OpenVixDiskLib/docs/reverse_engineering_procedure.md

Reverse-engineering procedure

This is the working method used to replace VDDK’s NBD path with Python. Protocol details live in docs/nfc_auth.md, docs/nfc_open.md, docs/nfc_read.md, and docs/nfc_write.md. The capture tool is described in docs/ssl_hook.md. This file is the sequence of steps, including dead ends, so later NFC work can follow the same loop instead of rediscovering it.

Scope so far: VixDiskLib_ConnectEx + VixDiskLib_Open + VixDiskLib_Read + VixDiskLib_Write against lab vCenter 8.0.1 / ESXi 8, transport nbd. Driver: tests/integration/ (TestBase creates a temporary empty VM with a 10 GiB disk in setUpClass and destroys it in tearDownClass).

Rule from AGENTS.md: reuse pyVmomi for every public VIM operation. Only reimplement what pyVmomi does not expose.

Loop

Each unknown stage (ticket SOAP, authd, NFC binary) went through:

1. Name it from VDDK logs and strings on the bundled libraries.
2. See it on the wire (or prove that tcpdump cannot).
3. Replay the smallest working subset in Python against the lab.
4. Write findings into a protocol doc and keep the hook out of the library path.

Do not skip (2). Log lines such as SESSIONID or useSSL=0 named the wrong wire command until the intercept existed.

Lab and artifacts

Item	Where / value
VDDK 8.0.2	.vddk/ (libvixDiskLib, libvddkVimAccess, libvim-types)
pyVmomi	.venv
Known-good VDDK client	tests/integration/test_vddk.py / tests/integration/vixdisklib.py
Verbose NFC logs	vixDiskLib.nfc.LogLevel=4 in a temp VDDK config
ctypes Open+Read driver	/tmp/vddk_open_trace.py (not in the library)
SSL / write hook	/tmp/sslhook.c → /tmp/sslhook.so

Always set LD_LIBRARY_PATH to .vddk/ so VDDK uses its own libssl.so.3. Unset LD_PRELOAD before running the Python replacement; a leftover write hook will crash pyVmomi’s TLS.

Step 1 — Map the public VDDK calls

tests/integration/test_vddk.py is the specification of what “success” looks like: login, open the temporary VM’s VMDK, write a known pattern, read it back.

Turn on VDDK verbose logging around InitEx / ConnectEx / Open. The logs split the work that the Python API hides:

• ConnectEx → VIM login only.
• Open → NFC ticket, authd, NFC handshake, AIO open, then I/O.
• transport_modes=nbd → URL form vpxa-nfc://[ds] path.vmdk@esxi:902.
• snapshot_ref does not appear on the ticket SOAP call.

That mapping is the table at the top of docs/nfc_auth.md. It tells you which stage to reverse next and which arguments belong there (VM moref on the ticket, VMDK path on NFC OPEN_FILE).

Step 2 — Strings and pyVmomi before any capture

strings -a on .vddk/*.so produced candidate tokens before a single packet was decoded:

• SOAP: NfcService, NfcGetVmFiles, nfcService, ha-nfc, HostServiceTicket.
• authd: SESSION, BANNER, THUMBPRINT_SHA2, PROXY, USER, PASS, SSL Required.
• NFC: NFC_HANDSHAKE, NFC_CONNECTION_DATA, NFC_AIO_MSG_*, NFC_DISK.

Then check whether pyVmomi already has the type:

from pyVmomi import vim
hasattr(vim, "NfcService")   # False
hasattr(vim, "HostServiceTicket")  # True

ServiceManager.QueryServiceList on the live vCenter does not list NFC. GET /sdk/nfcServiceVersions.xml does (urn:nfc 7.0.3.2). The moref is hardcoded in VDDK (nfcService on vCenter, ha-nfc on ESXi).

Anything public (SmartConnect, vim.VirtualMachine, HostServiceTicket) stays in pyVmomi. Missing managed types are registered with CreateManagedType on the same SOAP stub so cookies and serialization are not reimplemented.

