Server queries

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You can query information from a running game server using UDP/IP packets. This document describes the packet formats and protocol to access this data.

Data Types

All server queries consist of five basic types of data packed together into a data stream. All types are little endian, which is the default on most languages on x86 CPUs. Java users should use ByteBuffer to convert.

Name Description
byte 8 bit character or unsigned integer
short 16 bit signed integer
long 32 bit signed integer
float 32 bit floating point
long long 64 bit unsigned integer
string variable-length byte field, encoded in UTF-8, terminated by 0x00

Protocol

Steam uses a packet size of up to 1400 bytes + IP/UDP headers. If a request or response needs more packets for the data it starts the packets with an additional header.

2023-07-10: Rust has been observed breaking this, sending packets up to the maximum IP size (64KB).

Simple Response Format

Data Type Comment
Header long Always equal to -1 (0xFFFFFFFF). Means it isn't split.
Payload

Multi-packet Response Format

Goldsource Server

Data Type Comment
Header long Always equal to -2 (0xFFFFFFFE). Means the packet is split.
ID long Unique number assigned by server per answer.
Packet number byte Upper 4 bits represent the number of the current packet (starting at 0) and bottom 4 bits represent the total number of packets (2 to 15).
Payload

Source Server

Data Type Comment
Header long Always equal to -2 (0xFFFFFFFE). Means the packet is split.
ID long Same as the Goldsource server meaning. However, if the Wikipedia icon most significant bit is 1, then the response was compressed with bzip2 before being cut and sent. Refer to compression procedure below.
Total byte The total number of packets in the response.
Number byte The number of the packet. Starts at 0.
Size short (Orange Box Engine and above only.) Maximum size of packet before packet switching occurs. The default value is 1248 bytes (0x04E0), but the server administrator can decrease this. For older engine versions: the maximum and minimum size of the packet was unchangeable. AppIDs which are known not to contain this field: 215, 17550, 17700, and 240 when protocol = 7.
This is only present in the first packet of the response and only if the response is being compressed.
Data Type Comment
Size long Size of the whole response once it is decompressed.
CRC32 sum long CRC32 checksum of uncompressed response.

To uncompress this response: collect and assemble all the payloads of the received packets, then pass that into a function like BZ2_bzBuffToBuffDecompress() in C, bzdecompress() in PHP, or bz2.decompress() in Python. Compression appears to have been phased out in newer Source games, but is still present in ~2006-era engines.

Payload

Because UDP packets can arrive in different order or delayed, every packet should be verified by its "ID" and "Number"! Every request or response with or without this header is continued with an integer -1 (0xFFFFFFFF) and the user data. So by reading the first 4 bytes you can decide whether the data is split (-2) or in one packet (-1).

Note: Some titles when using the split packet method have been observed erroneously nesting the single message header inside the split payload.

Requests

The server responds to 5 queries:

A2S_INFO
Basic information about the server.
A2S_PLAYER
Details about each player on the server.
A2S_RULES
The rules the server is using.
A2A_PING
Ping the server. (DEPRECATED)
A2S_SERVERQUERY_GETCHALLENGE
Returns a challenge number for use in the player and rules query. (DEPRECATED)

Queries should be sent in UDP packets to the listen port of the server.

Challenge Mechanism

Servers may respond with the data immediately. However, since this reply is larger than the request, it makes the server vulnerable to a reflection amplification attack. Instead, the server may reply with a challenge to the client using S2C_CHALLENGE ('A' or 0x41). In that case, the client should repeat the request by appending the challenge number. This change was introduced in December 2020 to address the reflection attack vulnerability, and all clients are encouraged to support the new protocol. See this post for more info.

Previously the structure of this document made it seem that only A2S_INFO messages would be responded with in this manner, however ANY of the queries made MAY result in a `S2C_CHALLENGE` response if either the challenge was omitted, is wrong, or has expired.

