FTP - File Transfer Protocol TFTP – Trivial FTP CISC 856 – Fall 2008

FTP - File Transfer ProtocolTFTP – Trivial FTP

CISC 856 – Fall 2008Shriram Ganesh

University of Delaware

(some/most slides courtesy of Brian Lucas,

Umakanth Puppala, William BoyerVikram Rajan, Michael Haggerty, and Prof Amer)

ganesh@cis.udel.edu

File Transfer Protocol (RFC 959)• Why FTP?• FTP’s connections• FTP in action• FTP commands/responses

Trivial File Transfer Protocol (RFC 1350)• TFTP and TFTP’s message formats• FTP and TFTP compared

Overview

Network Use

Direct (e.g. telnet)

Indirect (e.g. FTP)

RFC 114 – April 1971 before TCP and IP existed - Used NCP to do FTP on ARPANETRFC 354 – July 1972 - Overall Communication ModelRFC 542 – August 1973 - Remarkably similar to today’s FTP - Still based on NCPRFC 765 – June 1980 - FTP over TCP/IP

A Bit of History - FTP

Purpose: To Transfer files between two computersGoals of FTP Service

• Promote sharing of files (programs and/or data)• Encourage indirect/implicit use of remote

computers• Shield users from variations in file storage among

hosts• Transfer data reliably and efficiently

Why do we need a FTP service?

• At first, file transfer may seem simple• Heterogeneous systems use different:

– Operating Systems– Character Sets– Naming Conventions– Directory Structures– File Structures and Formats

• FTP needs to address and resolve these problems

Problems of file transfer

UserInterface

User Data Transfer

Function

UserProtocol

Interpreter

ServerProtocol

Interpreter

Server Data Transfer

Function

client

server

ControlConnection

DataConnection

* Insulates users from “raw” FTP commandsServer is listening on port 21for connection requests

2

* Routes “raw” FTP commands * Receives server’s replies

* Persistent command and reply connection

Non-persistent data connection

21

20

*Server uses port 20for data connections

FTP’s 2 Connections

UserInterface

User Data Transfer

Function

UserProtocol

Interpreter

ServerProtocol

Interpreter

Server Data Transfer

Function

client

server

ControlConnection

DataConnection

ftp> open strauss.udel.eduConnected to strauss.udel.edu220 strauss FTP server ready.

USER ganesh

331 Password req for ganesh.Password:

PASS mypass

230 User ganesh logged in.ftp>

FTP’s 2 Connections - Establishment

128.4.40.17 (19×256)+137128.4.40.17:5001

User Data Transfer

Function

Server Data Transfer

Function

UserInterface

UserProtocol

Interpreter

ServerProtocol

Interpreter

client

server

ControlConnection

DataConnection

ls client.txt

Passive open onPort 5001

PORT 128,4,40,17,19,137200 Port Command SucessfulLIST client.txt150 Data Connection will be open shortly226 Closing Data Connection

-rw-r--r-- lucasb client.txt

Establish Data Connection

UserProtocol

Interpreter

ServerProtocol

Interpreter

128.4.40.17 19,137

FTP’s 2 Connections – Data Transfer

UserInterface

UserProtocol

Interpreter

ServerProtocol

Interpreter

client

server

ControlConnection

DataConnection

User Data Transfer

Function

Server Data Transfer

Function

bye

QUIT221 Service Closing

FTP’s 2 Connections – Connection Closing

ftp> open server SYN

SYN|ACKACK

220 Service Ready

ftp> USER ganesh

ACK

ACK331 User OK,password?

ACKftp> PASS mypass

ACK230 User login OK

ACK

Client Server

21Eph

FTP Connection

PORT 128,4,40,17,19,137

200 Command Successful

SYN

LIST client.txt

SYN-ACKACK 150 Data Connection

will be open shortly

NAME LIST

FINFIN-ACK

226 Closing Data Connection

ACK

ACK

ACK

ACK

ACK

Control connectionData Connection

Client Server

ACK

Eph

Eph

21

21

500120

5001 20

FTP – Data transfer (get command)

PORT 128,4,40,17,19,137

200 Command Successful

SYN

LIST client.txt

SYN-ACKACK 150 Data Connection

will be open shortlyClient.txt

FINFIN-ACK

226 Closing Data Connection

ACK

ACK

ACK

ACK

ACK

Control connectionData Connection

Client Server

ACK

Eph

Eph

21

21

500120

5001 20

FTP – Data transfer (put command)

Command Descriptionget filename Retrieve file from server

mget filename* Retrieve multiple files from server

put filename Copy local file to server

mput filename* Copy multiple local files to server

open server Begin login to server

bye / close / exit Logoff server

ls / dir List files in current remote dir on server

lcd Change local directory

cd Change remote directory

rhelp / remotehelp Lists commands the server accepts

FTP Client Commands (issued by user interface)

*Server sends list of matching files to client, Client protocol interpreter asks the user for operation on each matching file.

