Steam Server Browser

If your game is on Steam, implementing Steam’s server browser protocol gives players a built-in way to find and join servers without leaving the Steam client. It also enables third-party tools (GameTracker, Battlemetrics, etc.) to index and monitor your game’s servers.

A2S Query Protocol

The Steam server browser uses the A2S query protocol to retrieve information from game servers. It’s a simple UDP request/response protocol with three main query types:

Query	Response	Purpose
A2S_INFO	Server info (name, map, players, max players, game)	Overview for the server list
A2S_PLAYER	List of connected players with play time	Player detail view
A2S_RULES	Server configuration key/value pairs	Server settings for filters

Your server needs to listen for these queries on a UDP port (typically the game port +1) and respond with correctly formatted packets.

Implementing the Query Responder

The A2S protocol is well-documented by Valve. Each query type has a specific request byte and an expected response format.

At minimum, implement A2S_INFO. This is what the Steam server browser and third-party tools query most frequently. It should return:

Server name
Current map
Game directory / game name
Number of connected players
Maximum players
Server type (dedicated, listen)
OS (Linux, Windows)
Password protection status
VAC security status

A2S_PLAYER and A2S_RULES add depth (player list, server settings) and are worth implementing but lower priority.

Rate Limiting

A2S queries come from the internet. Your query responder should:

Rate-limit responses per source IP (e.g., max 1 response per second per IP)
Not allocate significant resources per query (keep responses pre-built or cached)
Ignore malformed packets silently

Without rate limiting, your query port can be used for UDP amplification attacks.

Server Browser Tags and Filtering

A2S_INFO responses include a tags field (also called “game tags” or “sv_tags”). This is a comma-separated string that the Steam client uses for filtering.

Use tags to expose:

Game mode (pvp, pve, creative)
Modded vs. vanilla
Server difficulty
Region identifier
Any other attribute players might filter by

Example: pvp,modded,hard,us-east

Keep tags concise and consistent. Document the tag vocabulary for your game so third-party tools and hosting platforms can set them correctly.

Master Server Registration

For your servers to appear in the Steam server browser, they must register with Steam’s master server. This happens automatically if your server uses the Steamworks Game Server API (SteamGameServer_Init).

The Steamworks Game Server API also provides:

VAC (Valve Anti-Cheat) integration
Player authentication via Session Tickets
Server-side Steamworks API access (stats, achievements)

If your server already initializes Steamworks for other features, master server registration comes with it. If not, you need to initialize the Game Server API at startup.

Requirements for Master Server Listing

Your game must have a Steam App ID
The server must call SteamGameServer()->SetProduct() and other metadata setters
The server must call SteamGameServer()->LogOnAnonymous() to connect to Steam
The server must call SteamGameServer()->SetAdvertiseServerActive(true) to stay listed

Servers that fail to send heartbeats are delisted after a timeout.

Dedicated Server App ID

If you distribute your server via SteamCMD create a separate App ID for the dedicated server tool. The server’s Game Server API should use your game’s main App ID for listing, but the dedicated server tool itself can have its own App ID for distribution purposes.

Testing

Use an online A2S query tool to verify your server responds correctly
- https://gamedig.github.io/ is a popular Node.js library for querying game servers, including A2S, and offers a CLI for testing
Check that your server appears in the Steam server browser under your game
Verify filtering works with your tags
Test with the server behind NAT and on non-default ports