A2S_INFO

Retrieves information about the server including, but not limited to: its name, the map currently being played, and the number of players.

l4d2 server 2.2.1.3 update:some server will send challenge number ,it caused hlsw cant get server info;

Request Format

Data Type Value
Header byte 'T' (0x54)
Payload string "Source Engine Query\0"
Challenge long 4-byte challenge (sometimes -- see below)
FF FF FF FF 54 53 6F 75 72 63 65 20 45 6E 67 69   ÿÿÿÿTSource Engi
6E 65 20 51 75 65 72 79 00                        ne Query.

Example for challenge number:

Server response:

FF FF FF FF 41 0A 08 5E EA

You need send :

FF FF FF FF 54 53 6F 75 72 63 65 20 45 6E 67 69 6E 65 20 51 75 65 72 79 00 0A 08 5E EA


NOTE: In the future, additional data may be appended to the request! Do not assume that if a request has extra data, that it is a bogus packet!

NOTE:changed 2021-6-19 13:05:43 :when l4d2 server received some packet from same ip address,it may deined replay

NOTE:some older titles have been observed sending the GoldSource INFO packet AND the newer INFO packet in reply to this.

Response Format

Note.pngNote:Both SiN 1 Multiplayer and Rag Doll Kung Fu utilise this response protocol.
Data Type Comment
Header byte Always equal to 'I' (0x49)
Protocol byte Protocol version used by the server.
Name string Name of the server.
Map string Map the server has currently loaded.
Folder string Name of the folder containing the game files.
Game string Full name of the game.
ID short Steam Application ID of game.
Players byte Number of players on the server.
Max. Players byte Maximum number of players the server reports it can hold.
Bots byte Number of bots on the server.
Server type byte Indicates the type of server:
  • 'd' for a dedicated server
  • 'l' for a non-dedicated server
  • 'p' for a SourceTV relay (proxy)
Environment byte Indicates the operating system of the server:
  • 'l' for Linux
  • 'w' for Windows
  • 'm' or 'o' for Mac (the code changed after L4D1)
Visibility byte Indicates whether the server requires a password:
  • 0 for public
  • 1 for private
VAC byte Specifies whether the server uses VAC:
  • 0 for unsecured
  • 1 for secured
These fields only exist in a response if the server is running The Ship:
Data Type Comment
Mode byte Indicates the game mode:
  • 0 for Hunt
  • 1 for Elimination
  • 2 for Duel
  • 3 for Deathmatch
  • 4 for VIP Team
  • 5 for Team Elimination
Witnesses byte The number of witnesses necessary to have a player arrested.
Duration byte Time (in seconds) before a player is arrested while being witnessed.
Version string Version of the game installed on the server.
Extra Data Flag (EDF) byte If present, this specifies which additional data fields will be included.
Only if if ( EDF & 0x80 ) proves true:
Data Type Comment
Port short The server's game port number.
Only if if ( EDF & 0x10 ) proves true:
Data Type Comment
SteamID long long Server's SteamID.
Only if if ( EDF & 0x40 ) proves true:
Data Type Comment
Port short Spectator port number for SourceTV.
Name string Name of the spectator server for SourceTV.
Only if if ( EDF & 0x20 ) proves true:
Data Type Comment
Keywords string Tags that describe the game according to the server (for future use.)
Only if if ( EDF & 0x01 ) proves true:
Data Type Comment
GameID long long The server's 64-bit GameID. If this is present, a more accurate AppID is present in the low 24 bits. The earlier AppID could have been truncated as it was forced into 16-bit storage.
Note.pngNote:Rag Doll Kung Fu servers always return 0 for "Server type."