Step 3 — Confirm tcpdump is the wrong tool for TLS stages

tcpdump on 443 and 902 shows TLS records only. That is enough to prove “something talks to vCenter then to ESXi:902”, and not enough for SOAP bodies, authd lines, or NFC headers.

VDDK logs name functions and AIO opId / type / size. They do not give magic numbers, path placement, or command spacing (BANNER \r\n).

An ESXi-impersonating service was considered (AGENTS.md) and not needed: the lab answers VDDK, so capturing the real client is simpler than simulating the server.

Step 4 — Interpose OpenSSL (authd and SOAP)

VDDK 8.0.2 still calls SSL_write / SSL_read. A small LD_PRELOAD library logs those buffers as hex, tagged with the SSL * pointer.

Run a minimal ctypes program (InitEx, ConnectEx, Open, Read) under:

export LD_LIBRARY_PATH=…/.vddk
export LD_PRELOAD=/tmp/sslhook.so
export SSLHOOK_LOG=/tmp/sslhook-open.log
python /tmp/vddk_open_trace.py

Parse offline:

1. Concatenate adjacent same-direction records (authd SSL_read is often one byte).
2. Split by SSL *. vCenter HTTPS contains POST /sdk and SOAP. ESXi:902 contains SESSION / PROXY.
3. An early hook without the pointer mixed both streams; always tag.

The vCenter stream identified the ticket as NfcGetVmFiles with moref nfcService and xmlns="urn:vim25" (not a guess from strings alone). The ESXi stream gave the authd command order, including the trailing space on BANNER and THUMBPRINT_SHA2 PlainText.

Hook implementation notes: docs/ssl_hook.md.

Step 5 — Probe SOAP, then replay only what VDDK sends

With a SmartConnect session, raw SOAP posts were used to learn parameter names and which moref vCenter accepts:

• ha-nfc on vCenter → ManagedObjectNotFound.
• NfcGetServerNfcLibVersion without hostForAccess → invalid argument; with a host moref → 11.
• NfcGetVmFiles(vm) → HostServiceTicket (VDDK read-only Open).
• NfcRandomAccessOpenReadonly(vm, diskDeviceKey, host) → same ticket type, disk-scoped read.
• NfcRandomAccessOpenDisk(vm, diskDeviceKey, host) → writable ticket. GetVmFiles plus OPEN_FILE flags 0x1a fails with VIX_E_FILE_READ_ONLY (0x0b).

The replacement registers those methods and calls them through pyVmomi. It does not ship a hand-rolled SOAP client for login or tickets.

Step 6 — Probe authd; record dead ends

Plaintext banner (220 … SSL Required), then ssl.wrap_socket. Commands before TLS drop the connection.

vCenter UID/password are not sent to port 902. Attempts that failed and must not be retried for this ticket type:

Attempt	Result
USER / PASS (vCenter account)	530 Login incorrect
USER / PASS with sessionId	530 Login incorrect
SESSIONID <sessionId>	530 Please login with USER and PASS
CONNECT_VPXA after TLS	530 Please login with USER and PASS
Wait for a reply after SESSION	Hang until BANNER / PROXY follow
THUMBPRINT_SHA2 with SHA-1 digest	501 Invalid arguments

The working sequence is in docs/nfc_auth.md. useSSL=0 in the VDDK log means skip a second NFCSSL wrap, not skip TLS on 902.

Replay: openvixdisklib/nfc_auth.py / tests/integration/test_nfc_auth.py. Stop at 200 Connect.

Step 7 — NFC binary: extend the hook to write / read

After PROXY, SSL_write on the ESXi SSL * goes silent. VDDK logs useSSL=0 / “plain-text connection is deprecated” and then NFC function names. The bytes are write(SSL_get_fd(ssl), …) / read on peer port 902.

The hook was extended to those syscalls, filtered with getpeername port 902, and mutex-locked (VDDK is multi-threaded). Skip TLS records (16 03 / 17 03) left over from the authd phase.

