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Packet Capture Command in ChromeOS: Network Debugging

The packet_capture command in ChromeOS provides a way to capture network packets for debugging purposes. This tool is essential for developers and network administrators diagnosing connectivity or protocol issues.

Command Overview

Syntax

packet_capture [options]

Options

  • --device <device>: Specify the network device for the capture.
  • --max-size <max size in MiB>: Set a maximum file size for the capture.
  • --frequency <frequency>: Specify the frequency for over-the-air captures.
  • --ht-location <above|below>: Define the HT channel location for over-the-air captures.
  • --vht-width <80|160>: Set the VHT channel width for over-the-air captures.
  • --monitor-connection-on <monitored_device>: Initiate an over-the-air capture based on the channel parameters of a currently connected device.

Notes

  • Over-the-air captures are not available for all 802.11 devices.
  • Only device-based captures (--device and --max-size options) are supported in verified mode. Switch to developer mode for additional options.

Use Cases

Capturing Device Traffic

To capture packets on a specific network device:

packet_capture --device wlan0 --max-size 50
This starts a capture on wlan0 with a maximum file size of 50 MiB.

Over-the-Air Packet Capture

To capture packets on a specific frequency:

packet_capture --frequency 2412 --ht-location above --vht-width 80
This captures traffic on the frequency 2412 MHz with HT channel location set to "above" and a VHT width of 80 MHz.

Monitoring a Connected Device

To capture packets based on the channel of a connected device:

packet_capture --monitor-connection-on wlan0
This monitors over-the-air traffic based on wlan0's channel parameters.

Best Practices

  1. Select the Correct Device
  2. Use --device to capture traffic from a specific network interface.

  3. Ensure the device supports packet capture features.

  4. Set File Size Limits

  5. Use --max-size to prevent storage issues during long captures.

  6. Use Developer Mode for Advanced Options

  7. Over-the-air captures require developer mode for full functionality.

  8. Analyze Results

  9. Use tools like Wireshark to analyze the .pcap files generated by this command.

Common Errors

Insufficient Permissions

Error: Unable to initiate capture.

Resolution: Ensure you are in developer mode for advanced options and have sufficient permissions to access the network device.

Unsupported Device

Error: Over-the-air capture not supported.

Resolution: Verify that your network device supports over-the-air captures and is properly configured.

File Size Exceeded

Error: Capture file size exceeds the limit.

Resolution: Increase the --max-size parameter or reduce capture duration.

Example Scenarios

Debugging Network Issues

  1. Start a packet capture on the Wi-Fi device: 
    packet_capture --device wlan0 --max-size 100
  2. Reproduce the network issue.
  3. Stop the capture and analyze the .pcap file.

Analyzing Over-the-Air Traffic

  1. Monitor traffic on a specific frequency: 
    packet_capture --frequency 5180 --ht-location below --vht-width 160
  2. Use the capture to analyze interference or rogue devices.

By leveraging the packet_capture command, you can effectively troubleshoot and diagnose complex network issues in ChromeOS environments.