Whisperly - Network Debugging Tool for Developers

The Developer's Reality

Most developers don't need to become networking experts to debug production issues. This guide focuses on the 20% of PCAP knowledge that solves 80% of real-world problems.

What is a PCAP File?

PCAP (Packet Capture) is a file format that stores network traffic data. Think of it as a recording of all the network communications happening on a network interface during a specific time period.

PCAP File Basics

What's in a PCAP

Raw network packets
Complete network frames with all protocol headers and data
Timestamps
Precise timing information for each packet
Metadata
Interface information, capture duration, file stats

Common File Formats

.pcap
Original format, widely supported
.pcapng
Next-generation format with enhanced features
.cap
Alternative extension, same as .pcap

When Do You Need PCAP Analysis?

PCAP analysis is particularly useful when other debugging methods fall short:

Network Performance Issues

High Latency

When application metrics show slowness but system metrics look normal

Intermittent Failures

Problems that occur sporadically and are hard to reproduce

Connection Problems

Issues with establishing or maintaining network connections

Security Investigations

Unusual Traffic Patterns

Unexpected data flows or connections to unfamiliar destinations

Data Exfiltration

Suspected unauthorized data transfers from your systems

Protocol Violations

Incorrect or malicious use of network protocols

Essential PCAP Analysis Concepts

You don't need to understand every protocol detail. Focus on these key concepts:

The OSI Model (Simplified)

Understanding the network layers helps you know what to look for:

Application Layer

HTTP, DNS, SMTP - What developers interact with directly

Transport Layer

TCP, UDP - Responsible for end-to-end communication

Internet Layer

IP - Handles routing packets across networks

TCP Connection Lifecycle

Most application traffic uses TCP, so understanding its connection process is crucial:

Three-Way Handshake

SYN → SYN-ACK → ACK (Establishes connection)

Data Transfer

Application data exchanged with sequence/acknowledgment numbers

Connection Termination

FIN → ACK → FIN → ACK (Graceful closure)

Common Patterns to Look For

These patterns often indicate specific types of problems:

Problem Indicators

Retransmitted packets (duplicate data)
Missing acknowledgments (unconfirmed data)
Connection timeouts (incomplete handshakes)
Unexpected resets (abrupt connection closures)

Normal Patterns

Consistent packet sequencing
Regular acknowledgment flow
Expected connection establishment
Graceful connection termination

How to Capture PCAP Files

Before you can analyze network traffic, you need to capture it. Here are the most common methods:

Capture Tools and Commands

1. tcpdump (Most Common)

# Capture all traffic on interface eth0
sudo tcpdump -i eth0 -w capture.pcap

# Capture only HTTP traffic
sudo tcpdump -i eth0 port 80 -w http-traffic.pcap

# Capture traffic to/from specific host
sudo tcpdump -i eth0 host example.com -w host-traffic.pcap

# Capture for specific duration
sudo timeout 60 tcpdump -i eth0 -w timed-capture.pcap

2. Wireshark (GUI Tool)

# Install Wireshark
sudo apt install wireshark # Ubuntu/Debian
brew install --cask wireshark # macOS

# Run Wireshark GUI
wireshark

# Use tshark (command-line version)
tshark -i eth0 -w capture.pcap

3. Cloud Platform Tools

# AWS VPC Traffic Mirroring
aws ec2 create-traffic-mirror-session ...

# GCP Packet Mirroring
gcloud compute packet-mirrorings create ...

# Azure Network Watcher
az network watcher packet-capture create ...

Analyzing PCAP Files: A Practical Approach

When analyzing PCAP files, focus on these practical steps rather than deep protocol knowledge:

Start with the Big Picture

Before diving into details, get an overview of the capture:

# Get basic statistics with capinfos
capinfos capture.pcap

# Get protocol hierarchy with tshark
tshark -r capture.pcap -q -z io,phs

# Get conversation statistics
tshark -r capture.pcap -q -z conv,tcp

Filter for Relevant Traffic

Narrow down to the specific traffic you're investigating:

# Filter by IP address
tshark -r capture.pcap -Y "ip.addr == 192.168.1.100"

# Filter by port
tshark -r capture.pcap -Y "tcp.port == 80"

# Filter by protocol
tshark -r capture.pcap -Y "http"

# Combine filters
tshark -r capture.pcap -Y "ip.addr == 192.168.1.100 && tcp.port == 80"

Look for Anomalies

Search for common indicators of problems:

# Find TCP retransmissions
tshark -r capture.pcap -Y "tcp.analysis.retransmission"

# Find duplicate ACKs
tshark -r capture.pcap -Y "tcp.analysis.duplicate_ack"

# Find connection timeouts
tshark -r capture.pcap -Y "tcp.flags.syn == 1 && tcp.flags.ack == 0"

# Find connection resets
tshark -r capture.pcap -Y "tcp.flags.reset == 1"

Real-World Example: Debugging a Slow API

Let's walk through a practical example of using PCAP analysis to debug a slow API call:

Scenario

Problem

• API response time: 5 seconds (normal: 200ms)
• Database queries: Normal performance
• Application logs: No errors
• System metrics: CPU/memory normal

Hypothesis

• Network latency between services
• TCP connection establishment delays
• Packet loss causing retransmissions

Analysis Process

Capture Traffic

Used tcpdump to capture traffic during slow API calls

Filter for API Traffic

Applied filters to focus on HTTP traffic to the API endpoint

Identify Anomalies

Found 47 TCP retransmissions during the 5-second period

Root Cause

Network congestion was causing packet loss, requiring retransmissions

Tools for PCAP Analysis

While you can analyze PCAP files with command-line tools, these GUI tools make it much easier:

Command-Line Tools

tcpdump
Capture and basic analysis of network traffic
tshark
Command-line version of Wireshark with powerful filtering
capinfos
Display metadata and statistics about PCAP files

GUI Tools

Wireshark
Industry-standard GUI tool with powerful analysis features
whisperly
AI-powered analysis that explains network issues in plain English
CloudShark
Web-based PCAP analysis tool with collaboration features

Best Practices for PCAP Analysis

Follow these practices to make your PCAP analysis more effective:

Capture Best Practices

Capture during problem occurrence, not normal operation
Use appropriate filters to limit capture size
Document the time and conditions of capture
Capture on the correct network interface

Analysis Best Practices

Start with high-level statistics before diving into packets
Use display filters to focus on relevant traffic
Look for patterns rather than individual anomalies
Correlate findings with application logs and metrics

Analyze PCAP Files in Seconds

Upload your PCAP file to whisperly and get instant insights without learning Wireshark. AI-powered network analysis for developers.

Try whisperly - Free PCAP Analysis PCAP Analysis Guide

PCAP Analysis for Developers: What You Actually Need to Know