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Lab - Using Wireshark to Examine FTP and TFTP Captures

2013 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public. Page 2 of 14

In Part 1 of this lab, you will use Wireshark open source tool to capture and analyze TCP protocol header fields for FTP file transfers between the host computer and an anonymous FTP server. The Windows command line utility is used to connect to an anonymous FTP server and download a file. In Part 2 of this lab, you will use Wireshark to capture and analyze UDP protocol header fields for TFTP file transfers between the host computer and Switch S1.

Note: The switch used is a Cisco Catalyst 2960s with Cisco IOS Release 15.0(2) (lanbasek9 image). Other switches and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the available commands and output produced might vary from what displays in the labs.

Note: Make sure that the switch has been erased and has no startup configurations. If you are unsure, contact your instructor.

Note: Part 1 assumes the PC has Internet access and cannot be performed using Netlab. Part 2 is Netlab compatible.

Required Resources Part 1 (FTP) 1 PC (Windows 7, Vista, or XP with command prompt access, Internet access, and Wireshark installed)

Required Resources Part 2 (TFTP) 1 Switch (Cisco 2960 with Cisco IOS Release 15.0(2) lanbasek9 image or comparable)

1 PC (Windows 7, Vista, or XP with Wireshark and a TFTP server, such as tftpd32 installed)

Console cable to configure the Cisco IOS devices via the console port

Ethernet cable as shown in the topology

Part 1: Identify TCP Header Fields and Operation Using a Wireshark FTP Session Capture

In Part 1, you use Wireshark to capture an FTP session and inspect TCP header fields.

Step 1: Start a Wireshark capture.

a. Close all unnecessary network traffic, such as the web browser, to limit the amount traffic during the Wireshark capture.

b. Start the Wireshark capture.

Step 2: Download the Readme file.

a. From the command prompt, enter ftp ftp.cdc.gov. b. Log into the FTP site for Centers for Disease Control and Prevention (CDC) with user anonymous and

no password.

c. Locate and download the Readme file.

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Page 3 of 14

a for address,

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Page 4 of 14

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The TCP source port number belongs to the TCP session host that opened a connection. The value is normally a random value above 1,023.

The TCP destination port number is used to identify the upper layer protocol or application on the remote site. The values in the range 01,023 represent the well-known ports and are associated with popular services and applications (as described in RFC 1700, such as Telnet, FTP, HTTP, and so on). The combination of the source IP address, source port, destination IP address, and destination port uniquely identifies the session to both sender and receiver.

Note: In the Wireshark capture below, the destination port is 21, which is FTP. FTP servers listen on port 21 for FTP client connections.

The Sequence number specifies the number of the last octet in a segment. The Acknowledgment number specifies the next octet expected by the receiver. The Code bits have a special meaning in session management and in the treatment of segments.

Among interesting values are: - ACK Acknowledgement of a segment receipt. - SYN Synchronize, only set when a new TCP session is negotiated during the TCP three-way

handshake. - FIN Finish, request to close the TCP session.

The Window size is the value of the sliding window; determines how many octets can be sent before waiting for an acknowledgement.

The Urgent pointer is only used with an Urgent (URG) flag when the sender needs to send urgent data to the receiver.

The Options has only one option currently, and it is defined as the maximum TCP segment size (optional value).

Using the Wireshark capture of the first TCP session startup (SYN bit set to 1), fill in information about the TCP header:

From the PC to CDC server (only the SYN bit is set to 1):

Source IP Address:

Destination IP Address:

Source port number:

Destination port number:

Sequence number:

Acknowledgement number:

Header length:

Window size:

In the second Wireshark filtered capture, the CDC FTP server acknowledges the request from the PC. Note the values of the SYN and ACK bits.

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Page 6 of 14

t emented

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Page 7 of 14

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Page 8 of 14

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Page 9 of 14

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Reflection This lab provided the opportunity to analyze TCP and UDP protocol operations from captured FTP and TFTP sessions. How does TCP manage communication differently than UDP?

Challenge Because neither FTP nor TFTP are secure protocols, all transferred data is sent in clear text. This includes any user IDs, passwords, or clear-text file contents. Analyzing the upper-layer FTP session will quickly identify the user ID, password, and configuration file passwords. Upper-layer TFTP data examination is a bit more complicated, but the data field can be examined and the configuration user ID and password information extracted.

Cleanup Unless directed otherwise by your instructor:

1) Remove the files that were copied to your PC.

2) Erase the configurations on switch S1. 3) Remove the manual IP address from the PC and restore Internet connectivity.

Source IP Address: Destination IP Address: Source port number: Destination port number: Sequence number: Acknowledgement number: Header length: Window size: Sou rce IP addres ss: Des tination IP ad dress: Sou rce port num ber: Des tination port n number: Seq uence numbe er: Ackn nowledgemen nt number: Hea der length: Win dow size: Sou rce IP addres s: Des tination IP ad dress_2: Sou rce port numb ber: Des tination port n number_2: Seq uence numbe er_2: Ackn nowledgemen nt number_2: Hea der length_2: Win dow size_2: Sou rce IP Addres ss: Des tination IP Ad dress: Sou rce Port Num ber: Des tination Port Number: UDP P Message Le ength: UDP P Checksum: Sou rce IP Addres ss_2: Des tination IP Ad dress_2: Sou rce Port Num ber_2: Des tination Port Number_2: UDP P Message Le ength_2: UDP P Checksum_2: 1: 2: 3:

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