From: Fletcher Dunn @ Valve
Date: Thu, 9 Aug 2012 11:32:58 -0500
Subject: [hlds_announce] New WebAPI: Is my server listed on the master server?
A new WebAPI was made available yesterday that is intended to help server admins answer for themselves a frequently-asked question: "Is my server listed on the master servers?". Here's an example:
http://api.steampowered.com/ISteamApps/GetServersAtAddress/v0001?addr=<SERVER IP HERE>&format=json
http://api.steampowered.com/ISteamApps/GetServersAtAddress/v0001?addr=<SERVER IP HERE>&format=xml
To avoid the temptation for abuse, the output does not list all server fields, but it should provide enough information to help troubleshoot a situation where your server is not showing up in the server browser. Most importantly, if you see a "reject" field, the status code indicates why your server was rejected. The most common reason is being out of date, less common is being banned. (Currently numeric codes are used, these will be replaced with human readable codes shortly.)
Please note: This WebAPI is intended for troubleshooting purposes only. Automated use of this WebAPI to scrape the server list or make excessively frequent checks (more frequent than once every 10 minutes or so) is considered abusive.
Simply replace the <SERVER IP HERE> part with your server IP and it will output a list of all server reporting on the Master Servers on that IP.