Server queries

The Source engine allows you to query information from a running game server using UDP/IP packets. This document describes the packet formats and protocol to access this data.

Basic Data Types

All server queries consist of 5 basic types of data packed together into a data stream. All types are little endian, so no conversion is needed on x86 CPUs.

Name	Description
byte	8 bit character
short	16 bit signed integer
long	32 bit signed integer
float	32 bit float value
string	variable length byte field, terminated by 0x00

Protocol

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

Data	Type	Comment
Type	integer	Is -2 (0xFFFFFFFE) to indicate that data is split over several packets
Request ID	integer	The server assigns a unique number to each split packet
Packet Number	byte	The lower four bits represent the number of packets (2 to 15) and the upper four bits represent the current packet starting with 0

Server rules usually exceed the 1400 bytes. Because UDP packets can arrive in different order or delayed, every packet should be verified by its Request ID and the Packet 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).

Query Types

The server responds to 4 queries:

• A2S_SERVERQUERY_GETCHALLENGE - Returns a challenge number for use in the player and rules query.
• A2S_INFO - Basic information about the server.
• A2S_PLAYER - Details about each player on the server.
• A2S_RULES - The rules the server is using.

Queries should be sent in UDP packets to the listen port of the server, which is typically port 27015.

A2S_SERVERQUERY_GETCHALLENGE

Request format

Challenge values are required for A2S_PLAYER and A2S_RULES requests, you can use this request to get one. Note: You can also send A2S_PLAYER and A2S_RULES queries with a challenge value of -1 (0xFF FF FF FF) and they will respond with a challenge value to use (using the reply format below).

FF FF FF FF 57

Reply format

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

Example reply:

FF FF FF FF 41 32 42 59 45

A2S_INFO

Request format

Server info can be requested by sending the following byte values in a UDP packet to the server.

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

Reply format

Data	Type	Comment
Type	byte	Should be equal to 'I' (0x49)
Version	byte	Network version
Hostname	string	The Servers name
Map	string	The current map being played
Game Directory	string	The Game type
Game Description	string	A friendly string name for the game type
AppID	short	Steam Application ID
Num players	byte	The number of players currently on the server
Max players	byte	Maximum allowed players for the server
Num of bots	byte	Number of bot players currently on the server
Dedicated	byte	'l' for listen, 'd' for dedicated
OS	byte	'l' for Linux, 'w' for Windows
Password	byte	If set to 1 a password is required to join this server
Secure	byte	if set to 1 this server is running VAC
Game Version	string	The version of the game

Example reply:

FF FF FF FF 49 02 67 61 6D 65 32 78 73 2E 63 6F    ....I.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.

A2S_PLAYER

Request format

FF FF FF FF 55 <4 byte challenge number>

The challenge number can either be set to -1 (0xFF FF FF FF) to have the server reply with S2C_CHALLENGE, or use the value from a previous A2S_SERVERQUERY_GETCHALLENGE request.

Reply format

The players response has two sections, the initial header:

Data	Type	Comment
Type	byte	Should be equal to 'D' (0x44)
Num Players	byte	The number of players reported in this response

Then for each player the following fields are sent:

Data	Type	Comment
Index	byte	The index into [0.. Num Players] for this entry
Player Name	string	Player's name
Kills	long	Number of kills this player has
Time connected	float	The time in seconds this player has been connected

A2S_RULES

Request format

FF FF FF FF 56 <4 byte challenge number>

The challenge number can either be set to -1 (0xFF FF FF FF) to have the server reply with S2C_CHALLENGE, or use the value from a previous A2S_SERVERQUERY_GETCHALLENGE request.

Reply format

The rules response has two sections, the initial header:

Data	Type	Comment
Type	byte	Should be equal to 'E' (0x45)
Num Rules	short	The number of rules reported in this response

Then for each rule the following fields are sent:

Data	Type	Comment
Rule Name	string	The name of the rule
Rule Value	string	The rules value

See Also

• Source Server Query Library
• [1] (Contains A2S_INFO reply format for HL1 servers (e.g. Counter-Strike).) Mind you that A2S_INFO replies start with -1 (long).