How to Decide Which Layer tcpdump Operates On, Layer-by-Layer Breakdown with tcpdump

Understanding tcpdump Across OSI Layers

This explains how tcpdump operates across different layers of the OSI model and how to interpret its output depending on the filter and protocol used.

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✨ Overview

tcpdump is a powerful command-line packet capture tool used to observe network traffic. It captures packets from the network interface and displays headers and payloads depending on the specified options. It can operate across multiple OSI layers, depending on what data is being captured. tcpdump itself is not bound to one OSI layer — it captures and displays packets that contain information across multiple layers. The layer you’re observing depends entirely on:

• The protocol you’re filtering for
• The level of detail in the capture
• Whether the traffic is encrypted

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🌌 OSI Layer-Based Operation

OSI Layer	Captured by tcpdump?	What It Sees
Layer 2 - Data Link	Yes	Ethernet headers: MAC addresses, EtherType, frame checks
Layer 3 - Network	Yes	IP headers: Source/destination IPs, TTL, protocol (TCP, UDP, ICMP)
Layer 4 - Transport	Yes	TCP/UDP headers: Ports, flags, sequence numbers
Layer 5–6 - Session/Presentation	Indirectly	TLS handshakes, session resumption, encryption metadata (not content)
Layer 7 - Application	Only if unencrypted	HTTP, DNS, FTP content if traffic is not encrypted (e.g., port 80, not 443)

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🔍 Layer-by-Layer Examples

Layer 2 (Data Link Layer)

tcpdump -e -i eth0 

Shows Ethernet headers, MAC addresses (Ethernet headers: source/destination MACs).

Layer 3 (Network Layer)

tcpdump ip or tcpdump host 192.168.1.10 

Filters by IP packet or specific IP address (IP headers: source/destination IP, TTL, protocol).

Layer 4 (Transport Layer)

tcpdump tcp port 443 

Captures TCP handshakes and encrypted TLS payloads(TCP/UDP headers: ports, flags, seq/ack numbers). 

Layer 7 (Application Layer)

tcpdump -A port 80

Displays readable HTTP request/response data (if not using HTTPS), Application data (e.g., HTTP headers) – only if unencrypted

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🔒 Encrypted Traffic Consideration

If the traffic is encrypted (e.g., HTTPS on port 443), tcpdump will only show TLS handshake metadata. You cannot see HTTP headers or payloads unless you:

• Use a tool like Wireshark with decryption keys.
• Have access to TLS termination proxy with decrypted logs. 

🔐 Important Note:

tcpdump never decrypts encrypted traffic. It only shows raw packet bytes.To analyze Layer 7 for HTTPS, you need decryption tools like:

• Wireshark + SSLKEYLOGFILE
• TLS termination proxy (MITM-like setup in dev)
• Reverse proxy logs (e.g., Apache or NGINX access logs)

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🔎 How to Determine Which Layer You're Observing

Command/Filter	Primary Layer Observed
tcpdump -e	Layer 2 (Ethernet addresses, frame type)
tcpdump host <IP>	Layer 3 (IP addresses, protocol (TCP/UDP))
tcpdump tcp port 443	Layer 4 (TCP/UDP port numbers, flags)
tcpdump -A port 80	Layer 7 (HTTP, DNS, FTP (if unencrypted))
tcpdump port 443 with TLS handshake	Layer 6 (TLS Handshake info (SNI, version))

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✅ Summary

• tcpdump can observe Layer 2 to Layer 4 directly.
• Layer 7 content is only visible if unencrypted.
• TLS introduces encryption at Layer 6, hiding application content.
• Use appropriate flags and filters to target the layer and protocol you want to analyze.

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For further inspection and visualization, consider exporting tcpdump captures using -w and opening them in Wireshark.