protocol-reverse-engineering

# Protocol Reverse Engineering Comprehensive techniques for capturing, analyzing, and documenting network protocols for security research, interoperability, and debugging. ## Traffic Capture ### Wireshark Capture ```bash # 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 ```bash # 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 ```bash # 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 ```bash # 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 ```python 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... | +--------+--------+--------+--------+ ``` Before binary protocol or encryption analysis, read [parsers, structure identification, and TLS techniques](references/binary-encryption-analysis.md). Before documenting a protocol or writing a dissector, read [the specification template and Lua example](references/protocol-documentation.md). ## Active Testing ### Fuzzing with Boofuzz ```python 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 ```python 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 1. **Capture traffic**: Multiple sessions, different scenarios 2. **Identify boundaries**: Message start/end markers 3. **Map structure**: Fixed header, variable payload 4. **Identify fields**: Compare multiple samples 5. **Document format**: Create specification 6. **Validate understanding**: Implement parser/generator 7. **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