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Revision 0.5 (Draft)
November 1995
This specification describes the basics of writing an ISAPI (Internet Server API) filter for the Microsoft® Internet Information Server. An ISAPI filter is a replaceable dynamic-link library (DLL) the server calls on every HTTP (Hypertext Transfer Protocol) request. When the filter is loaded, it tells the server what sort of notifications it is interested in. After that, whenever the selected events occur, the filter is called and given the opportunity to process that event.
ISAPI filters are powerful enough to allow for the following applications:
Note that once a filter has expressed interest in a request, it will receive that data regardless of whether the request is for a file, a CGI (Common Gateway Interface, a common HTTP server extension) application, or an ISAPI application.
ISAPI filters can be used to enhance the Microsoft Internet Information Server with custom features such as enhanced logging of HTTP requests, custom encryption, and compression schemes, or new authentication methods. The filter applications sit between the network connection to the clients and the HTTP server. Depending on the options that the filter application chooses, it can act on several server actions, including reading raw data from the client, processing the headers, communications over a secure port (PCT--Private Communication Technology, SSL--Secure Sockets Layer, and so on), or several other stages in the processing of the HTTP request.
The setup program that installs the ISAPI filter should add it to the registry under "HKEY_LOCAL_MACHINE\ System\ CurrentControlSet\ Services\ W3Svc\ Parameters\ Filter DLLs". This value is a comma-separated list. The filter should be added to the end of the list. When uninstalling the filter, remove the string that you added, taking care to handle the situation where other ISAPI filters have been added or removed since your DLL was installed.
Using the ISAPI filter APIs requires the filter application's path to be installed as described above. When the server starts up, it reads this value and loads the DLLs listed. It then calls the GetFilterVersion entry point to exchange version information and determine the notifications desired and priority to deliver them. As the events happen, the server will notify each filter application (by calling the HttpFilterProc entry points) that registered for that event in the priority order requested by GetFilterVersion. In the event of a tie, the order listed in the registry is used.
Every ISAPI filter DLL must export at least two entry points. These are GetFilterVersion and HttpFilterProc. The GetFilterVersion entry point is passed a HTTP_FILTER_VERSION structure that must be filled out with version information, requested events, and priority level. ISAPI filter applications should register only for the events that are required. Registering for extraneous events can have a significant negative impact on performance and scalability. After this exchange, every time the server processes one of the available events, it will call any filters that have registered for that event. The order in which the server will call the filters depends first on the priority specified in the dwFlags member of HTTP_FILTER_VERSION by GetFilterVersion. In the event that two or more different filters have registered for the same event at the same priority, the order that the filters were loaded from the registry determines the order in which they will be called.
When the HttpFilterProc entry point is called, the filter will typically perform a switch on the NotificationType parameter to determine what action to take. For example, an encryption or compression filter will probably register for reading and writing raw data, while a logging filter will probably only register for the log event. Most filters will also register for the end of net session event. This event is a good time to recycle any buffers used by that client request. For performance reasons, most filters will probably keep a pool of filter buffers and only allocate or free when the pool becomes empty or too large, to save on the overhead of the memory management. One useful callback is the AllocMem callback in the HTTP_FILTER_CONTEXT structure. This will allocate memory that is automatically freed when the communication with the client is terminated. As noted, this can have a negative impact on performance, but with careful use it can be a valuable tool.
BOOL WINAPI GetFilterVersion(
PHTTP_FILTER_VERSION pVer
);
Parameters
pVer
The HTTP_FILTER_VERSION structure pointed to by this parameter contains version information for the server and fields for the client to indicate version number, notifications, and priority desired. There is also a space for the filter application to register a small description of itself.
Return Value
The return code indicates if the filter was properly loaded. If the filter returns FALSE, the filter application will be unloaded and will not receive any notifications.
Remarks
This API, implemented in the ISAPI filter application, is the first entry point called by the Internet Information Server. It is important to specify only the necessary notifications in the pVer->dwFlags member. Some notifications can have a strong impact on performance and scalability. In addition to the notification flags described in the HttpFilterProc documentation, there are also priority flags to specify which order to call the filter:
![[HTT1458B 3028 bytes ]](images/HTT1458B.GIF)
See Also:
HttpFilterProc, HTTP_FILTER_VERSION
DWORD WINAPI HttpFilterProc(
PHTTP_FILTER_CONTEXT pfc,
DWORD NotificationType,
LPVOID pvNotification );
Parameters
pfc
The HTTP_FILTER_CONTEXT structure pointed to by this parameter contains context information. The pFilterContext member can be used by the filter to associate any context information with the HTTP request. The SF_NOTIFY_END_OF_NET_SESSION notification can be used to release any such context information.
