majiayu000 Protocol Reverse Engineering
Comprehensive techniques for capturing, analyzing, and documenting network protocols for security research, interoperability, and debugging.
Overview
Focus on repeatable capture, structured analysis, and clear documentation so findings are usable by others.
Quick Start
- Capture traffic with tcpdump or Wireshark and export a clean pcap.
- Identify protocol boundaries, message types, and state transitions.
- Document message formats, fields, and validation rules.
Progressive Details
Use the detailed sections below for full workflows, templates, and advanced techniques.
Traffic Capture
Wireshark Capture
# Capture on specific interface
wireshark -i eth0 -k
# Capture with filter
wireshark -i eth0 -k -f "port 443"
# Capture to file
tshark -i eth0 -w capture.pcap
# Ring buffer capture (rotate files)
tshark -i eth0 -b filesize:100000 -b files:10 -w capture.pcap
tcpdump Capture
# Basic capture
tcpdump -i eth0 -w capture.pcap
# With filter
tcpdump -i eth0 port 8080 -w capture.pcap
# Capture specific bytes
tcpdump -i eth0 -s 0 -w capture.pcap # Full packet
# Real-time display
tcpdump -i eth0 -X port 80
Man-in-the-Middle Capture
# mitmproxy for HTTP/HTTPS
mitmproxy --mode transparent -p 8080
# SSL/TLS interception
mitmproxy --mode transparent --ssl-insecure
# Dump to file
mitmdump -w traffic.mitm
# Burp Suite
# Configure browser proxy to 127.0.0.1:8080
Protocol Analysis
Wireshark Analysis
# Display filters
tcp.port == 8080
http.request.method == "POST"
ip.addr == 192.168.1.1
tcp.flags.syn == 1 && tcp.flags.ack == 0
frame contains "password"
# Following streams
Right-click > Follow > TCP Stream
Right-click > Follow > HTTP Stream
# Export objects
File > Export Objects > HTTP
# Decryption
Edit > Preferences > Protocols > TLS
- (Pre)-Master-Secret log filename
- RSA keys list
tshark Analysis
# Extract specific fields
tshark -r capture.pcap -T fields -e ip.src -e ip.dst -e tcp.port
# Statistics
tshark -r capture.pcap -q -z conv,tcp
tshark -r capture.pcap -q -z endpoints,ip
# Filter and extract
tshark -r capture.pcap -Y "http" -T json > http_traffic.json
# Protocol hierarchy
tshark -r capture.pcap -q -z io,phs
Scapy for Custom Analysis
from scapy.all import *
# Read pcap
packets = rdpcap("capture.pcap")
# Analyze packets
for pkt in packets:
if pkt.haslayer(TCP):
print(f"Src: {pkt[IP].src}:{pkt[TCP].sport}")
print(f"Dst: {pkt[IP].dst}:{pkt[TCP].dport}")
if pkt.haslayer(Raw):
print(f"Data: {pkt[Raw].load[:50]}")
# Filter packets
http_packets = [p for p in packets if p.haslayer(TCP)
and (p[TCP].sport == 80 or p[TCP].dport == 80)]
# Create custom packets
pkt = IP(dst="target")/TCP(dport=80)/Raw(load="GET / HTTP/1.1\r\n")
send(pkt)
Protocol Identification
Common Protocol Signatures
HTTP - "HTTP/1." or "GET " or "POST " at start
TLS/SSL - 0x16 0x03 (record layer)
DNS - UDP port 53, specific header format
SMB - 0xFF 0x53 0x4D 0x42 ("SMB" signature)
SSH - "SSH-2.0" banner
FTP - "220 " response, "USER " command
SMTP - "220 " banner, "EHLO" command
MySQL - 0x00 length prefix, protocol version
PostgreSQL - 0x00 0x00 0x00 startup length
Redis - "*" RESP array prefix
MongoDB - BSON documents with specific header
Protocol Header Patterns
+--------+--------+--------+--------+
| Magic number / Signature |
+--------+--------+--------+--------+
| Version | Flags |
+--------+--------+--------+--------+
| Length | Message Type |
+--------+--------+--------+--------+
| Sequence Number / Session ID |
+--------+--------+--------+--------+
| Payload... |
+--------+--------+--------+--------+
Binary Protocol Analysis
Structure Identification
