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Netcraft determines the operating system of the queried host by looking in detail at the network characteristics of the HTTP reply received from the web site. Some of these SLBs handle the TCP connection themselves, so we detect the SLB's operating system. The site is using a TCP connection-level proxy firewall , such as provided in the TIS Gauntlet , BorderWare , Raptor , CyberGuard or IBM SecureWay firewalls, or some other kind of HTTP level proxy. In these cases we will receive data from the intermediate machine rather than the web server, so detect the intermediate machine's operating system. The site load shares across multiple servers using different operating systems with round robin DNS .