Systems and methods are disclosed for detecting mobile devices. In one implementation, a system includes a web server. The web server receives mobile device detection strings from a DNS server in response to a DNS request and stores the mobile device detection strings on the server. The web server uses the mobile device detection strings to determine if a client device making a request to the web server is a mobile device. The web server may determine if the client device is a mobile device by comparing a user agent string in the request to the mobile device detection strings. The web server may determine whether the client device is a mobile device before sending the client device's request to any server-side scripts stored at the web server.