Correlate each frame with the verbose log line that has the same type and size (NfcAioSendMessage: opId = … type = … size = …). Name the types from the consecutive NFC_AIO_MSG_* string table in libvixDiskLib.so. Lengths 4 and 7 on the connection-data message are the ASCII strings vddk and nbdmode sent in the next two writes.

Classic NFC uses a 264-byte padded struct; AIO uses a 16-byte header (magic 0xA100DA7A) plus payload; path / DDB key / sector data are extra writes not included in size.

Step 8 — Replay the smallest subset, then compare to VDDK

Python must dup the authd fd and send NFC as raw TCP. SSLSocket.send would encrypt; unwrap() would SSL_shutdown. VDDK does neither.

openvixdisklib/nfc_open.py replays handshake + AIO OPEN_SESSION / sockopts / resource pool / OPEN_FILE. VDDK’s extra DDB_GET keys were omitted once a file handle was enough to read. Proof of open: tests/integration/test_nfc_open.py.

Do not copy every VDDK message. Copy what the server requires for the Python API you are replacing.

Step 9 — Vary VixDiskLib_Read until IO fields stop moving

A single-sector read is not enough to decode NFC_AIO_MSG_IO. Drive VDDK with several (startSector, numSectors) pairs in one Open (including n=128 = 64 KiB and n=129) under the write/read hook.

What that comparison showed:

• Wire units are bytes (offset = start * 512, length = n * 512).
• Request size stays 44; data is extra after the payload.
• VDDK sends one request even when length > 65536. The server replies with several type-7 messages that share opId, each with a chunk length at payload offset 32 (max 65536).
• Treating offset 36 as NFC_DISK (2) was a 1-sector coincidence; VDDK repeats the byte length there.
• Zeros on the wire are real transferred zeros, not a sparse skip.

Replay: NfcDisk.read loops on fragments until length bytes arrive. Proof: tests/integration/test_nfc_read_write.py writes a known pattern (including a 129-sector read that must assemble two fragments) and checks the bytes that came back.

Step 10 — Writes from the same IO message

VixDiskLib_Write uses the same 44-byte NFC_AIO_MSG_IO layout as read. The direction field at offset 8 is 0 instead of 1, and the sector bytes are sent after the payload (like the path on OPEN_FILE). Writable OPEN_FILE flags are 0x1a (the captured read-only flags 0x1e with bit 0x04 cleared, matching VIXDISKLIB_FLAG_OPEN_READ_ONLY in .vddk/vixDiskLib.h).

Writable OPEN_FILE still failed with VIX_E_FILE_READ_ONLY until the ticket switched from NfcGetVmFiles to NfcRandomAccessOpenDisk (libvim-types.so: vmodl randomAccessOpen ↔ WSDL NfcRandomAccessOpenDisk). Integration tests create a temporary empty 10 GiB VM for the run so writes cannot land on other lab disks.

The Python client splits writes larger than 64 KiB; it does not send a single oversized write the way VDDK sends an oversized read. Details: docs/nfc_write.md. Proof: write then read in tests/integration/test_nfc_read_write.py and the VDDK cross-check in tests/integration/test_crosscheck.py.

What to write down

After a stage works:

Document	Contents
docs/nfc_auth.md	Ticket SOAP + authd wire format
docs/nfc_open.md	Classic NFC + AIO open
docs/nfc_read.md	AIO IO / VixDiskLib_Read
docs/nfc_write.md	AIO IO / VixDiskLib_Write
docs/ssl_hook.md	Capture tool only
docs/reverse_engineering_procedure.md	This procedure (update when the method changes)

Keep the hook and ctypes driver under /tmp. They are not part of the replacement library.

Next stages (same procedure)

Not yet reversed, same loop as above:

• DDB_GET / disk geometry, compression, encrypted disks
• NFC_DELTA_DISK, CBT / QueryAllocatedBlocks
• VixDiskLib_GetInfo capacity
• Host-switch AIO messages
• useSSL=1 (second NFCSSL wrap)
• Direct ESXi ha-nfc without vCenter vpxa-nfc