Example response for Counter-Strike: Source:

FF FF FF FF 49 02 67 61 6D 65 32 78 73 2E 63 6F    ÿÿÿÿI.game2xs.co
6D 20 43 6F 75 6E 74 65 72 2D 53 74 72 69 6B 65    m Counter-Strike
20 53 6F 75 72 63 65 20 23 31 00 64 65 5F 64 75     Source #1.de_du
73 74 00 63 73 74 72 69 6B 65 00 43 6F 75 6E 74    st.cstrike.Count
65 72 2D 53 74 72 69 6B 65 3A 20 53 6F 75 72 63    er-Strike: Sourc
65 00 F0 00 05 10 04 64 6C 00 00 31 2E 30 2E 30    e......dl..1.0.0
2E 32 32 00                                        .22.

Example response for The Ship:

FF FF FF FF 49 07 53 68 69 70 20 53 65 72 76 65    ÿÿÿÿI.Ship Serve
72 00 62 61 74 61 76 69 65 72 00 73 68 69 70 00    r.batavier.ship.
54 68 65 20 53 68 69 70 00 60 09 01 05 00 6C 77    The Ship.`....lw
00 00 01 03 03 31 2E 30 2E 30 2E 34 00             .....1.0.0.4.

Example response for SiN 1 Multiplayer:

FF FF FF FF 49 2F 53 65 6E 73 65 6D 61 6E 6E 20    ÿÿÿÿI/Sensemann 
53 69 4E 20 44 4D 00 70 61 72 61 64 6F 78 00 53    SiN DM.paradox.S
69 4E 20 31 00 53 69 4E 20 31 00 1D 05 00 10 00    iN 1.SiN 1......
6C 77 00 00 31 2E 30 2E 30 2E 30 00                lw..1.0.0.0.

Example response for Rag Doll Kung Fu:

FF FF FF FF 49 FC 54 68 65 20 44 75 64 65 27 73     ÿÿÿÿIüThe Dude's
20 64 6F 6A 6F 00 53 6F 63 63 65 72 00 52 44 4B      dojo.Soccer.RDK
46 53 6F 63 63 65 72 00 52 61 67 44 6F 6C 6C 4B     FSoccer.RagDollK
75 6E 67 46 75 3A 20 53 6F 63 63 65 72 00 EA 03     ungFu: Soccer.ê.
01 04 00 00 77 00 00 32 2E 33 2E 30 2E 30 00        ....w..2.3.0.0.


Obsolete GoldSource Response

GoldSource servers reply using the same response as Source servers. Older and pre-Steam servers still reply with the response format below. GoldSource can be distinguished from Source via AppIDs.

Data Type Comment
Header byte Always equal to 'm' (0x6D)
Address string IP address and port of the server.
Name string Name of the server.
Map string Map the server has currently loaded.
Folder string Name of the folder containing the game files.
Game string Full name of the game.
Players byte Number of players on the server.
Max. Players byte Maximum number of players the server reports it can hold.
Protocol byte Protocol version used by the server.
Server type byte Indicates the type of server:
  • 'D' for dedicated server
  • 'L' for non-dedicated server
  • 'P' for a HLTV server
Environment byte Indicates the operating system of the server:
  • 'L' for Linux
  • 'W' for Windows
Visibility byte Indicates whether the server requires a password:
  • 0 for public
  • 1 for private
Mod byte Indicates whether the game is a mod:
These fields are only present in the response if "Mod" is 1:
Data Type Comment
Link string URL to mod website.
Download Link string URL to download the mod.
NULL byte NULL byte (0x00)
Version long Version of mod installed on server.
Size long Space (in bytes) the mod takes up.
Type byte Indicates the type of mod:
  • 0 for single and multiplayer mod
  • 1 for multiplayer only mod
DLL byte Indicates whether mod uses its own DLL:
  • 0 if it uses the Half-Life DLL
  • 1 if it uses its own DLL
VAC byte Specifies whether the server uses VAC:
  • 0 for unsecured
  • 1 for secured
Bots byte Number of bots on the server.
Note.pngNote:HLTV servers may respond with a late request that is either incomplete or incorrect if the number of slots is 255.