# Common patterns in binary protocols
# Length-prefixed message
struct Message {
uint32_t length; # Total message length
uint16_t msg_type; # Message type identifier
uint8_t flags; # Flags/options
uint8_t reserved; # Padding/alignment
uint8_t payload[]; # Variable-length payload
};
# Type-Length-Value (TLV)
struct TLV {
uint8_t type; # Field type
uint16_t length; # Field length
uint8_t value[]; # Field data
};
# Fixed header + variable payload
struct Packet {
uint8_t magic[4]; # "ABCD" signature
uint32_t version;
uint32_t payload_len;
uint32_t checksum; # CRC32 or similar
uint8_t payload[];
};
Python Protocol Parser
import struct
from dataclasses import dataclass
@dataclass
class MessageHeader:
magic: bytes
version: int
msg_type: int
length: int
@classmethod
def from_bytes(cls, data: bytes):
magic, version, msg_type, length = struct.unpack(
">4sHHI", data[:12]
)
return cls(magic, version, msg_type, length)
def parse_messages(data: bytes):
offset = 0
messages = []
while offset < len(data):
header = MessageHeader.from_bytes(data[offset:])
payload = data[offset+12:offset+12+header.length]
messages.append((header, payload))
offset += 12 + header.length
return messages
# Parse TLV structure
def parse_tlv(data: bytes):
fields = []
offset = 0
while offset < len(data):
field_type = data[offset]
length = struct.unpack(">H", data[offset+1:offset+3])[0]
value = data[offset+3:offset+3+length]
fields.append((field_type, value))
offset += 3 + length
return fields
Hex Dump Analysis
def hexdump(data: bytes, width: int = 16):
"""Format binary data as hex dump."""
lines = []
for i in range(0, len(data), width):
chunk = data[i:i+width]
hex_part = ' '.join(f'{b:02x}' for b in chunk)
ascii_part = ''.join(
chr(b) if 32 <= b < 127 else '.'
for b in chunk
)
lines.append(f'{i:08x} {hex_part:<{width*3}} {ascii_part}')
return '\n'.join(lines)
# Example output:
# 00000000 48 54 54 50 2f 31 2e 31 20 32 30 30 20 4f 4b 0d HTTP/1.1 200 OK.
# 00000010 0a 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 .Content-Type: t
Encryption Analysis
Identifying Encryption
# Entropy analysis - high entropy suggests encryption/compression
import math
from collections import Counter
def entropy(data: bytes) -> float:
if not data:
return 0.0
counter = Counter(data)
probs = [count / len(data) for count in counter.values()]
return -sum(p * math.log2(p) for p in probs)
# Entropy thresholds:
# < 6.0: Likely plaintext or structured data
# 6.0-7.5: Possibly compressed
# > 7.5: Likely encrypted or random
# Common encryption indicators
# - High, uniform entropy
# - No obvious structure or patterns
# - Length often multiple of block size (16 for AES)
# - Possible IV at start (16 bytes for AES-CBC)
TLS Analysis
# Extract TLS metadata
tshark -r capture.pcap -Y "ssl.handshake" \
-T fields -e ip.src -e ssl.handshake.ciphersuite
# JA3 fingerprinting (client)
tshark -r capture.pcap -Y "ssl.handshake.type == 1" \
-T fields -e ssl.handshake.ja3
# JA3S fingerprinting (server)
tshark -r capture.pcap -Y "ssl.handshake.type == 2" \
-T fields -e ssl.handshake.ja3s
# Certificate extraction
tshark -r capture.pcap -Y "ssl.handshake.certificate" \
-T fields -e x509sat.printableString
Decryption Approaches
# Pre-master secret log (browser)
export SSLKEYLOGFILE=/tmp/keys.log
# Configure Wireshark
# Edit > Preferences > Protocols > TLS
# (Pre)-Master-Secret log filename: /tmp/keys.log
# Decrypt with private key (if available)
# Only works for RSA key exchange
# Edit > Preferences > Protocols > TLS > RSA keys list
Custom Protocol Documentation
Protocol Specification Template
# Protocol Name Specification
## Overview
Brief description of protocol purpose and design.
