Network simulator : tutorial, Application mode, FTP client and server

Open the TP05_05.xml file and move to the Application mode.
The two stations are connected to the Internet via an Internet service provider.
Their IP address was obtained automatically from the ISP, and their gateway is correctly configured.
A FTP client is installed on st1, as well as a FTP server on st2.
Watch the configuration of the two software components.
Choose "hard disk" in st2’s pop-up menu: two files can be downloaded, going to the c:\ftp folder.
Choose "hard disk" in st1’s pop-up menu: in the c:\folder, a file has been set for sending to the FTP server.
Choose the "step by step" simulation mode, and launch the FTP client on st1.
Click on the "connect" button.
Immediately after connection, the client sends the "ls" command, and the server sends back the content of its "document root" folder.

FTP client window

Select the "ftp02.txt" file, and click on the button "<<": the file is downloaded from the server.
Note: for each exchange, the server opens a port that will be used to transmit the data.
Port 21 is used to transmit the commands.
Click on the "disconnect" button.
Close the FTP client.
Edit the "ftp02.txt" file on st1’s hard disk:

File edition

Start again the FTP client, and select the "active client" mode.
Click on the "connect" button. Now, when the "ls" command is sent, the client provides a port number required for transmission of data.
The server does not open a port, but uses port 20. You can make sure it does, using the balloon help displayed in TCP textbox:

FTP in active mode

Download again the "ftp02.txt" file by clicking on the button "<<".
Close the FTP client, and make sure that the file has been replaced on st1’s hard disk.
Start again the FTP client, and connect yourself to the server as an anonymous user.
After the execution of the "ls" command, select the "upload.txt" file in the list located on the left, and click on the ">>" button. The server rejects the request as anonymous connections do not have any write permission.
Try to connect yourself once again, but this time as a "root" user: the file is indeed uploaded to the server.

Open the TP05_06.xml file, and move to the Application mode.
The FTP client (on st1) is in a local area network that accesses the Internet through a router.
As is the case in most situations, the FTP server (on st2) is directly connected to the Internet.
Launch the FTP client on st1, make sure that the "passive client" mode is selected, click on the "trace" checkbox, and then on the "connect" button.
When the exchanges are completed, the list of the commands is displayed. The address translations and port translations carried out by the router for each command are displayed:

Tracing natpat translations

Let us detail the case of the "ls" command (the number of the ports which are used change from one execution to another one):

The message initially goes from the address "10.0.0.2" (port produced by the client) to the address "172.12.0.3" (port 21).
rt1 NatPat OUT (172.11.0.3:4421) -> (172.12.0.3:21): When it sends its connection to the Internet, the router relocates the source of the message by putting his own address IP, before storing this translation in its NatPat table.
When the server sends back the opened data port, the router reads its NatPat table, and relocates the destination.
rt1 NatPat IN (172.12.0.3:21) -> (10.0.0.2:7228): the message is re-directed to st1.
The client uses the returned port to send the "ready" message. In fact, this message corresponds to the negotiation of the TCP exchange at the transport level.
rt1 NatPat OUT (172.11.0.3:9690) -> (172.12.0.3:8029): the router relocates the source of the message by putting his own address IP, before storing the translation in its NatPat table.
The server sends back the list of files, using its data port.
rt1 NatPat IN (172.12.0.3:8029) -> (10.0.0.2:3352): the message is re-directed to st1.

Disconnect the FTP client, select the "active client mode", then, check the "trace" checkbox, and finally, click on the "connect" button.
Everything is fine, until the server tries to send its list of files...
When you are in the active mode, it is the client who indicates the number of the data port; while the server still uses port 20.

Tracing a FTP client in active mode

Let us detail the case of the "ls" command (the number of the ports which are used change from one execution to another):

The message initially goes from the address "10.0.0.2" (port produced by the client, in this case, 7085) to the address "172.12.0.3" (port 21).
rt1 NatPat OUT (172.11.0.3:5432) -> (172.12.0.3:21): the router relocates the source of the message by putting his own address IP, before storing this translation in its NatPat table.
The server sends back the list of files by using port 20, this time targeting port 4989 (the one transmitted by the client).
rt1 reads to its NatPat table: the only connection which is specified concerns port 7085, which is used to send the LS command: you cannot relocate a message made to be sent to port 4989.

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