August 7, 2013 | By kiranbadi1991 | Filed in: Development, Environment, Others.

Recently someone asked me the importance of http.sys component of IIS 6 Server hosted on Windows 2003 Server. HTTP.SYS was the new addition to IIS 6 and it does some functions which was done earlier by INETINFO.EXE process of IIS 5.0. In IIS 6.0, http.sys is just a listener and in no way processes any of the requests which it gets,

So I am writing this short information about http.sys which I am aware of,

  • HTTP.Sys is more of the Listener service. Its job is listen to the incoming requests and pass it down to downstream IIS Stack.
  • HTTP.Sys often runs in Kernel Mode, which gives it an access to TCP/IP Stack and various other hardware resources of the server.
  • Since HTTP.Sys runs in Kernel Mode, it has got high priority than other threads, so jobs allocated to http.sys gets higher priority by the native OS Threads.
  • HTTP.SYS also queues the requests till application or downstream components comes back with response. Technically it queues the requests till worker process picks the request for further     processing.
  • HTTP.SYS rarely crashes due to bad code though in very rare cases you might see the crash in badly malformed requests.
  • HTTP.SYS also caches the response and can fetch the response from Memory thereby increasing the performance of the application.
  • HTTP.SYS is the component which handles the TCP connections of the IIS Server like opening and closing the connection, bandwidth throttling etc.
  • HTTP.SYS is the component which routes the requests to the appropriate application pool.
  • Errors related to HTTP.Sys are logged into the HTTP Err logs.

More information about HTTP.Sys can be found at below URL,


If you are IIS Administrator, it helps to know and understand as what are the various functions done by core component of IIS. It helps to troubleshoot and fix odd bugs.

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