Limit Capture Size in Tshark

Introduction

In this lab, you will learn to control packet capture size in Wireshark using Tshark commands. You'll explore two essential techniques: limiting total packets with -c 500 and restricting packet length with -s 128 to capture only headers while saving storage space.

Through practical exercises on the eth1 interface, you'll capture traffic into limited.pcap and observe how different settings affect your captures. This hands-on experience will help you optimize packet collection for various analysis scenarios.

Skills Graph

Set Packet Count with -c 500

In this step, you'll learn how to control packet capture duration by limiting the number of packets Wireshark collects. The -c option is particularly valuable when you need a representative sample of network traffic without overwhelming your system with continuous capture.

1. Begin by opening the terminal in your LabEx VM. The system automatically places you in the correct working directory (~/project), so you can start immediately.

2. The command structure uses this basic pattern:

tcpdump -c [number] [other options]

Here, [number] determines exactly when the capture stops by specifying the maximum packets to collect. This prevents indefinite runs that could fill your storage.

3. Let's capture 500 packets as our main exercise. Execute this command:

sudo tcpdump -c 500 -i eth1

Notice we specify eth1 as the network interface. This tells tcpdump which network connection to monitor - crucial when your machine has multiple network adapters.

4. As packets flow, you'll see real-time output. After exactly 500 packets, tcpdump automatically terminates and displays statistics:

500 packets captured
500 packets received by filter
0 packets dropped by kernel

These numbers confirm successful capture (first line), proper filtering (second line), and no system resource issues (zero drops).

5. Before the full 500-packet capture, test with a smaller sample to understand the mechanism. This 10-packet trial helps verify your setup:

sudo tcpdump -c 10 -i eth1

Define Snapshot Length with -s 128

In this step, you will learn how to set the snapshot length (snaplen) when capturing packets using the -s option. This determines how much of each packet is captured, with 128 bytes being a common value that captures packet headers while conserving storage space. The snapshot length is particularly useful when you only need packet headers for analysis rather than full packet contents.

1. First, ensure you're in the default working directory ~/project in your terminal. This is where we'll run all our capture commands to keep things organized.

2. The snapshot length option (-s) limits how much of each packet is captured by specifying the number of bytes to record. Smaller values save disk space but may miss important payload data. The basic syntax is:

tcpdump -s [length] [other options]

3. Let's capture packets with a snapshot length of 128 bytes combined with the packet count from the previous step. This command will capture the first 128 bytes of each packet until it reaches 500 packets:

sudo tcpdump -c 500 -s 128 -i eth1

4. You'll see output showing the first 128 bytes of each packet. The capture will automatically stop after 500 packets as specified by the -c option. This combination helps manage both capture size and duration.

5. To better understand how snapshot length affects captures, try these comparison commands. Notice how the amount of data displayed changes with different -s values:

sudo tcpdump -c 5 -s 64 -i eth1 ## Captures only 64 bytes per packet
sudo tcpdump -c 5 -s 0 -i eth1  ## Captures entire packets (default)

The -s 0 setting tells tcpdump to capture the full packet, which is useful when you need complete packet contents but consumes more storage space.

Capture Traffic with -i eth1

In this step, we'll focus on capturing network traffic from a specific interface using Tshark's -i option. Network interfaces are the physical or virtual connections your computer uses to communicate with networks. When you have multiple interfaces (like Wi-Fi and Ethernet), specifying the correct one is crucial for targeted packet analysis.

1. Open your terminal and navigate to the working directory:

cd ~/project

2. The -i flag tells Tshark which network interface to monitor. The basic command structure is:

tcpdump -i [interface] [other options]

Here, [interface] should be replaced with your actual interface name, typically 'eth1' for Ethernet or 'wlan0' for Wi-Fi.

3. Now let's combine this with what we've learned about packet limits (-c) and snapshot length (-s). This command will capture 500 packets from the eth1 interface, saving only the first 128 bytes of each packet:

sudo tcpdump -c 500 -s 128 -i eth1

The sudo is required because packet capture needs administrative privileges.

4. If you're unsure which interfaces are available on your system, run:

tcpdump -D

This displays a numbered list of all active network interfaces, helping you identify the correct one for your capture.

5. To generate test traffic while capturing, open another terminal and run:

ping -c 3 google.com

This sends 3 ICMP packets to Google's servers, which should appear in your capture. Watching these known packets helps verify your capture is working correctly.

6. The capture will automatically stop after 500 packets, but you can manually stop it anytime by pressing:

Ctrl+C

This keyboard interrupt safely terminates the capture process while preserving any captured data.

Save to File with -w limited.pcap

In this step, you'll learn how to save captured network traffic to a file for later analysis. The -w option in tcpdump creates a packet capture (pcap) file that preserves all the captured network data. This is particularly useful when you need to examine traffic patterns offline or share captures with colleagues.

1. Before starting, make sure your terminal is in the correct working directory. Type:

cd ~/project

This ensures all your capture files will be saved in the designated project folder.

2. The -w flag tells tcpdump where to store the captured packets. The basic command structure is:

tcpdump -w [filename] [other options]

The filename should end with .pcap extension, which is the standard format for packet capture files.

3. Now let's combine all the options we've learned so far into a practical example. This command will:

• Capture exactly 500 packets (-c 500)
• Limit each packet to 128 bytes (-s 128)
• Listen on the eth1 interface (-i eth1)
• Save everything to a file called limited.pcap (-w limited.pcap)

sudo tcpdump -c 500 -s 128 -i eth1 -w limited.pcap

4. While tcpdump is running, open another terminal window to generate some test traffic. These commands will create typical network activity:

ping -c 3 google.com
curl http://example.com

This simulated traffic will be captured by our running tcpdump session.

5. After capturing 500 packets (or press Ctrl+C to stop earlier), verify your capture file exists and check its size:

ls -lh limited.pcap

The output shows the file details including size (56K in this example) and creation time:

-rw-r--r-- 1 root root 56K Aug 10 15:30 limited.pcap

6. To review your captured packets later, use the -r option to read the pcap file:

tcpdump -r limited.pcap

This displays the packet contents exactly as they were captured, allowing you to analyze the traffic at your convenience.

Summary

In this lab, you have learned to control packet capture size in Tshark using essential command-line parameters. You practiced limiting packet counts with -c 500 and restricting packet length with -s 128, observing how these options affect capture behavior and storage efficiency.

The exercises demonstrated combining these techniques with interface selection (-i eth1) for targeted traffic analysis. Through commands like sudo tcpdump -c 500 -s 128 -i eth1, you gained hands-on experience in applying multiple capture constraints simultaneously.