Analyzing Network Packets with Scapy
Overview
Scapy is a Python packet manipulation library that enables crafting, sending, sniffing, and dissecting network packets at granular protocol layers. This skill covers using Scapy for security-relevant tasks including TCP/UDP/ICMP packet crafting, pcap file analysis, protocol field extraction, SYN scan implementation, DNS query analysis, and detecting anomalous traffic patterns such as unusually fragmented packets or malformed headers.
When to Use
- When investigating security incidents that require analyzing network packets with scapy
- When building detection rules or threat hunting queries for this domain
- When SOC analysts need structured procedures for this analysis type
- When validating security monitoring coverage for related attack techniques
Detection Gaps & Validation
- Offline analysis misses what was never captured:
rdpcap() only sees what the tap/SPAN forwarded. Truncated snaplen (tcpdump -s 96) strips payload so DNS-tunnel qry.name entropy and HTTP bodies vanish — capture full frames (-s 0) before scoring.
- Scapy reassembles nothing by default: a SYN-flood ratio check on raw packets misreads retransmits; sessionize with
sessions()/packet.sessions() and follow streams before judging TCP flag ratios, or fragmented C2 (IP frag/MF bit set) is read as benign noise.
- Encrypted blind spots: DNS-over-HTTPS/DoT (443/853) and TLS exfil have no plaintext
DNSQR.qname to measure — entropy heuristics silently return nothing. Pivot on SNI from the TLS ClientHello, JA3 hashing, and packet-size/timing rather than concluding "no tunneling."
- Validate the detector actually fires: craft a positive sample —
IP()/UDP()/DNS(qd=DNSQR(qname="<60-char-base64>.evil.com")) written with wrpcap() — feed it back through your analysis script, and confirm the high-entropy/long-label rule flags it. No flag means the threshold or layer filter is wrong.
- FP tuning: CDN hostnames, DNS SRV/TXT lookups, and chatty mDNS produce long/odd labels legitimately; baseline
qname length distribution per host and whitelist known resolvers before alerting.
Prerequisites
- Python 3.8+ with
scapy library installed (pip install scapy)
- Root/administrator privileges for raw socket operations (sniffing, sending)
- Npcap (Windows) or libpcap (Linux) for packet capture
- Authorization to perform packet operations on target network
Steps
- Read and parse pcap/pcapng files with
rdpcap() for offline analysis
- Extract protocol layers (IP, TCP, UDP, DNS, HTTP) and field values
- Compute traffic statistics: top talkers, protocol distribution, port frequency
- Detect SYN flood patterns by analyzing TCP flag ratios
- Identify DNS exfiltration indicators via query length and entropy analysis
- Craft custom probe packets for authorized network testing
- Export findings as structured JSON report
Expected Output
JSON report containing packet statistics, protocol distribution, top source/destination IPs, detected anomalies (SYN floods, DNS tunneling indicators, fragmentation attacks), and per-flow summaries.
1---2name: analyzing-network-packets-with-scapy3description: Craft, send, sniff, and dissect network packets using Scapy for protocol analysis, network reconnaissance, and traffic anomaly detection in authorized security testing4license: Apache-2.05---67# Analyzing Network Packets with Scapy89## Overview1011Scapy is a Python packet manipulation library that enables crafting, sending, sniffing, and dissecting network packets at granular protocol layers. This skill covers using Scapy for security-relevant tasks including TCP/UDP/ICMP packet crafting, pcap file analysis, protocol field extraction, SYN scan implementation, DNS query analysis, and detecting anomalous traffic patterns such as unusually fragmented packets or malformed headers.121314## When to Use1516- When investigating security incidents that require analyzing network packets with scapy17- When building detection rules or threat hunting queries for this domain18- When SOC analysts need structured procedures for this analysis type19- When validating security monitoring coverage for related attack techniques2021## Detection Gaps & Validation2223- **Offline analysis misses what was never captured:** `rdpcap()` only sees what the tap/SPAN forwarded. Truncated snaplen (`tcpdump -s 96`) strips payload so DNS-tunnel `qry.name` entropy and HTTP bodies vanish — capture full frames (`-s 0`) before scoring.24- **Scapy reassembles nothing by default:** a SYN-flood ratio check on raw packets misreads retransmits; sessionize with `sessions()`/`packet.sessions()` and follow streams before judging TCP flag ratios, or fragmented C2 (IP `frag`/`MF` bit set) is read as benign noise.25- **Encrypted blind spots:** DNS-over-HTTPS/DoT (443/853) and TLS exfil have no plaintext `DNSQR.qname` to measure — entropy heuristics silently return nothing. Pivot on SNI from the `TLS` ClientHello, JA3 hashing, and packet-size/timing rather than concluding "no tunneling."26- **Validate the detector actually fires:** craft a positive sample — `IP()/UDP()/DNS(qd=DNSQR(qname="<60-char-base64>.evil.com"))` written with `wrpcap()` — feed it back through your analysis script, and confirm the high-entropy/long-label rule flags it. No flag means the threshold or layer filter is wrong.27- **FP tuning:** CDN hostnames, DNS SRV/TXT lookups, and chatty mDNS produce long/odd labels legitimately; baseline `qname` length distribution per host and whitelist known resolvers before alerting.2829## Prerequisites3031- Python 3.8+ with `scapy` library installed (`pip install scapy`)32- Root/administrator privileges for raw socket operations (sniffing, sending)33- Npcap (Windows) or libpcap (Linux) for packet capture34- Authorization to perform packet operations on target network3536## Steps37381. Read and parse pcap/pcapng files with `rdpcap()` for offline analysis392. Extract protocol layers (IP, TCP, UDP, DNS, HTTP) and field values403. Compute traffic statistics: top talkers, protocol distribution, port frequency414. Detect SYN flood patterns by analyzing TCP flag ratios425. Identify DNS exfiltration indicators via query length and entropy analysis436. Craft custom probe packets for authorized network testing447. Export findings as structured JSON report4546## Expected Output4748JSON report containing packet statistics, protocol distribution, top source/destination IPs, detected anomalies (SYN floods, DNS tunneling indicators, fragmentation attacks), and per-flow summaries.
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