NotificationType
Indicates the type of event being processed. Valid types are:
![[HTT1458C 6164 bytes ]](images/HTT1458C.GIF)
pvNotification
Notification-specific structure
![[HTT1458D 3799 bytes ]](images/HTT1458D.GIF)
Return Code
Indicates how the application handled the event. Possible return codes are:
![[HTT1458E 6538 bytes ]](images/HTT1458E.GIF)
Remarks
This is where the core work of the ISAPI filter applications is done. The various structures pointed to by pvNotification contain data and function pointers specific to these operations. See the structure details for more information.
See Also
HTTP_FILTER_CONTEXT, HTTP_FILTER_RAW_DATA, HTTP_FILTER_PREPROC_HEADERS, HTTP_FILTER_AUTHENT, HTTP_FILTER_URL_MAP, HTTP_FILTER_LOG
typedef struct _HTTP_FILTER_VERSION
{
DWORD dwServerFilterVersion;
DWORD dwFilterVersion;
CHAR lpszFilterDesc[SF_MAX_FILTER_DESC_LEN+1];
DWORD dwFlags;
} HTTP_FILTER_VERSION, *PHTTP_FILTER_VERSION;
Members
dwServerFilterVersion [IN]
Version of the spec used by the server. The version of the current header file is HTTP_FILTER_REVISION.
dwFilterVersion [OUT]
Version of the spec used by the server. The version of the current header file is HTTP_FILTER_REVISION.
lpszFilterDesc[OUT]
Location to store a short string description of ISAPI filter application.
dwFlags [OUT]
Combination of SF_NOTIFY_* flags to specify which events this application is interested in. See HttpFilterProc for a list of valid flags.
Remarks
This structure is passed to the application's HttpFilterProc entry point by the server.
See Also
typedef struct _HTTP_FILTER_CONTEXT
{
DWORD cbSize;
DWORD Revision;
PVOID ServerContext;
DWORD ulReserved;
BOOL fIsSecurePort;
PVOID pFilterContext;
BOOL (WINAPI * GetServerVariable) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszVariableName,
LPVOID lpvBuffer,
LPDWORD lpdwSize
);
BOOL (WINAPI * AddResponseHeaders) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszHeaders,
DWORD dwReserved
);
BOOL (WINAPI * WriteClient) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPVOID Buffer,
LPDWORD lpdwBytes,
DWORD dwReserved
);
VOID * (WINAPI * AllocMem) (
struct _HTTP_FILTER_CONTEXT * pfc,
DWORD cbSize,
DWORD dwReserved
);
BOOL (WINAPI * ServerSupportFunction) (
struct _HTTP_FILTER_CONTEXT * pfc,
enum SF_REQ_TYPE sfReq,
PVOID pData,
DWORD ul1,
DWORD ul2
);
} HTTP_FILTER_CONTEXT, *PHTTP_FILTER_CONTEXT;
Members
cbSize [IN]
Size of this structure, in bytes.
Revision [IN]
Revision level of this structure. This is less than or equal to the version of the spec, HTTP_FILTER_REVISION.
ServerContext [IN]
Reserved for server use.
ulReserved [IN]
Reserved for server use.
fIsSecurePort [IN]
TRUE indicates that this event is over a secure port.
pFilterContext [IN/OUT]
A pointer to be used by the filter for any context information that the filter wants to associate with this request. Any memory associated with this request can be safely freed during the SF_NOTIFY_END_OF_NET_SESSION notification.
BOOL (WINAPI * GetServerVariable) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszVariableName,
LPVOID lpvBuffer,
LPDWORD lpdwSize
);
Pointer to a function to retrieve information about the server and this connection. See the ISAPI application documentation for GetServerVariable for details. Function parameters:
pfc: pfc passed to HttpFilterProc.
lpszVariableName: Server variable to retrieve.
lpvBuffer: Buffer to store value of variable.
lpdwSize: Size of buffer pointed to lpvBuffer.
