protocol-reverse-engineering

Overview of protocol-reverse-engineering skill

What the protocol-reverse-engineering skill is for

The protocol-reverse-engineering skill helps an agent work through the practical steps of capturing, analyzing, and documenting unknown or poorly documented network protocols. It is best used when your real goal is not “explain packets” in the abstract, but “help me figure out how this client and server actually talk so I can debug, interoperate, test, or document it.”

Who should install it

This skill is a strong fit for:

• security researchers inspecting proprietary traffic
• developers debugging custom client/server behavior
• engineers building compatible integrations without full protocol docs
• analysts who already have packet captures and need a structured investigation path

It is especially useful for protocol-reverse-engineering for Debugging, where the pain point is usually finding message boundaries, request/response patterns, state transitions, or field meanings quickly.

What it adds beyond a generic prompt

A normal prompt may tell an agent to “analyze this PCAP.” The protocol-reverse-engineering skill is more useful because it anchors the workflow around actual capture and inspection methods: Wireshark, tshark, tcpdump, and MITM-style collection for HTTP/HTTPS traffic. That makes it better for installation decisions and practical usage, not just theory.

What users usually care about first

Before installing, most users want to know:

1. whether it helps with real packet capture and filtering
2. whether it is useful for unknown proprietary protocols
3. whether it can support debugging, not just security research
4. whether they need existing PCAP files first

For this skill, the answer is: it is most valuable once you already have traffic, can collect traffic, or can describe the target protocol context clearly.

Main limitation to know up front

This skill is documentation-heavy rather than automation-heavy. There are no helper scripts, parsers, or bundled dissectors in the skill folder. If you want one-command decoding, this is not that. If you want a structured protocol-reverse-engineering guide that helps an agent reason about captures, filters, streams, and protocol structure, it is a better fit.

How to Use protocol-reverse-engineering skill

Install context for protocol-reverse-engineering

Install the skill from the repository:

npx skills add https://github.com/wshobson/agents --skill protocol-reverse-engineering

After installation, trigger it when your task involves traffic capture, protocol dissection, stream inspection, or producing protocol notes from observed network behavior.

Read this file first

Start with:

• SKILL.md

This skill is essentially all in one file, so there is little repo archaeology required. That is good for speed, but it also means you should read the sections that match your stage:

• capture setup if you do not have traffic yet
• analysis filters if you already have a PCAP
• documentation/dissection guidance if you are turning observations into a reusable protocol description

What inputs the skill needs

The protocol-reverse-engineering usage quality depends heavily on the inputs you provide. Best inputs include:

• a pcap or pcapng file
• protocol transport details like TCP/UDP port, hostnames, IPs, or process name
• whether traffic is plaintext, compressed, framed, or encrypted
• a sample client action timeline such as “login, fetch list, send command, disconnect”
• any known message examples, magic bytes, headers, or error codes

If you do not provide these, the agent can still suggest a workflow, but it will be much less specific.

Turn a rough goal into a strong prompt

Weak prompt:

Analyze this protocol.

Better prompt:

Use the protocol-reverse-engineering skill to help me reverse engineer traffic in capture.pcap. The suspected service runs on TCP port 8080. I need message boundaries, request/response pairs, likely field meanings, and anything useful for debugging intermittent client failures after login. Assume I can inspect streams in Wireshark and run tshark filters if needed.

Why this works:

• it names the artifact
• it narrows the transport
• it states the output format you want
• it gives the debugging objective, not just the analysis topic

Best workflow for first-pass analysis

A practical protocol-reverse-engineering guide with this skill usually looks like:

1. identify the right interface or capture source
2. collect full packets, not truncated snapshots when possible
3. filter by relevant port/host/process
4. isolate a single session or stream
5. map request/response order
6. look for repeated headers, lengths, counters, IDs, and status fields
7. document hypotheses before trying to fully decode every byte

That sequence matters. Many reverse-engineering attempts fail because users jump straight into field guessing before they have session boundaries and message sequencing.

Capture methods the skill supports well

The skill gives practical collection paths for:

• Wireshark live capture
• tshark file capture and ring buffers
• tcpdump for lightweight CLI capture
• MITM collection with mitmproxy or proxy-based interception for HTTP/HTTPS-style traffic

This makes it useful if your blocker is still “how do I get the traffic safely and completely?” rather than only “how do I decode it?”

When Wireshark is the right starting point

Use Wireshark first when you need:

• stream following
• visual packet inspection
• quick display filters
• side-by-side comparison of repeated transactions

For many proprietary protocols, “Follow TCP Stream” is the fastest way to detect whether the payload is plaintext, length-prefixed binary, or mixed control/data traffic.