Command DescriptionLIST [filelist ] List files or directories (ls / dir)

USER username Send username to server

PASS password Password on server

PORT h1,h2,h3,h4,p1,p2 Client IP and port number

RETR filename Retrieve (get) filename

STOR filename Store (put) filename

TYPE (A, I, E, N or T) Defines the file type or print format

A-PDU FTP Commands

Reply Description1yz

2yz3yz

4yz

5yz

Positive preliminary reply.The action is being started but expect another reply before sending another cmd.Positive completion reply. A new cmd can be sent.Positive intermediate reply. The cmd has been accepted but another cmd must be sent.Transient negative completion reply. The requested action did not take place but can be sent laterPermanent negative completion reply. Cmd not accepted and should not be reissued.

x0zx1zx2z

x3z

x4zx5z

SyntaxInformationConnections. Replies referring to control or data connections.

Authentication and accounting

UnspecifiedFilesystem status

FTP Response Format

• 120 Service will be ready shortly• 200 Command OK• 230 User login OK• 331 User name OK; password is needed• 421 Service not available• 530 User not logged in• 552 Requested action aborted; exceeded storage

allocation

Example FTP Responses

• FTP has 2 connections- Control (persistent connection)

- Server issues a passive open on well-known 21- Client uses an ephemeral port to issue active open- Server ultimately closes control connection

- Data (ephemeral connection)- Client issues passive open on an ephemeral port- Client sends this port to server via PORT

command- Server receives the port number and issues active

open using its well-known 20 to the received ephemeral port

Summary of FTP Connections

• PORT does not always work…why?• Instead, use PASV command

– Client sends PASV command to server– Server chooses ephemeral port: passive open– Server responds with IP, Port in reply (227)– Client issues active open to server’s port

• Ultimately, the data sender closes connection

Data Connection

User Data Transfer

Function

Server Data Transfer

Function

UserInterface

UserProtocol

Interpreter

ServerProtocol

Interpreter

client

server

ControlConnection

DataConnection

ls client.txt

Passive open onPort 5125

PASV227 Entering Passive Mode (128,4,40,42,20,5) LIST client.txt150 Data Connection will be open shortly226 Closing Data Connection

-rw-r--r-- lucasb client.txt

Establish Data Connection

UserProtocol

Interpreter

ServerProtocol

Interpreter

FTP Passive Data Transfer

• Used only to read and write files from/to a remote server– Cannot list directories

• Useful for bootstrapping diskless systems

TFTP

UDP

IP

Ethernet

Physical

Trivial FTP (TFTP)

Diagrams from McGraw-Hill

TFTP Message Formats

Diagram from McGraw-Hill

TFTP Connection Establishment

Client Server69

Passive open

a. Passive open by server

Client Server69

b. Active open by client

Active open

50032

Client Server62000

c. Rest of communication

5003269

Diagram from McGraw-Hill

TFTP Data Transfer

Read Request RRQ “fullOS”

DATA 1ACK 1

Timeout

ACK 2

Client Server

DATA 2

DATA 2

Timeout

ACK 3

DATA 3

DATA 3

First Block of 512 Bytes Sent

Block 2 Lost

Block 3 Damaged

ACK 4DATA 4

Timeout

ACK 4ACK 4 Lost

Eph69

Eph

Timer running

TFTP Connection - Timers

DATA 5

ACK 5 Timeout

Client Server

DATA 5

ACK 5 DATA 6

ACK 6 DATA 6

ACK 6 DATA 7

ACK 7 DATA 7

ACK 7 DATA 8

ACK 8 DATA 8

ACK 8

Block 8 is the LastBlock (383 Bytes)

ACK 5 is Slow

Discard Duplicate

Resend Data 6

Data is SentTwice, Known asThe Sorcerer's Apprentice Bug

TFTP Connection (Cont’d)

FTP TFTP2 connections: control

(21) and data (20)1 connection (69), stop

and wait flowReliable service using

TCPUses UDP, handles own

retransmissionsMany commands 5 message types, only

reads/writes filesMinimal security using

logon procedureNo logon or security

Larger code size, full-featured

Lightweight, designed to fit on ROM

FTP vs. TFTP

Security IssuesFTP Bounce Attack

FTP Server

Attacker Victim

10.0.18.30 10.0.18.35

sthuy

LoginPORT 10.0.18.35:5000LIST

Data Connection

Control messages

• According to FTP protocol, client is *supposed* to specify its own IP address and port number.

• Port Scan Attack – Attacker gathers information on ports of target machine

FTP Bounce Attack (cont’d)

Attacker: 10.0.18.30 FTP login account: sthuy Target: 10.0.18.35

FTP Bounce Attack (cont’d)

• Method of data transfer which uses the FTP protocol’s PASV mode.

• Transfer data from one remote server to another (inter-server) without routing this data through the client's connection.

• Enabling this can make a server vulnerable to the FTP bounce attack.

File Exchange Protocol (FXP)