DarkflameServer/docs/CaptureReplay.md

Packet capture and replay

Staff record every packet of one account, one character, or everything, on all servers at once; play the recording back on the dashboard; and replay it against a throwaway server to see how the server answers now. The same replay takes the 2014 live captures, which makes them a conformance test for the server. The dashboard side is described in Dashboard.md; this document is how it works and the rules it follows.

Playback across worlds

An account or character capture puts every server on one timeline, so one character's packets move from one world server to the next when they transfer. CaptureTools::Worlds lists each character's world visits in time order: one entry each time the packets their client sends (never the server's, since the old world can still send a few after the client reached the new one) carry another zone or instance in the record header; zone 0 is character select. The dashboard serves them with the zone's name from the client's locale (/api/inspector/sessions/:id/worlds, and worlds in /positions, which takes ?zone=all for every zone's movement). The capture page marks each change on its slider; World 3D switches its scene to the followed character's world at that time (world3d-core.js captureSwitch, worldMarkers).

Captures, bundles and fixtures are player data. None of them is ever committed: captures/, *.bundle and tests/fixtures-local/ are in .gitignore.

Capturing

Staff arm a capture on the Packet Captures page (dev_message_inspector; arming, stopping and exporting are audited). The dashboard sends MESSAGE_CAPTURE_CONTROL with the appended ARM action to master, which passes it to every world, auth and chat, and arms itself. The dashboard repeats it every 10 seconds (servers that started since, and characters the account made since, are picked up) and sends DISARM at the end; every server also stops on its own at the time limit. Up to 8 captures run at once; each has one bit (its slot) in every record's mask.

Target	What is recorded
Account	Its connections from its next login (or now, if online): auth, character select, every zone, chat, and its master link messages. Packets of a connection before it is known whose it is (the handshake, the login request, a world's session check) are held per connection and added once auth or the world names the account.
Character	The same, from when the character is picked in a world.
Everything	Every packet on every server's listening socket, and master's server-to-server traffic (not the dashboard's).

Where each server taps (dNet/PacketCapture.*, all on the server's main thread, since RakNet isn't thread safe):

• received: dServer::Receive and dServer::ReceiveFromMaster;
• sent: a hook in RakPeer::Send (g_RakPeerSendHook, set only while armed), so replica constructions and serializations that RakNet's ReplicaManager sends are seen too;
• who a packet belongs to: auth binds the connection when the login names the account, worlds when the session is validated and when a character is picked; chat reads the player's object ID each chat packet starts with; master link messages are matched by account name (session keys), by request (zone transfers answered later), by the connection being handled when they are sent (player added and removed), and instance-wide ones (migration) go to every captured player in that world. A capture's own traffic (MESSAGE_CAPTURE_*) is never recorded.

Secrets are never stored

Packets that carry secrets are rewritten before they are recorded, per struct: the decoder registry (dNet/PacketDecoder.cpp) declares a redactor for each such packet, which reads it with the struct's Deserialize, blanks the secret fields and writes it again with Serialize. A packet that declares secrets but doesn't read is dropped, never stored as is. Today that is:

Packet	Blanked
AUTH LOGIN_REQUEST	username and password
CLIENT LOGIN_RESPONSE	user key (the session key), CDN key
WORLD VALIDATION	session key
MASTER SET_SESSION_KEY, SESSION_KEY_RESPONSE, NEW_SESSION_ALERT	session key

Auth records only the handshake and the login request and response; anything else auth sees is left out. A new packet with a secret opts in by adding its redactor next to its decoder. PacketCaptureTests checks that a captured login and session never contain the test account's password, user key or session key.

Why not start capturing at character select? The login is where most "can't log in" reports happen, and its response code, stamps and timing are what staff need. With redaction by struct there is nothing secret left in it, and the replay fills in its own account and session key anyway, so recording it costs nothing. Everything from the world's validation on is recorded the same way.