When tcpdump or tshark is better

Use tcpdump or tshark when:

• you need remote or headless capture
• GUI access is inconvenient
• you want repeatable capture commands
• traffic volume is high and you need rotation or file-based workflows

This is one of the stronger practical aspects of the protocol-reverse-engineering skill: it does not assume a GUI-only workflow.

How to prompt for protocol-reverse-engineering for Debugging

For debugging-focused use, ask the agent to produce:

• transaction timeline
• expected versus failing exchange
• suspected state machine transitions
• points where sequence numbers, flags, or lengths become inconsistent
• candidate root causes such as framing mismatch, timeout behavior, or malformed fields

Example:

Use the protocol-reverse-engineering skill for Debugging. Compare successful and failed sessions on port 44321. Focus on where the protocol diverges after authentication, and list field-level or sequencing hypotheses I should test.

Practical tips that improve output quality

A few details materially change the result:

• capture full packets with -s 0 when using tcpdump
• separate a single representative session before asking for deep analysis
• annotate what user action caused each burst of traffic
• include both successful and failing samples when debugging
• mention if TLS, compression, or app-layer encoding is involved

Without that context, the agent may overfit on byte patterns that are incidental rather than structural.

protocol-reverse-engineering skill FAQ

Is this protocol-reverse-engineering skill good for beginners

Yes, if you already understand basic networking concepts like TCP streams, ports, and request/response flow. It is not a beginner networking course. It is more useful for guided investigation than for teaching packet fundamentals from scratch.

Do I need a PCAP before installing

No, but you need either:

• a way to capture traffic, or
• enough system context for the agent to suggest a capture plan

If you have neither, the skill will still be readable, but the practical value is lower.

Can it handle encrypted protocols

Partially. The protocol-reverse-engineering skill can help you identify encrypted sessions, capture metadata, and suggest MITM-style workflows where appropriate. It does not magically decrypt unknown TLS traffic or bypass application protections on its own.

How is it different from a normal reverse-engineering prompt

A generic prompt often stays abstract. This skill gives the agent a concrete protocol-analysis frame: capture tools, filters, stream inspection, and documentation mindset. That usually reduces guesswork when the task is operational, not academic.

When is this skill a poor fit

Skip it if your problem is mainly:

• binary executable reversing with no network component
• malware unpacking unrelated to wire protocols
• application-layer logic that never leaves process memory
• a need for automated dissector generation out of the box

This is a network-protocol investigation skill, not a universal reverse-engineering toolkit.

Does it fit modern debugging workflows

Yes. The strongest fit is mixed debugging work where you need to move between CLI capture, GUI packet inspection, and protocol note-taking. That makes it a practical choice for protocol-reverse-engineering usage inside real incident, interoperability, or QA workflows.

How to Improve protocol-reverse-engineering skill

Give the agent a narrower target

The fastest way to improve protocol-reverse-engineering skill results is to reduce ambiguity. Provide:

• exact port or endpoint
• one clean session
• the user action that triggered it
• what “success” and “failure” look like

This helps the agent infer structure instead of searching the whole capture blindly.

Ask for hypotheses, not certainty too early

Good reverse engineering is iterative. Ask the agent for:

• likely message framing
• probable field candidates
• confidence levels
• tests to confirm or reject each hypothesis

That produces better next steps than demanding a full protocol spec from the first pass.

Compare known-good and bad traffic

For protocol-reverse-engineering for Debugging, the highest-leverage input is often two captures:

• one working session
• one failing session

That lets the agent spot divergence points in ordering, field values, lengths, retries, or timing. A single broken trace is much harder to interpret.

Provide decoded context around the packets

Even small bits of external context improve accuracy:

• “this packet happens right after login”
• “this app sends a heartbeat every 5 seconds”
• “we expect a list of 12 records here”
• “the server closes when payload exceeds 4 KB”

These clues help distinguish protocol semantics from random payload differences.

Common failure modes to avoid

Users often get weaker results when they:

• provide huge noisy captures with no target stream
• omit whether traffic is compressed or encrypted
• ask for full field meaning without any behavioral context
• ignore transport-layer issues like retransmits and segmentation

The skill is strongest when the session has already been narrowed to the traffic that matters.

Iterate after the first output

After the first run, ask the agent to refine one layer deeper:

• identify recurring header layout
• propose field names and lengths
• isolate state transitions
• draft a protocol note or mini spec
• suggest filters or captures to validate uncertain fields

That is the best way to turn the initial protocol-reverse-engineering guide into something usable for debugging, documentation, or interoperability work.