out the status of the connection
List commands or get information about a specific command
End the session
Here is a slightly edited sample session using several FTP commands. This session was created by using
TELNET to connect to the destination server at port 21.
220 nobody.nowhere.org FTP server (Version 6.00LS) ready.
user joe
331 Password required for joe.
pass hushhush
230 User joe logged in.
syst
215 UNIX Type: L8 Version: BSD-199506
stat
211- nobody.nowhere.org FTP server status:
Version 6.00LS
Connected to joe (10.0.36.71)
Logged in as joe
TYPE: ASCII, FORM: Nonprint; STRUcture: File; transfer MODE: Stream
No data connection
211 End of status
help
214- The following commands are recognized (* =>'s unimplemented).
USER
PORT
TYPE
MLFL*
MRCP*
DELE
SYST
RMD
STOU
PASS
LPRT
STRU
MAIL*
ALLO
CWD
STAT
XRMD
SIZE
ACCT*
EPRT
MODE
MSND*
REST
XCWD
HELP
PWD
MDTM
SMNT*
PASV
RETR
MSOM*
RNFR
LIST
NOOP
XPWD
REIN*
LPSV
STOR
MSAM*
RNTO
NLST
MKD
CDUP
QUIT
EPSV
APPE
MRSQ*
ABOR
SITE
XMKD
XCUP
214 Direct comments to ftp-bugs@nobody.nowhere.org.
quit
221 Goodbye.
This example shows an FTP login to nohow.nowhere.org. At first glance, this looks a lot like the email
sessions. But if you try some of the other commands such as LIST, you’ll discover that most don’t work.
The reason is that FTP opens separate connections to transfer information, something that a TELNET client
can’t cope with. This means that we are going to have to turn to other tools to see how FTP works,
specifically packet capture. (Still, using TELNET can be helpful. For example, it can be used to confirm
that the FTP server is operational or to investigate which modes it will support.)
Lab
For each of the following steps describe your results, give the syntax of the command you used, and where
appropriate, the output produced. Include screen captures as needed in your output. Be sure to label your
results carefully and organize your results in the order the steps are given here and to answer each question
in your report.
With this project, you will capture output on your local machine and capture traffic with Ethereal. With
Ethereal, traffic can be captured at either end of the connection. Use whichever is easier. As with previous
labs, you will connect to the remote machine maud.
1.
Use TELNET to connect to maud, capture the session, and include it in your report. Use your name
and account editing out your password. Try some commands like HELP, NLST, PWD, etc. paying
attention to the reply codes. The first digit indicates general success and failure while the second and
third digits provide additional information. For example, a bad password will result in the reply code
530—a 5 for permanent negative completion, the 3 for authentication and accounting, and the 0 for the
specific error. Summarize the possible values for the first digit in the return codes. (See RFC 959 for
the details.)
2.
Using an FTP client, download a small file. (See the documentation for FTP on your system if you
aren’t familiar with it.) Capture the session using Ethereal. What filter do you need so that you only
see traffic between the FTP client and server? Use the Follow TCP Stream option to display the
session. Explain each command used in the session.
3.
Include a screen capture in your report that shows all the packets that are exchanged. Explain in some
detail the file transfer session from the previous step.
4.
Stream, block, and compressed transfers are often used with FTP. However, not every implementation
of FTP supports all three modes. You can discover which modes a server supports by simply trying to
put it into the appropriate mode and seeing how it responds. TELNET to maud (port 21) and try the
commands MODE B, MODE C, and MODE S. Which modes does maud support?
5.
(Optional) Software tools like GetRight allow you to stop and, at a later time, resume an FTP
connection. How does GetRight manage to resume a session?
Please notice that some of these parts may ask several questions. Be sure you answer every question for
every part.
Copyright © 2003, Dr. Joseph D. Sloan