openfsd

openfsd is an open-source multiplayer flight simulation server implementing the modern VATSIM FSD protocol. It connects pilots and air traffic controllers in a shared virtual environment.

About

Flight Sim Daemon (colloquially known as FSD) is the software/protocol responsible for connecting home flight simulator clients to a single, shared multiplayer world on hobbyist networks such as VATSIM and IVAO. FSD was originally written in the late 90's by Marty Bochane for SATCO, later to be forked and taken closed-source by VATSIM in 2001. As of May 2025, FSD is still used to facilitate over 140,000 active members connecting their flight simulators to the network.

Features

• Multiplayer flight simulation with VATSIM protocol compatibility
• Web-based management for users, settings, and connections
• SQLite for persistent storage (single file; easy backups)
• Single binary — FSD and web share one process and one database; enable services with CLI flags

Package layout

cmd/openfsd/          # Binary entrypoint (FSD + web; image CMD is /openfsd)
pkg/protocol/         # Pure wire format (parse/marshal; no I/O)
pkg/fsdclient/        # Mock/real FSD client for e2e and tools
pkg/twrfiles/         # Pure .apt/.air parse + format (sweatbox + editor)
internal/server/      # TCP accept, login, handlers, service HTTP
internal/session/     # Per-connection state + outbound send worker
internal/postoffice/  # Callsign registry + geospatial index
internal/geo/         # Pure haversine / bounding box
internal/auth/        # JWT + VATSIM client auth
internal/metar/       # METAR worker pool (injectable HTTP)
internal/db/          # Shared repositories + migrations
internal/serviceapi/  # Pure JSON DTOs for FSD service HTTP (shared by server + web)
internal/web/         # Gin MPA + /api/v1
internal/sweatbox/    # Ground/taxi sim (stdlib + geo)
cmd/aptdat2apt/       # Optional: convert XP12 apt.dat → sweatbox .apt (user-fetched data)

Airport layout data (sweatbox)

openfsd does not ship X-Plane Global Airports apt.dat or bulk generated airport files. Operators fetch layout data themselves and convert locally:

go run ./cmd/aptdat2apt -download -out generated-apt -quiet

Sources, licensing (Gateway / Global Airports GPLv2), and packaging rules: docs/xplane-airport-data.md.

Airport editor (web)

Administrators can author paired .apt (geometry) and .air (scenario aircraft) files in the browser at /airport-editor:

• Map-first editing (Leaflet; JS required for the map; text paste + echo-download works without JS)
• No server persistence of APT/AIR — open/download only (Blob download with JS; form echo-download without)
• Validate tab: live client parse + soft cross-file warnings; optional Confirm with server (POST /api/v1/editor/validate-apt and validate-air)
• Workflow: download files → load them on /sweatbox (editor never pushes into a live session)

Design notes: docs/design/apt-air-editor.md.

Build and run

go build -o openfsd ./cmd/openfsd

./openfsd              # both FSD and web (default)
./openfsd -fsd         # FSD only (:6809 + service HTTP :13618)
./openfsd -web         # web only (:8000)

Flag	Effect
(none)	Both services
-fsd	FSD only
-web	Web only
-fsd -web	Both (same as default)

Environment

Variable	Default	Notes
DATABASE_SOURCE_NAME	openfsd.db?_pragma=busy_timeout(5000)&_pragma=journal_mode(WAL)	SQLite path (preferred) or :memory:; must be shared by both services. Bare :memory: is rewritten to a process-shared DSN when FSD+web run together.
DATABASE_DRIVER	sqlite	Compatibility only; must be sqlite or unset (Postgres removed)
DATABASE_AUTO_MIGRATE	true	FSD and web apply migrations on startup (idempotent)
FSD_LISTEN_ADDRS	:6809	FSD TCP listen address(es)
SERVICE_HTTP_LISTEN_ADDR	:13618	Internal FSD admin HTTP
FSD_HTTP_SERVICE_ADDRESS	http://127.0.0.1:13618	Web → FSD service HTTP
LISTEN_ADDR	:8000	Web UI + /api/v1
LOG_DEBUG	(unset)	Set true for slog debug logging (default is info / release)
GIN_MODE	release	Gin mode for web + FSD service HTTP; set debug for Gin debug output

Colocated mode (default) uses the shared DB and in-process service HTTP. For -web against a remote FSD, set FSD_HTTP_SERVICE_ADDRESS.

Quick start (Docker)

Preferred for operators. See the Deployment Wiki (source: wiki/).

Images: ghcr.io/renorris/openfsd (:latest, :dev, sha-*) published by CI on every push to main and dev.

Upgrading from PostgreSQL? openfsd is SQLite-only. Use openfsd-migrate-to-sqlite and follow Migrating from PostgreSQL.

git clone https://github.com/renorris/openfsd.git
cd openfsd
docker compose up -d          # pull/build single image; both services
# or: docker compose up -d --build

1. Open http://localhost:8000
2. Log in with the default admin credentials (printed in container logs on first startup)
3. Configure Server — see the Configuration wiki
4. Connect a client — Client Connection Wiki

Service selection

# Both (default CMD)
docker run --rm -p 6809:6809 -p 8000:8000 ghcr.io/renorris/openfsd:latest

# FSD only
docker run --rm -p 6809:6809 ghcr.io/renorris/openfsd:latest /openfsd -fsd

# Web only (remote FSD)
docker run --rm -p 8000:8000 \
  -e FSD_HTTP_SERVICE_ADDRESS=http://fsd-host:13618 \
  ghcr.io/renorris/openfsd:latest /openfsd -web

Local smoke

docker compose up -d --build
curl -fsS -o /dev/null -w "%{http_code}\n" http://localhost:8000/login
nc -z localhost 6809 && echo "fsd:6809 open"
docker compose down

Tests

go test -race ./...
bash scripts/check-coverage.sh 80    # overall ≥80%; pure-pkg floors (see AGENTS.md)
bash scripts/check-webjs.sh          # Node ≥20 unit tests for airport-editor JS modules
go test -bench=. -benchmem ./internal/postoffice/ ./pkg/protocol/
go test -tags=stress -count=1 -timeout=120s ./internal/server/ -run TestStress -v

E2E: internal/server/e2e_test.go via pkg/fsdclient + StartTestServer. Stress is optional (CI schedule / workflow_dispatch).

API

/api/v1 covers auth, users, config, and FSD connections. See internal/web.

Protocol docs

Unofficial reverse-engineered FSD protocol docs live under docs/:

pip install mkdocs
mkdocs serve