ftp protocol details

1

Tips for the assignmentTips for the assignment

2

Socket: a door between application process and end-end-transport protocol (UDP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

Internet

Socket Programming using TCPSocket Programming using TCP

4

client

Client socket

server

Welcome socket

Connection socket

time

Client socket

bytes

Client Sockets & Server SocketsClient Sockets & Server Sockets

5

Client/server socket interaction: Client/server socket interaction: TCPTCP

wait for incomingconnection requestconnectionSocket =accept()

create socket,port=x, forincoming request:welcomeSocket =

Socket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

ServerServer (running on hostid) ClientClient

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

Application 2-5

6

socket

bind

listen

loop "forever"

{

accept /* by creating new socket */

/* process the client’s request */

loop until done

{

receive/send

}

closesocket(newsocket)

}

Simple Server Pseudo-CodeSimple Server Pseudo-Code

7

socket

connect

/* process the client’s request */

loop until done

{

send/receive

}

closesocket(socket)

Simple Client Pseudo-CodeSimple Client Pseudo-Code

8

socket

bind

listen

loop "forever"

{

accept /* by creating new socket */

/* process the clients request */

loop until done

{

receive/send

}

closesocket(newsocket)

}

Proper Ordering of Proper Ordering of SendSend and and RecvRecv

socket

connect

/* process the clients request */

loop until done

{

send/receive

}

closesocket(socket)

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Test the Test the TCPTCP Client-Server Client-Server CodesCodes

Run ServerWindows.cServerWindows.c Compile ClientWindows.cClientWindows.c, look for the

executable. Run ClientWindows.cClientWindows.c from the command

prompt to connect to the server: ClientWindows localhost ClientWindows localhost 12341234 Alternatively, use IpConfig to find out what your IP

address is: (e.g. 130.123.123.111), then connect to the server using:• ClientWindows ClientWindows 130.123.123.111130.123.123.111 12341234

Let’s see how to use the compiler’s Let’s see how to use the compiler’s debug debug mode mode to trace the exchange of messages.to trace the exchange of messages.

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FTP (Multiple Clients)FTP (Multiple Clients)

In TCP, the Server should be running already prior to a Client connecting to it

SSServerServer

TCP Control SocketTCP Control Socket

CCTCP Control SocketTCP Control Socket

Listening Listening SocketSocket

DIRDIR

TCP Active Data SocketTCP Active Data Socket

Port 127,0,0,1,Port 127,0,0,1,6,116,11

QuitQuit

After file transferAfter file transfer

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FTP OperationFTP Operation

Source: Computer Networking and the Internet (5/e) by Fred Halsall

CLIENTCLIENT

SERVERSERVER

Control connection

Data connection

Note that Ports 1216 & 1217 are ephemeral ports. Any port [1024, 65535] will work.

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FTP CommandsFTP Commands

COMMAND DESCRIPTION

USER username User name on the FTP server

PASS password User’s password on the FTP server

SYST Type of operating system requested

TYPE type File type to be transferred: A (ASCII), I (Image/Binary)

PORT n1,n2,n3,n4,n5,n6 Client IP address (n1-n4) and port number (n5, n6)

RETR filename.type Retrieve (get) a file

STOR filename.type Store (put) a file

LIST filelist List files or directories

QUIT Log off from server


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FTP Reply CodesFTP Reply Codes

REPLY DESCRIPTION

11yz Positive reply, wait for another reply before sending a new command

22yz Positive reply, a new command can be sent

33yz Positive reply, another command is awaited

44yz Negative reply, try again

55yz Negative reply, do not retry

x00z Syntax

x11z Information

x22z Control or data connection

x33z Authentication

x44z Unspecified

x55z File status


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ActiveActive FTP (or Standard ) FTP (or Standard )

Standard (or PORT or Active)The Standard mode FTP client sends PORT commands to the FTP server. These commands are sent over the FTP command channel when establishing the FTP session.

OperationOperationStandard mode FTP clients first establish a connection to TCP port 21 on the FTP server. This connection establishes the FTP command channel. The client sends a PORT command over the FTP command channel when the FTP client needs to send or receive data, such as a folder list or file. The PORT command contains information about which port the FTP client receives the data connection on. In Standard mode, the FTP server always starts the data connection from TCP port 20. The FTP server must open a new connection to the client when it sends or receives data, and the FTP client requests this by using the PORT command again.

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PassivePassive FTP (or PASV) FTP (or PASV)

Passive (or PASV)The Passive mode client sends PASV commands to the FTP Server.

