Http.sys registry settings for Windows

This article describes Http.sys registry settings for Windows.

Original product version:   Windows 8, Windows Server 2012, 2008 R2, 2008
Original KB number:   820129


In Windows Server 2008 and later versions, Http.sys is the kernel mode driver that handles Hypertext Transfer Protocol (HTTP) requests. Several registry values can be configured according to specific requirements. The table in the Registry keys section contains the following information about these registry values:

  • Registry key names
  • Default values
  • Valid value ranges
  • Registry key functions
  • WARNING codes (where applicable)


See the Warning codes section for information about potential risks when you create and configure registry values by using settings other than the default settings.

This article is intended for advanced users and assumes knowledge of the registry and of the risks that are involved when the registry is changed.

Registry keys


This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows.

You can create the following DWORD registry values under the following registry key:

Registry key Default value Valid value range Registry key function WARNING code
AllowRestrictedChars 0 Boolean If nonzero, Http.sys accepts hex-escaped chars in request URLs that decode to U+0000 - U+001F and U+007F - U+009F ranges. 0
EnableAggressiveMemoryUsage 0 0
Preallocate nonpaged pool memory. By default, the HTTP service stops accepting connections when less than 20 megabytes (MB) of nonpaged pool memory is available. After you add this value to the registry, the HTTP service stops accepting connections when less than 8 MB of nonpaged pool memory is available. Setting this registry value may reduce the number of Connections_refused and 503 errors in the Httperr.log file. 0
EnableNonUTF8 1 Boolean If zero, Http.sys accepts only UTF-8-encoded URLs. If nonzero, Http.sys also accepts ANSI- or DBCS-encoded URLs in requests. 0
FavorUTF8 1 Boolean If nonzero, Http.sys always tries to decode a URL as UTF-8 first; if that conversion fails and EnableNonUTF8 is nonzero, Http.sys then tries to decode it as ANSI or DBCS. If zero (and EnableNonUTF8 is nonzero), Http.sys tries to decode it as ANSI or DBCS; if that is not successful, it tries a UTF-8 conversion. 0
MaxBytesPerSend 65536 1-0xFFFFF (Bytes) Overrides the TCP window size that is used by Http.sys. A higher value may enable higher download speeds in network environments that have high bandwidth and high latency. 0
MaxConnections MAX_ULONG 1024 (1k) - 2031616 (2 MB) connections Overrides the MaxConnections calculation in the driver. This is primarily a function of memory. 1
MaxEndpoints 0 0 - 1024 The maximum number of current endpoint objects that are permitted. The default value of zero implies that the maximum is computed from available memory. 1
MaxFieldLength 16384 64 - 65534 (64k - 2) bytes Sets an upper limit for each header. See MaxRequestBytes. This limit translates to approximately 32k characters for a URL. 1
MaxRequestBytes 16384 256 - 16777216 (16 MB) bytes Determines the upper limit for the total size of the Request line and the headers.
Its default setting is 16 KB. If this value is lower than MaxFieldLength, the MaxFieldLength value is adjusted.
PercentUAllowed 1 Boolean If nonzero, Http.sys accepts the % uNNNN notation in request URLs. 0
UrlSegmentMaxCount 255 0 - 16,383 segments Maximum number of URL path segments. If zero, the count bounded by the maximum value of a ULONG. 1
UriEnableCache 1 Boolean If nonzero, the Http.sys response and fragment cache are enabled. 0
UriMaxUriBytes 262144 (bytes) 4096 (4k) - 16777216 (16 MB) bytes Any response that is greater than this value is not cached in the kernel response cache. 1
UriScavengerPeriod 120 (seconds) 10 - 0xFFFFFFFF seconds Determines the frequency of the cache scavenger. Any response or fragment that has not been accessed in the number of seconds equal to UriScavengerPeriod is flushed. 1
UrlSegmentMaxLength 260 0 - 32,766 chars Maximum number of characters in a URL path segment (the area between the slashes in the URL). If zero, it is the length that is bounded by the maximum value of a ULONG. 1
DisableServerHeader 0 0 - 2 This key controls how http.sys behaves with regards to appending the http response header Server for responses that it sends to clients. A value of 0, which is the default value, will use the header value the application provides to http.sys, or will append the default value of Microsoft-HTTPAPI/2.0 to the response header. A value of 1 will not append the Server header for responses generated by http.sys (responses ending in 400, 503, and other status codes). A value of 2 will prevent http.sys from appending a Server header to the response. If a Server header is present on the response, it will not be removed, if one is not present, it will not be added. 0

You may experience slow performance in Internet Information Services (IIS) when Internet Server API (ISAPI) applications or Common Gateway Interface (CGI) applications that are hosted on IIS send responses. If you experience this issue, you can add the MaxBufferedSendBytes DWORD value to the registry.

In Windows Server 2008 and later versions, you can also create the following DWORD value under the following registry key:

Registry key Default value Valid value range Registry key function WARNING code
CertChainCacheOnlyUrlRetrieval 1 0
By default, the AIA hints are not followed during chain validation when IIS is configured to use Client Certificates. This behavior is for performance and security reasons. For example, this behavior can help prevent DoS attacks. However, this behavior can also lead to unexpected certificate rejections when AIA retrieval is needed. To override this behavior, you can set the DWORD parameter CertChainCacheOnlyUrlRetrieval to 0 (zero) under the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTPFilter\Parameters registry key. Not applicable

Warning codes

  • 0: No risks.
  • 1: Changing this registry key is considered dangerous. This key causes Http.sys to use more memory and may increase vulnerability to malicious attacks.
  • 2: A low value may cause the cache to be flushed more frequently. If this behavior occurs, it may affect performance.
  • 3: A low value may affect performance for static content.

Changes that are made to the registry will not take effect until you restart the HTTP service. Additionally, you may have to restart any related IIS services.

To restart the HTTP service, type and all related IIS services, follow these steps:

  1. Select Start, select Run, type Cmd, and then select OK.

  2. At the command prompt, type net stop http, and then press Enter.

  3. At the command prompt, type net start http, and then press Enter.

  4. At the command prompt, type net stop iisadmin /y, and then press Enter.


    Any IIS services that depend on the IIS Admin Service service will also be stopped. Notice the IIS services that are stopped when you stop the IIS Admin Service service. You will restart each service in the next step.

  5. Restart the IIS services that were stopped in step 4. To do this, type net start servicename at the command prompt and then press Enter. In the command, servicename is the name of the service that you want to restart. For example, to restart the World Wide Web Publishing Service service, type net start World Wide Web Publishing Service, and then press Enter.