Example response:

FF FF FF FF 6D 37 37 2E 31 31 31 2E 31 39 34 2E    ÿÿÿÿm77.111.194.
31 31 30 3A 32 37 30 31 35 00 46 52 20 2D 20 56    110:27015.FR - V
65 72 79 47 61 6D 65 73 2E 6E 65 74 20 2D 20 44    eryGames.net - D
65 61 74 6D 61 74 63 68 20 2D 20 6F 6E 6C 79 20    eatmatch - only 
73 75 72 66 5F 73 6B 69 20 2D 20 6E 67 52 00 73    surf_ski - ngR.s
75 72 66 5F 73 6B 69 00 63 73 74 72 69 6B 65 00    urf_ski.cstrike.
43 6F 75 6E 74 65 72 2D 53 74 72 69 6B 65 00 0C    Counter-Strike..
12 2F 64 6C 00 01 77 77 77 2E 63 6F 75 6E 74 65    ./dl..www.counte
72 2D 73 74 72 69 6B 65 2E 6E 65 74 00 00 00 01    r-strike.net....
00 00 00 00 9E F7 0A 00 01 01 00                   ....ž÷.....

A2S_PLAYER

This query retrieves information about the players currently on the server. It needs an initial step to acquire a challenge number.

Note.pngNote:SourceTV does not respond to this request.

Request Format

Data Type Value
Header byte 'U' (0x55)
Challenge int Challenge number, or -1 (0xFFFFFFFF) to receive a challenge number.

Challenge response format:

Data Type Value
Header byte 'A' (0x41)
Challenge int Challenge number.

Example challenge request:

FF FF FF FF 55 FF FF FF FF                         ÿÿÿÿUÿÿÿÿ"       

Example challenge response:

FF FF FF FF 41 4B A1 D5 22                         ÿÿÿÿAÿÿÿÿ"       

Example A2S_PLAYER request with the received challenge number:

FF FF FF FF 55 4B A1 D5 22                         ÿÿÿÿUÿÿÿÿ"       

Response Format

Data Type Comment
Header byte Always equal to 'D' (0x44)
Players byte Number of players whose information was gathered.
For every player in "Players" there is this chunk in the response:
Data Type Comment
Index byte Index of player chunk starting from 0.
Name string Name of the player.
Score long Player's score (usually "frags" or "kills".)
Duration float Time (in seconds) player has been connected to the server.

The Ship additional player info (comes after the basic information in the packet):

Data Type Comment
Deaths long Player's deaths
Money long Player's money
Note.pngNote:When a player is trying to connect to a server, they are recorded in the number of players. However, they will not be in the list of player information chunks.
Warning.pngWarning:CS:GO Server by default returns only max players and server uptime. You have to change server cvar "host_players_show" in server.cfg to value "2" if you want to revert to old format with players list.



Example response:

FF FF FF FF 44 02 01 5B 44 5D 2D 2D 2D 2D 3E 54    ÿÿÿÿD..[D]---->T
2E 4E 2E 57 3C 2D 2D 2D 2D 00 0E 00 00 00 B4 97    .N.W<----.....´—
00 44 02 4B 69 6C 6C 65 72 20 21 21 21 00 05 00    .D.Killer !!!...
00 00 69 24 D9 43                                  ..i$ÙC

The Ship response:

FF FF FF FF 44 13 00 53 68 69 70 6D 61 74 65 31    ▒▒▒▒D..Shipmate1
00 00 00 00 00 00 00 80 BF 01 53 68 69 70 6D 61    .......▒▒.Shipma
74 65 32 00 00 00 00 00 00 00 80 BF 02 53 68 69    te2.......▒▒.Shi
70 6D 61 74 65 33 00 00 00 00 00 00 00 80 BF 03    pmate3.......▒▒.
53 68 69 70 6D 61 74 65 34 00 00 00 00 00 00 00    Shipmate4.......
80 BF 04 53 68 69 70 6D 61 74 65 35 00 00 00 00    ▒▒.Shipmate5....
00 00 00 80 BF 07 28 31 29 4C 61 6E 64 4C 75 62    ...▒▒.(1)LandLub
62 65 72 00 00 00 00 00 D3 8E 68 45 00 00 00 00    ber.....ӎhE....
C4 09 00 00 00 00 00 00 C4 09 00 00 00 00 00 00    ▒.......▒.......
C4 09 00 00 00 00 00 00 C4 09 00 00 00 00 00 00    ▒.......▒.......
C4 09 00 00 00 00 00 00 C4 09 00 00                ▒.......▒...    

