Automating Network Packet Capture for an Ethical Hacking Robot

In my last article, TCPDump & Python, we explored using the tcpdump command to capture local network traffic. But for my ethical hacking robot, I need to take things a step further. Instead of just capturing packets from a single machine, I want to monitor the entire subnet the robot has access to.

The ultimate goal? One day, the robot should be able to autonomously scan networks, analyzing traffic without human intervention. But before we get ahead of ourselves, there’s an important question: What about permission?

To ensure that the robot isn’t capturing unauthorized data, I’ve implemented a simple safeguard—it checks for a permission file before starting tcpdump. If the file exists, the robot assumes it has permission to scan. Otherwise, it shuts down packet capturing quietly. With that in place, let’s dive into how the robot actually captures packets.

The Capture Script

To automate packet capture, I created a Bash script called pcap-capture.sh. This script does a few key things:

1. Checking for Permission

Before doing anything else, the script verifies whether a permission file exists. If it doesn’t, the script exits immediately.

# Check permission file
if [[ ! -f "$PERMISSION_FILE" ]]; then
    echo "Permission file not found! Exiting."
    exit 1
fi

2. Loading Configuration Variables

The script reads a configuration file to set important variables like output directories and capture settings.

# Load config
CONFIG_FILE="/etc/pcap-capture.conf"
if [[ ! -f "$CONFIG_FILE" ]]; then
    echo "Config file $CONFIG_FILE not found!"
    exit 1
fi
source "$CONFIG_FILE"

3. Preparing the Output Directory

To keep things organized, the script ensures that an output directory exists, sets the correct permissions, and cleans up any old .pcap files before starting a new capture session.

# Ensure output directory exists
mkdir -p "$OUTPUT_DIR"

# Set proper permissions for output directory (owned by root but accessible)
chmod 777 "$OUTPUT_DIR"

# Remove old .pcap files before starting a new capture session
echo "Clearing previous capture files in $OUTPUT_DIR..."
find "$OUTPUT_DIR" -name "*.pcap" -type f -delete

4. Identifying the Network Subnet

The robot needs to determine which subnet it has access to before capturing traffic. A function is included in the script to find this information dynamically.

# Function to get subnet for an interface
get_subnet() { 
   # Implementation to determine the network range
}

5. Starting the Packet Capture

Once the subnet is identified, the script launches tcpdump for each network interface to capture traffic. It writes the captured packets to files, rotating them based on size and time limits.

echo "tcpdump -i "$IFACE" net "$SUBNET" -w "$FILE" -C "$FILE_SIZE_MB" -G "$ROTATE_SECONDS" -z gzip"

See full code here

With the script ready, the next step is ensuring it runs automatically whenever the robot starts up.

Running the Script at Startup with systemd

To make sure the packet capture starts on boot, I’ve created a systemd service that runs pcap-capture.sh as a background process.

[Service]
Type=forking
ExecStart=/usr/local/bin/pcap-capture.sh
Restart=always

After defining the service, I enable it with:

systemctl daemon-reload
systemctl enable pcap-capture.service

Now, whenever the robot starts up, it will check for permission, and if allowed, it will begin capturing packets automatically. In the future, I plan to add network triggers so that the robot starts capturing packets only when there are network changes.

Why Capture Network Traffic?

So, why does the robot need to capture network traffic? There are a few good reasons:

1. Security Monitoring – By analyzing network traffic, the robot can help audit networks for unusual or suspicious activity.
2. Hacking Exploration – From an ethical hacking perspective, capturing packets can help discover active devices and their communication patterns.
3. Storage Estimation – Unlike netstat or nmap, tcpdump generates a massive amount of data.I need to determine how much storage the robot will require in its final build, given that tcpdump can generate a massive volume of data.

If you were building a hacking robot, what’s the first thing you’d make it do? Let me know—I’m always looking for creative ideas! 🚀