OperationPassive mode FTP clients also start by establishing a connection to TCP port 21 on the FTP server to create the control channel. When the client sends a PASV command over the command channel, the FTP server opens an ephemeral port (between 1024 and 5000) and informs the FTP client to connect to that port before requesting data transfer. As in Standard mode, the FTP client must send a new PASV command prior to each new transfer, and the FTP server will await a connection at a new port for each transfer.

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p. 50p. 50, RFC 959, RFC 959

19Source: Computer Networking and the Internet (5/e) by Fred Halsall

ACTIVEACTIVE FTP Operation FTP Operation

CLIENTCLIENTSERVERSERVER

PortPort2020

PortPort2121

Data connection control connection

timetime time time

Passive open

PortPort11201120

PortPort11211121

Data connectioncontrol connection




PortPort2020

PortPort2121


timetime time time

Passive openActive open

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time


TCP control connection to Port 21 established

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time



220 FTP server ready

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>

331 password required

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>

220 user <username> logged in

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>


SYST

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>


SYST215 UNIX Type:X Version:Y

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>


get <filename.type>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>


PORT n1-n6 get <filename.type>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>


PORT n1-n6200 PORT command successful

get <filename.type>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>



get <filename.type>

RETR <filename.type>

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>



get <filename.type>


150 opening ASCII mode data connection

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>



get <filename.type>


150 opening ASCII mode data connection

Server does an active open to Port n5, n6

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time time




USER <username>


PASS <password>



get <filename.type>


150 opening ASCII mode data connectionSend file through data connection


PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


timetime time




USER <username>


PASS <password>



get <filename.type>




226 File transfer complete

User prompted for a conmand

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


time




USER <username>


PASS <password>



get <filename.type>




226 File transfer complete


QUIT

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


time




USER <username>


PASS <password>



get <filename.type>




226 File transfer complete,


QUIT221 Goodbye

PortPort11201120

PortPort11211121





PortPort2020

PortPort2121


PortPort11201120

PortPort11211121


time




USER <username>


PASS <password>



get <filename.type>




226 File transfer complete,


QUIT221 GoodbyeServer closes data connection

first , then control connection


PASSIVEPASSIVE FTP OperationFTP Operation


PortPort20242024

PortPort2121


timetime time time

Passive open

PortPort11201120

PortPort11211121



PASSIVEPASSIVE FTP OperationFTP Operation


PortPort20242024

PortPort2121


timetime time time


PortPort11201120

PortPort11211121



PASSIVE PASSIVE FTP OperationFTP Operation


PortPort20242024

PortPort2121


timetime time time



PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121

Data connection control connection Data connection

timetime time time




USER <username>


PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


PortPort11201120

PortPort11211121

timetime time time




USER <username>


PASS <password>


SYST





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


SYST215 UNIX Type:X Version:Y

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV

227 Entering PASV mode (192,168,150,90,195,149)

Port2024

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV


Port2024

dir

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV


Port2024

LIST dir

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV


Port2024

LIST dir150 Opening ASCII mode data connection for file list

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV


Port n5, n6(or Port 2024 in this

example)


Client does an active open to Port n5,n6

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime time time




USER <username>


PASS <password>


PASV



example)



Send file through data connection

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime




USER <username>


PASS <password>


PASV



example)



Send file through data connection226 File transfer complete

User is prompted for a command

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime




USER <username>


PASS <password>


PASV



example)




User is prompted for a commandQUIT

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime




USER <username>


PASS <password>


PASV



example)





221 Goodbye

PortPort11201120

PortPort11211121





PortPort20242024

PortPort2121


timetime




USER <username>


PASS <password>


PASV



example)





221 GoodbyeServer closes data connection first , then control connection

PortPort11201120

PortPort11211121


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Security IssuesSecurity Issues

Firewall administrators may not want to use Passive Passive mode FTP servers mode FTP servers because the FTP server can open any ephemeral port number.

Many FTP servers are configured with an ephemeral ephemeral port rangeport range of 10241024 through 6553565535.

Firewall configurations that allow full access to all ephemeral ports for unsolicited connections may be considered unsecuredunsecured.

http://support.microsoft.com/kb/323446

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How to change the Internet Explorer How to change the Internet Explorer FTP Client modeFTP Client mode

Start Internet Explorer. On the Tools menu, click Internet Options. Click the Advanced tab. Under Browsing, click to clear the Enable folder

view for FTP sites check box. Click to select the Use Passive FTP Passive FTP (for firewall

and DSL modem compatibility) check box. Internet Explorer behaves as a Standard mode FTP client if you select

the Enable folder view for FTP sites check box, even if you also select the Use Passive FTP check box. If you clear the Enable folder view for FTP sites check box and then select the Use Passive FTP check box, Internet Explorer behaves as a Passive mode FTP client.

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The EndThe End

ftp protocol details

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