# Competition Pcap Protocol

> Analyze PCAP files by reconstructing TCP/UDP sessions, decoding application-layer protocols, and correlating packet sequences with host or malware behavior for CTF challenges.

- Skill: `zhaoxuya520/competition-pcap-protocol` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add zhaoxuya520/competition-pcap-protocol`
- Raw SKILL.md: https://api.skillmd.com/api/skills/zhaoxuya520/competition-pcap-protocol/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security, Data & Analytics, Data Analysis, Penetration Testing
- Tags: Ctf, Malware Analysis, Network Forensics, Packet Capture, Pcap, Protocol Decoding, Session Reconstruction, Tcp Reassembly
- Author: zhaoxuya520 (https://skillmd.com/u/zhaoxuya520)
- Updated: 2026-08-22
- Page: https://skillmd.com/skills/zhaoxuya520/competition-pcap-protocol

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# Competition PCAP Protocol

Use this skill only as a downstream specialization after `$ctf-sandbox-orchestrator` is already active and has established sandbox assumptions, node ownership, and evidence priorities. If that has not happened yet, return to `$ctf-sandbox-orchestrator` first.

Use this skill when the decisive evidence sits inside packet order, protocol framing, or stream reconstruction rather than a single IOC or host log.

Reply in Simplified Chinese unless the user explicitly requests English.

## Quick Start

1. Establish the capture boundaries first: hosts, time span, interfaces, missing packets, retransmits, and stream count.
2. Group traffic into sessions before decoding payload semantics.
3. Record protocol framing, sequence, timing, and transferred artifacts together instead of as isolated packets.
4. Correlate packet evidence with host, malware, or app behavior only after the session is reconstructed.
5. Reproduce the smallest decoded stream or transferred artifact that proves the challenge path.

## Workflow

### 1. Build The Session Map

- Identify endpoints, protocols, ports, TLS handshakes, DNS lookups, websocket upgrades, and long-lived streams.
- Note missing capture coverage, asymmetric routing, packet loss, or reassembly issues before drawing conclusions.
- Separate control channels, bulk transfers, keepalives, and noise.

### 2. Decode The Protocol Boundary

- Reassemble TCP streams or UDP conversations before interpreting fields.
- Recover framing, message order, custom headers, binary fields, compression, encryption boundaries, and object transfers.
- Keep payload direction, timing, and session state aligned with each decoded message.

### 3. Tie Packets To Behavior

- Show which packet sequence maps to which host event, malware branch, login flow, upload, exfiltration step, or command channel.
- Distinguish protocol recognition from artifact recovery: naming HTTP, DNS, or a custom C2 is not enough without decoded content or proven downstream effect.
- If the task becomes mostly a host timeline problem after decode, switch to the tighter forensic timeline skill.

## Read This Reference

- Load `references/pcap-protocol.md` for the session checklist, decode checklist, and evidence packaging.
- If the hard part is a WebSocket or SSE handshake, subscription flow, realtime frames, or frame-driven state, prefer `$competition-websocket-runtime`.
- If the hard part is a custom handshake, framing, checksum, sequence dependency, or deterministic replay harness, prefer `$competition-custom-protocol-replay`.

## What To Preserve

- Stream IDs, endpoint pairs, packet ranges, timestamps, protocol framing, and object boundaries
- Decoded requests, responses, commands, transferred files, and the session that carried them
- The exact packet sequence or reconstructed stream that proves the challenge behavior