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Overview

Connection packets handle the initial handshake between clients and servers, including authentication, join requests, and disconnect notifications.

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FSNETCMD_LOGON

Packet ID: 1 | Direction: Client → Server (Server → Client for acknowledgment) Sent by clients to log into a server. The server replies with an empty LOGON packet to acknowledge successful login.

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Location

lib/PacketManager/packets/FSNETCMD_LOGON.py

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Fields

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username
str
Player’s username (max 16 characters in short form)
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version
int
YSFlight version identifier for compatibility checking
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alias
str
Full username if longer than 16 characters, otherwise same as username

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Methods

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decode
method
Decodes the LOGON packet into readable fieldsExample:
from lib.PacketManager.packets import FSNETCMD_LOGON

packet = FSNETCMD_LOGON(data, should_decode=True)
print(f"User: {packet.username}")
print(f"Alias: {packet.alias}")
print(f"Version: {packet.version}")
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encode
static method
Creates a LOGON packetParameters:
  • username (str): Player username
  • version (int): YSFlight version number
  • with_size (bool): Include size header
Returns: bytes - Encoded packetExample:
packet = FSNETCMD_LOGON.encode(
    username="Pilot123",
    version=20180930,
    with_size=True
)
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alter_version
static method
Modifies the version number in an existing LOGON packetParameters:
  • buffer (bytes): Original packet data
  • new_version (int): New version number
Returns: bytes - Modified packetExample:
# Change client version for compatibility
modified = FSNETCMD_LOGON.alter_version(data, 20180930)

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Usage Example

# In proxy.py - handling client login
if packet_type == "FSNETCMD_LOGON":
    decode = FSNETCMD_LOGON(packet)
    player.login(decode)
    info(f"{decode.alias} logged in (version: {decode.version})")
    
    # Check version compatibility
    if decode.version < 20180930:
        warning(f"Old client version: {decode.version}")

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FSNETCMD_LOGOFF

Packet ID: 2 | Direction: Client → Server Sent when a client logs off from the server. Appears to be unused by YSFlight in practice.

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Location

lib/PacketManager/packets/FSNETCMD_LOGOFF.py

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Fields

No payload - empty packet.

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Methods

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encode
static method
Creates a LOGOFF packetExample:
packet = FSNETCMD_LOGOFF.encode(with_size=True)

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FSNETCMD_JOINREQUEST

Packet ID: 8 | Direction: Client → Server (Server → Client for readback) Sent by clients to request joining the simulation with a specific aircraft and starting position.

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Location

lib/PacketManager/packets/FSNETCMD_JOINREQUEST.py

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Fields

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iff
int
IFF (Identification Friend or Foe) team identifier. Note: The value sent is 1 less than the player’s selected IFF.
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aircraft
str
Aircraft identifier/name (max 32 characters)
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start_pos
str
Starting position identifier (STP name, max 32 characters)
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fuel
int
Initial fuel amount
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smoke
int
Initial smoke configuration

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Methods

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decode
method
Decodes the join request packetExample:
from lib.PacketManager.packets import FSNETCMD_JOINREQUEST

packet = FSNETCMD_JOINREQUEST(data, should_decode=True)
print(f"Aircraft: {packet.aircraft}")
print(f"Start: {packet.start_pos}")
print(f"IFF: {packet.iff}")
print(f"Fuel: {packet.fuel}")
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encode
static method
Creates a JOINREQUEST packetParameters:
  • iff (int): Team identifier
  • aircraft (str): Aircraft name
  • start_pos (str): Starting position
  • fuel (int): Initial fuel
  • smoke (int): Smoke configuration
  • with_size (bool): Include size header
Example:
packet = FSNETCMD_JOINREQUEST.encode(
    iff=0,
    aircraft="F-16C",
    start_pos="CARRIER",
    fuel=5000,
    smoke=0,
    with_size=True
)

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Usage Example

# In proxy.py - handling join request
if packet_type == "FSNETCMD_JOINREQUEST":
    decode = FSNETCMD_JOINREQUEST(packet)
    info(f"{player.username} requesting {decode.aircraft}")
    info(f"Start position: {decode.start_pos}")
    info(f"IFF: {decode.iff}, Fuel: {decode.fuel}")
    
    # Modify aircraft before forwarding
    if decode.aircraft in RESTRICTED_AIRCRAFT:
        decode.aircraft = "F-15C"  # Force different aircraft

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Aircraft Replacement Plugin Example

# From aircraft_replacer plugin
from lib.PacketManager.packets import FSNETCMD_JOINREQUEST

def replace_aircraft(data, player):
    packet = FSNETCMD_JOINREQUEST(data)
    
    # Replace old aircraft with new ones
    if packet.aircraft == "F-4E":
        packet.aircraft = "F-15C"
    
    # Re-encode and forward
    new_packet = FSNETCMD_JOINREQUEST.encode(
        iff=packet.iff,
        aircraft=packet.aircraft,
        start_pos=packet.start_pos,
        fuel=packet.fuel,
        smoke=packet.smoke,
        with_size=True
    )
    return new_packet

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FSNETCMD_JOINAPPROVAL

Packet ID: 9 | Direction: Server → Client Sent by the server after receiving the readback from the client’s ADDOBJECT packet. Signals that the client is approved to join.

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Location

lib/PacketManager/packets/FSNETCMD_JOINAPPROVAL.py

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Fields

No payload - empty packet.

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Methods

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encode
static method
Creates a JOINAPPROVAL packetExample:
packet = FSNETCMD_JOINAPPROVAL.encode(with_size=True)

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Join Sequence

The complete join sequence is:
  1. Client sends JOINREQUEST to server
  2. Server sends ADDOBJECT to client (add your aircraft)
  3. Client sends READBACK acknowledging ADDOBJECT
  4. Server sends JOINAPPROVAL ← This packet
  5. Client officially enters the simulation

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FSNETCMD_REJECTJOINREQ

Packet ID: 10 | Direction: Server → Client Sent by the server to reject a join request.

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Location

lib/PacketManager/packets/FSNETCMD_REJECTJOINREQ.py

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Usage

from lib.PacketManager.packets import FSNETCMD_REJECTJOINREQ

# Reject a player's join request
reject_packet = FSNETCMD_REJECTJOINREQ.encode(with_size=True)
message_to_client.append(reject_packet)

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Connection Flow Diagram

Client                          Proxy                          Server
  |                               |                               |
  |--- LOGON ------------------->|--- LOGON ------------------->|
  |                               |                               |
  |<-- LOGON (ack) --------------|<-- LOGON (ack) --------------|
  |                               |                               |
  |--- JOINREQUEST ------------->|--- JOINREQUEST ------------->|
  |                               |                               |
  |<-- ADDOBJECT ----------------|<-- ADDOBJECT ----------------|
  |                               |                               |
  |--- READBACK ---------------->|--- READBACK ---------------->|
  |                               |                               |
  |<-- JOINAPPROVAL -------------|<-- JOINAPPROVAL -------------|
  |                               |                               |
  |<-- AIRPLANESTATE (stream)    |<-- AIRPLANESTATE (stream)    |
  |                               |                               |

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Version Compatibility

YSFlight uses version numbers to ensure client-server compatibility:
  • 20180930: YSFlight 2018 / YSCE
  • 20191124: YSFlight 20191124
  • Older versions: May not be compatible
The proxy can modify version numbers using FSNETCMD_LOGON.alter_version() to maintain compatibility.

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Error Handling

try:
    packet = FSNETCMD_LOGON(data)
except Exception as e:
    warning(f"Failed to decode LOGON packet: {e}")
    # Send rejection or error packet

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