## Transport
- Layer: TCP/UDP
- Port: XXXX
- Encryption: TLS 1.2+
## Message Format
### Header (12 bytes)
| Offset | Size | Field | Description |
| ------ | ---- | ------- | ----------------------- |
| 0 | 4 | Magic | 0x50524F54 ("PROT") |
| 4 | 2 | Version | Protocol version (1) |
| 6 | 2 | Type | Message type identifier |
| 8 | 4 | Length | Payload length in bytes |
### Message Types
| Type | Name | Description |
| ---- | --------- | ---------------------- |
| 0x01 | HELLO | Connection initiation |
| 0x02 | HELLO_ACK | Connection accepted |
| 0x03 | DATA | Application data |
| 0x04 | CLOSE | Connection termination |
### Type 0x01: HELLO
| Offset | Size | Field | Description |
| ------ | ---- | ---------- | ------------------------ |
| 0 | 4 | ClientID | Unique client identifier |
| 4 | 2 | Flags | Connection flags |
| 6 | var | Extensions | TLV-encoded extensions |
## State Machine
[INIT] --HELLO--> [WAIT_ACK] --HELLO_ACK--> [CONNECTED] | DATA/DATA | [CLOSED] <--CLOSE--+
## Examples
### Connection Establishment
Client -> Server: HELLO (ClientID=0x12345678) Server -> Client: HELLO_ACK (Status=OK) Client -> Server: DATA (payload)
Wireshark Dissector (Lua)
-- custom_protocol.lua
local proto = Proto("custom", "Custom Protocol")
-- Define fields
local f_magic = ProtoField.string("custom.magic", "Magic")
local f_version = ProtoField.uint16("custom.version", "Version")
local f_type = ProtoField.uint16("custom.type", "Type")
local f_length = ProtoField.uint32("custom.length", "Length")
local f_payload = ProtoField.bytes("custom.payload", "Payload")
proto.fields = { f_magic, f_version, f_type, f_length, f_payload }
-- Message type names
local msg_types = {
[0x01] = "HELLO",
[0x02] = "HELLO_ACK",
[0x03] = "DATA",
[0x04] = "CLOSE"
}
function proto.dissector(buffer, pinfo, tree)
pinfo.cols.protocol = "CUSTOM"
local subtree = tree:add(proto, buffer())
-- Parse header
subtree:add(f_magic, buffer(0, 4))
subtree:add(f_version, buffer(4, 2))
local msg_type = buffer(6, 2):uint()
subtree:add(f_type, buffer(6, 2)):append_text(
" (" .. (msg_types[msg_type] or "Unknown") .. ")"
)
local length = buffer(8, 4):uint()
subtree:add(f_length, buffer(8, 4))
if length > 0 then
subtree:add(f_payload, buffer(12, length))
end
end
-- Register for TCP port
local tcp_table = DissectorTable.get("tcp.port")
tcp_table:add(8888, proto)
Active Testing
Fuzzing with Boofuzz
from boofuzz import *
def main():
session = Session(
target=Target(
connection=TCPSocketConnection("target", 8888)
)
)
# Define protocol structure
s_initialize("HELLO")
s_static(b"\x50\x52\x4f\x54") # Magic
s_word(1, name="version") # Version
s_word(0x01, name="type") # Type (HELLO)
s_size("payload", length=4) # Length field
s_block_start("payload")
s_dword(0x12345678, name="client_id")
s_word(0, name="flags")
s_block_end()
session.connect(s_get("HELLO"))
session.fuzz()
if __name__ == "__main__":
main()
Replay and Modification
from scapy.all import *
# Replay captured traffic
packets = rdpcap("capture.pcap")
for pkt in packets:
if pkt.haslayer(TCP) and pkt[TCP].dport == 8888:
send(pkt)
# Modify and replay
for pkt in packets:
if pkt.haslayer(Raw):
# Modify payload
original = pkt[Raw].load
modified = original.replace(b"client", b"CLIENT")
pkt[Raw].load = modified
# Recalculate checksums
del pkt[IP].chksum
del pkt[TCP].chksum
send(pkt)
Best Practices
Analysis Workflow
- Capture traffic: Multiple sessions, different scenarios
- Identify boundaries: Message start/end markers
- Map structure: Fixed header, variable payload
- Identify fields: Compare multiple samples
- Document format: Create specification
- Validate understanding: Implement parser/generator
- Test edge cases: Fuzzing, boundary conditions
Common Patterns to Look For
- Magic numbers/signatures at message start
- Version fields for compatibility
- Length fields (often before variable data)
- Type/opcode fields for message identification
- Sequence numbers for ordering
- Checksums/CRCs for integrity
- Timestamps for timing
- Session/connection identifiers