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Align about, protocol, CAPS, auth token, enumerations, and VATSIM auth pages with current wire behavior, openfsd notes, and clearer structure.
2.4 KiB
About
The FSD protocol functions primarily as a message forwarder. Aside from a few direct client–server interactions (login, METAR, capability advertisement, range fan-out, auth challenges), its main purpose is to relay messages between flight-simulator clients via a centralized server. This architecture is not peer-to-peer; all communication is routed through the central server.
FSD is a plaintext protocol that can be intercepted and analyzed with tools such as Wireshark. By default it operates on TCP port 6809, and its functionality can be tested with telnet:
telnet <FSD server address> 6809
Various implementations of FSD exist, each with unique protocol nuances. This site documents the modern VATSIM dialect. openfsd-specific behavior is called out where the server intentionally differs or is incomplete.
Wire conventions
- FSD messages are plaintext lines ending with CR/LF (
\r\n,0x0d0a). - Effective character set on the wire is ISO/IEC 8859-1 (“Latin alphabet no. 1”).
- Each message begins with a short packet identifier, followed by colon (
:) delimited fields. - Numerical values are base-10 (or occasionally base-16) ASCII; no raw binary payload.
- Clients are identified by plaintext aviation callsigns (e.g.
N7938C,KSFO_TWR). - Most packets include From and To (source and recipient). To may be a callsign,
SERVER, or a special group token (see Special Recipients).
Example: Server Identification ($DI)
$DISERVER:CLIENT:VATSIM FSD V3.43:d95f57db664f\r\n
Hexadecimal representation
00000000 24 44 49 53 45 52 56 45 52 3a 43 4c 49 45 4e 54 $DISERVE R:CLIENT
00000010 3a 56 41 54 53 49 4d 20 46 53 44 20 56 33 2e 34 :VATSIM FSD V3.4
00000020 33 3a 64 39 35 66 35 37 64 62 36 36 34 66 0d 0a 3:d95f57 db664f..
Explanation
| Piece | Value | Notes |
|---|---|---|
| Packet type | $DI |
Server identification |
| From | SERVER |
Reserved callsign for the server |
| To | CLIENT |
Placeholder used before the client callsign is known |
| Version string | VATSIM FSD V3.43 |
Human-readable server software version (format varies by server) |
| Initial challenge | d95f57db664f |
Random hex data for VATSIM Auth |
| Terminator | \r\n |
0x0d 0x0a |
Full packet catalog: Protocol.