Buffering, flushing and overhead

Nothing is written or sent per packet. Each server appends records to one preallocated chunk; the chunk is sealed when it reaches capture_flush_bytes (default 256 KB) or capture_flush_interval_ms has passed (default 1000), and sealed chunks are sent to master (master sends its own straight to the dashboard) from the main loop. While master can't take them they are kept up to capture_buffer_max_mb (default 16); past that the oldest are dropped, the next batch says how many, and the dashboard writes a gap marker ("N packets lost here"). All three are shared settings (Settings, Packet capture). With nothing armed, a received packet costs one flag check and a sent one a null check.

The dashboard appends each batch to the capture's file with one write, and saves the session row (counts, end) every 5 seconds. Measured by PacketCaptureTest.OverheadOfCapturingEverything (one core, unoptimised build, 300,000 position updates with EVERYTHING armed):

	per packet	throughput
server tap (record, redact check, buffer)	about 580 ns	about 1.7 million packets/s, 173 MB/s of records
dashboard: append batch to file	about 43 ns
dashboard: a SQLite row per packet, one transaction per batch (for comparison)	about 3,500 ns

A busy world sends a few thousand packets a second, so capturing everything there costs well under 1% of a core. Appending to a file is about 80 times cheaper than a database row per packet, so packets go to files and the database keeps only the session row (the game message inspector keeps its rows as before).

The bundle format

One format for the dashboard's capture files, exported bundles and converted live captures (dNet/CaptureBundle.h):

"DLUBNDL1"            8 bytes: the format and its version
u32 length            little endian
metadata              UTF-8 JSON, `length` bytes
records               to the end of the file

Each record is a 52 byte little-endian header (PacketRecordHeader in dNet/PacketRecord.h: time in µs, per-server sequence, capture mask, source server, direction, flags, peer, account, character, zone, instance, clone, full size in bits, stored length) followed by the packet's bytes exactly as they went over RakNet (up to 256 KB each; longer ones are cut and flagged). Flags: master link, broadcast, cut, gap.

Metadata keys:

Key
format	1
origin	dlu-capture or live-2014
server	version, commit of the server that recorded it
target, captureId, startedAt, exportedAt, scenario	where it came from
zones	map id -> mapChecksum from its LOAD_STATIC_ZONEs
fdbChecksum	the client data checksum from VALIDATION
portable, anonymised	see below
ids	char#1, account#1, ... -> placeholder
setup.characters	per character: symbol, placeholder, name, xml (its saved character XML, without the account)

Portability

A bundle made on one server replays on another (a copy, a fresh install, a friend's server):

• IDs are symbolic. Exporting replaces each captured character's object ID, wherever it appears in a packet's bytes and in the headers, with a placeholder (0x1FEDC00000000000 + n, listed as char#n in ids); accounts become account#n. The replay maps placeholders to the IDs the target gave the characters it made. Object IDs the server makes (spawned objects, loot) are learned during the replay by pairing the target's replica constructions with the recorded ones by LOT and order.
• Setup travels with it. setup.characters holds what the target needs to make the characters: their saved XML (appearance, level, stats, inventory, missions, flags), from the database for DLU captures and from CREATE_CHARACTER for live ones. Account names and secrets are never in a bundle; the replay uses its own account.
• Mismatches are reported, not diffed. The bundle names the server version and commit it was recorded on, and the zone and client data checksums. The replay report lists zones whose checksum differs on the target, so different data isn't read as a server bug.
• Anonymised bundles (Export anonymised, CaptureTool anonymise) also replace character names and what players typed (chat, whispers, character names in lists) with as many x, so packet sizes stay the same.

Replaying

CaptureTool (built next to the servers) replays bundles:

CaptureTool replay <bundle>... --client <game client folder> [--cdserver <CDServer.sqlite>] [--mode setup|as-is]
                               [--speed 4] [--port 41000] [--sandbox-root <dir>] [--keep | --keep-on-failure] [--report <file.json>]
CaptureTool import-live <folder> <out-dir>      convert live captures
CaptureTool anonymise <in> <out>                make a fixture
CaptureTool info|decode <bundle>                look inside

The sandbox

Every replay runs in its own sandbox and never touches a real game database:

• The tool makes a folder (under --sandbox-root, default the system's temporary folder), copies the server binaries into it (they read their settings and database from their own folder), links the migrations and navmeshes, and copies CDServer.sqlite. The game client's files are shared read-only.
• The settings are rewritten there: replay_sandbox=1, database_type=sqlite, a fresh sqlite_database_path=resServer/sandbox.sqlite, the live database's path as replay_live_sqlite_path, ports from --port on (master, auth +10, chat +20, worlds +100), prestarted servers, no dashboard. The tool reads the settings back as the servers will and refuses to start if any of them didn't take, and clears the environment variables that could override them.
• The sandbox database is always SQLite, a new file per replay, whatever the source or target server normally runs on; there are no throwaway MySQL schemas.
• Enforced by the servers: with replay_sandbox=1 every server refuses to connect to a database that isn't SQLite, isn't inside its own folder, or is the file named by replay_live_sqlite_path (Database::Connect).
• The tool runs itself inside the sandbox (sandbox-setup, which also refuses to run without replay_sandbox=1) to apply the migrations, make the replay account and, in setup mode, the bundle's characters; then starts master and waits for auth. Afterwards the whole stack is stopped (one process group) and the folder deleted (--keep, --keep-on-failure keep it). Nothing from a sandbox is merged anywhere.
• replay-target replays against a server you run yourself; it refuses unless given --i-know-this-is-not-a-sandbox, and says loudly that it isn't one. Never point it at a live server.

The fake client

A headless RakNet client (dCaptureTool/FakeClient.*). The recording is split into connections (each starts with the client's VERSION_CONFIRM). It logs in on the target itself when the recording has no login, and picks the character itself when the recording starts in a zone (with a new plain character, made by the server's own character creation, when the bundle has no character data). Before each client packet goes out it fills in what must differ: the target account and password, the session key the target's auth gave, and the target's IDs. Timing follows the recording (4 times faster by default, at most 3 seconds between packets), and it waits for what a real client waits for: the handshake answer, the character list, the zone, and, before each packet, the answer the recorded server had sent just before it (10 seconds the first time; an answer the target never sends isn't waited for again).

The diff

Only what the server answered (auth and world packets to the client) is compared. Each recorded answer pairs with the target's next answer of the same name (constructions: the same LOT). Paired answers compare by their decoded fields, leaving out what legitimately differs between runs: object and request IDs, handles, timestamps and stamps, instance and clone IDs, server addresses and ports, player IDs and account names (CaptureTools::IsVolatileField); packets without decoded fields compare by size. The report counts same, different, missing and extra answers by name, with examples, plus notes (answers waited for in vain, zone data that differs).

Live captures

CaptureTool import-live <folder> <out-dir> converts every folder of *_traffic.zip under <folder> (the 2014 captures as extracted from the packet capture archives: one packet per .bin, named <n>_<from port>-<to port>[_<part>|_joined]_[<header bytes>]...bin) into one bundle per folder, zips in play order (auth, char, world, world1, ...). Split packets are taken from their joined file. Only those zips are read: raw .pcap files and encrypted captures (key files, XML exports) are left alone. Secrets are removed as when capturing, and CREATE_CHARACTER gives the setup section. Converted bundles stay local like any other.

Results against this branch

Local fixtures

Export a capture anonymised (or CaptureTool anonymise) and put it in tests/fixtures-local/ (never committed). CaptureFixtureTests.RecordedPacketsRoundTrip (in dGameTests) reads every packet of every fixture whose struct the decoder registry knows and checks it writes back to the same bytes; without fixtures it is skipped.

To check in game

• Arm an account capture, log in with a real client: the auth, character select and zone packets appear on one timeline, the login request shows a blank username and password, and chat (a whisper, a friend request) shows up from the chat server.
• Arm a character capture before picking another character of the same account: nothing is recorded until the captured character is picked.
• Arm everything on a busy test server for a minute: no stutter; the capture's size grows about once a second.
• Play a capture with movement back and open World 3D: the player moves with the playhead.
• Capture a character that changes worlds (a rocket or a portal): the capture page's slider has a mark at each change named after the new zone; in World 3D, following the character switches the scene at the mark and the camera stays on them, forward and when seeking back.
• Export a bundle, replay it with CaptureTool replay, and open a kept sandbox's logs.