A2S_RULES

Returns the server rules, or configuration variables in name/value pairs. This query requires an initial challenge step.

The Console variables included are the ones marked with FCVAR_NOTIFY as well as any additional ones listed in the server configuration.

Request Format

Data Type Value
Header byte 'V' (0x56)
Challenge long Challenge number, or -1 (0xFFFFFFFF) to receive a challenge number.

Challenge response format:

Data Type Value
Header byte 'A' (0x41)
Challenge long Challenge number.

Example challenge request:

FF FF FF FF 56 FF FF FF FF                         ÿÿÿÿVÿÿÿÿ"       

Example challenge response:

FF FF FF FF 41 4B A1 D5 22                         ÿÿÿÿVK¡Õ"       
Warning.pngWarning:CS:GO Rules reply is broken since update CSGO 1.32.3.0 (Feb 21, 2014). Before the update rules got truncated when exceeding MTU, after the update rules reply is not sent at all. You can also see a console message: [NET] Cannot send connectionless packet to xxx.xxx.xxx.xxx:xxxxx '0x45' exceeding MTU (2644)

Final request that includes the received challenge number:

FF FF FF FF 56 4B A1 D5 22                         ÿÿÿÿVK¡Õ"       

Response Format

Note.pngNote:Responses to this query are usually multi-packeted.
Data Type Comment
Header byte Always equal to 'E' (0x45)
Rules short Number of rules in the response.
For every rule in "Rules" there is this chunk in the response:
Data Type Comment
Name string Name of the rule.
Value string Value of the rule.

A2A_PING

Ping the server to see if it exists, this can be used to calculate the latency to the server.

Warning.pngWarning:According to Valve (see Talk Page), A2A_PING is no longer supported on Counter-Strike: Source and Team Fortress 2 servers, and is considered a deprecated feature.

Request Format

Data Type Value
Header byte 'i' (0x69)
FF FF FF FF 69                                     ÿÿÿÿi           

Response Format

Goldsource Server
Data Type Value
Header byte 'j' (0x6A)
Payload string Null
FF FF FF FF 6A 00                                  ÿÿÿÿj.           
Source Server
Data Type Value
Heading byte 'j' (0x6A)
Payload string '00000000000000.'
FF FF FF FF 6A 30 30 30 30 30 30 30 30 30 30 30    ÿÿÿÿj00000000000
30 30 30 00                                        000.

A2S_SERVERQUERY_GETCHALLENGE

A2S_PLAYER and A2S_RULES queries both require a challenge number. Formerly, this number could be obtained via an A2S_SERVERQUERY_GETCHALLENGE request. In newer games it no longer works. Instead, issue A2S_PLAYER or A2S_RULES queries with an initial challenge of -1 (0xFFFFFFFF) to receive a usable challenge number.

On some engines (confirmed AppIDs: 17510, 17530, 17740, 17550, 17700) it can be used. The format is:

Request Format

Data Type Value
Header byte 'W' (0x57)
FF FF FF FF 57                                     ÿÿÿÿW           

Response Format

Data Type Comment
Header byte Should be equal to 'A' (0x41.)
Challenge long The challenge number to use.

Example response:

FF FF FF FF 41 4B A1 D5 22                         ÿÿÿÿAK¡Õ"       

Implementations

See also