Glossary

Short definitions, in the guide's own terms. Everything here is used in the chapters; the chapters carry the proof.

API key — The token generated with a device's config (<apikey> in config.xml) that authorises calls to its REST API and GUI. No key, no answer: the API replies CSRF Error, HTTP 403.

Block — The unit files travel in. A file is cut into blocks, each hashed; devices request only the blocks they lack, which is why small edits to large files are cheap.

Conflict file — The losing version of a file edited two ways while devices were apart, kept whole beside the winner with a name like front-page.sync-conflict-20261021-025700-VDKRCPU.md. It is an ordinary file: it syncs, it waits, a human resolves it.

Device — One running Syncthing, meaning one key pair. A device is a program on a machine, not the machine itself; two on one machine (the lab) are two devices.

Device ID — A device's name and proof of identity in one string, derived from its certificate: VDKRCPU-UXHNS6G-…. Not issued by anybody, not forgeable without the private key, and safe to share — an ID alone grants no access.

Discovery — The ways devices find each other's addresses when the address isn't fixed: global (public servers that map ID to address), local (announcements on your LAN). Off in the lab; the helpers never see file contents.

Folder — The unit of sharing: a directory, a folder ID that must match on every sharing device, and a per-device path, label, type, and ignore list that need not match.

Folder marker (.stfolder) — A directory Syncthing creates in each managed folder as proof the right directory is present. Missing marker, stopped folder — protection against syncing an unmounted drive's emptiness.

Folder type — Per device, per folder: sendreceive (send and accept), sendonly (send, never apply incoming), receiveonly (accept, never send, plus access to revert).

Global / local state — A folder's two counts: global is the folder's agreed content across devices, local is what this device's disk actually holds. Equal means converged; drifting apart is information.

Ignore (.stignore) — A per-device, per-folder file of patterns naming what that device will not sync. It is itself never synced; #include another file to share patterns, as the lab shares them via .stglobalignore.

Index — The catalogue of a folder's files, blocks, and versions that devices exchange. Sync is two devices comparing indexes and fetching the blocks the comparison says are missing.

Introducer / auto-accept — Trust shortcuts: an introducer device can add further devices to your config, and auto-accept takes folders a known device offers. Both off in the lab; the safe default is introductions by hand.

Pairing — The mutual introduction: each device is given the other's device ID (and, in the lab, a fixed address). Both sides must add each other before anything connects.

Relay — A public helper server that carries encrypted traffic between two devices that cannot connect directly. It forwards bytes it cannot read. Off in the lab.

Revert — A receive-only folder's verb: discard all local changes — edits and additions — and match the shared content exactly.

Versioning (.stversions) — A per-folder setting that files the old copy of a file into .stversions when a sync change replaces or deletes it on that device. Trash can (the scheme run here) keeps the latest replaced generation under its original name; staggered keeps several timestamped generations on a thinning schedule.