When working with network packet analysis, engineers often need to convert binary .cap or .pcap files into human-readable text formats for further processing. Wireshark provides several built-in methods and command-line tools for this conversion.

The most powerful method is using Wireshark's command-line companion, tshark. Here's a basic conversion command:

tshark -r input.cap -V > output.txt

This command:

• -r specifies the input file
• -V enables verbose packet details
• > redirects output to a text file

For more structured output that's easier to parse programmatically:

tshark -r input.cap -T fields -e frame.number -e ip.src -e ip.dst -e tcp.port > structured_output.csv

This generates CSV-formatted data with specific fields. You can customize the fields using Wireshark's display filter field names.

For automated processing, you can call tshark from Python:

import subprocess

def convert_pcap_to_text(input_file, output_file):
    command = f"tshark -r {input_file} -V"
    with open(output_file, 'w') as f:
        subprocess.run(command, shell=True, stdout=f, text=True)
        
convert_pcap_to_text('network_trace.cap', 'analysis_output.txt')

The Python scapy library provides another approach:

from scapy.all import *

packets = rdpcap('input.cap')
with open('output.txt', 'w') as f:
    for pkt in packets:
        f.write(pkt.show(dump=True))

For large files, consider processing packets in batches:

tshark -r large_capture.cap -Y 'frame.number <= 1000' -V > first_1000_packets.txt

This uses a display filter (-Y) to limit output to the first 1000 packets.

For modern applications, JSON output might be preferable:

tshark -r input.cap -T json > packet_data.json

This creates a JSON document that can be easily parsed with Python's json module or similar libraries in other languages.

When processing large files, these optimizations help:

• Use -c to limit packet count
• Filter packets with -Y before conversion
• Pipe output directly to processing scripts

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Wireshark primarily uses the .pcap (Packet CAPture) file format, though you might also encounter .pcapng (next generation) files. These binary formats contain raw network packet data with full protocol information.

Wireshark's command-line companion tshark provides the most straightforward conversion method:

tshark -r input.pcap -V > output.txt

Key options:

• -r: Read input file
• -V: Verbose packet details
• -T fields -e frame.number -e ip.src: Extract specific fields

For programmatic access in Python, Scapy provides excellent packet manipulation capabilities:

from scapy.all import *

packets = rdpcap("input.pcap")
with open("output.txt", "w") as f:
    for i, pkt in enumerate(packets):
        f.write(f"Packet {i}:\n")
        f.write(pkt.show(dump=True))
        f.write("\n\n")

To extract HTTP requests specifically:

tshark -r input.pcap -Y "http.request" -T json > http_requests.json

Or with Python:

http_packets = [pkt for pkt in packets if pkt.haslayer('HTTP')]

Other conversion options include:

• text2pcap (reverse conversion)
• Wireshark's File → Export Packet Dissections → As Plain Text
• capinfos for metadata extraction

For large capture files (500MB+), use stream processing:

from scapy.all import PcapReader

with PcapReader("large.pcap") as pcap_reader:
    with open("output.txt", "w") as f:
        for pkt in pcap_reader:
            # Process packet here
            pass