BOOL (WINAPI * AddResponseHeaders) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszHeaders,
DWORD dwReserved
);
Pointer to a function that adds a header to the HTTP response. See the ISAPI documentation on ServerSupportFunction, HSE_SEND_RESPONSE_HEADER for details. Function parameters:
pfc: pfc passed to HttpFilterProc.
lpszHeaders: Pointer string containing headers to add.
dwReserved: Reserved for future use. Must be 0.
BOOL (WINAPI * WriteClient) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPVOID Buffer,
LPDWORD lpdwBytes,
DWORD dwReserved
);
Pointer to a function that sends raw data back to the client. See the ISAPI documentation on WriteClient for details. Function parameters:
pfc: pfc passed to HttpFilterProc.
Buffer: Buffer containing data to send to the client.
lpdwBytes: Size of the buffer pointed to by Buffer.
dwReserved: Reserved for future use.
VOID * (WINAPI * AllocMem) (
struct _HTTP_FILTER_CONTEXT * pfc,
DWORD cbSize,
DWORD dwReserved
);
Pointer to a function used to allocate memory. Any memory allocated with this function will automatically be freed when the request is completed. Function parameters:
pfc: pfc passed to HttpFilterProc.
cbSize: Size of the buffer to allocate.
dwReserved: Reserved for future use.
BOOL (WINAPI * ServerSupportFunction) (
struct _HTTP_FILTER_CONTEXT * pfc,
enum SF_REQ_TYPE sfReq,
PVOID pData,
DWORD ul1,
DWORD ul2
);
Pointer to a function used to extend the ISAPI filter APIs. Parameters are specific to the extensions. Possible values for sfReq are SF_REQ_SEND_RESPONSE_HEADER, SF_REQ_ADD_HEADERS_ON_DENIAL, and SF_REQ_SET_NEXT_READ_SIZE.
SF_REQ_SEND_RESPONSE_HEADER
Sends a complete HTTP server response header including the status, server version, message time, and MIME version. Server extensions should append other information, such as Content-type, Content-length, and so on, followed by an extra '\r\n'. Parameters:
If the server denies the HTTP request, add the specified headers to the server error response. This allows an authentication filter to advertise its services without filtering every request. Generally the headers will be WWW-Authenticate headers with custom authentication schemes, but no restriction is placed on what headers may be specified. Parameter:
Only used by raw data filters that return SF_STATUS_READ_NEXT. Parameter:
typedef struct _HTTP_FILTER_RAW_DATA
{
PVOID pvInData;
DWORD cbInData;
DWORD cbInBuffer;
DWORD dwReserved;
} HTTP_FILTER_RAW_DATA, *PHTTP_FILTER_RAW_DATA;
Members
pvInData [IN]
Pointer to the data buffer (input or output).
cbInData [IN]
Amount of data in the buffer pointed to by pvInData.
cbInBuffer [IN]
Size of the buffer pointed to by pvInData.
dwReserved [IN]
Reserved for future use.
Remarks
This structure is passed to the SF_NOTIFY_READ_RAW_DATA and SF_NOTIFY_SEND_RAW_DATA notification routines.
See Also
typedef struct _HTTP_FILTER_PREPROC_HEADERS
{
BOOL (WINAPI * GetHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPVOID lpvBuffer,
LPDWORD lpdwSize
);
BOOL (WINAPI * SetHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPSTR lpszValue
);
BOOL (WINAPI * AddHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPSTR lpszValue
);
DWORD dwReserved;
} HTTP_FILTER_PREPROC_HEADERS, *PHTTP_FILTER_PREPROC_HEADERS;
Members
BOOL (WINAPI * GetHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPVOID lpvBuffer,
LPDWORD lpdwSize
);
Pointer to a function that retrieves the specified header value. Header names should include the trailing colon (':'). The special values 'method', 'url', and 'version' can be used to retrieve the individual portions of the request line. Function parameters:
pfc: Filter context for this request from the pfc passed to the HttpFilterProc.
lpszName: The name of the header to retrieve.
lpvBuffer: Pointer to a buffer of size lpdwSize where the value of the header will be stored.
BOOL (WINAPI * SetHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPSTR lpszValue
);
Pointer to a function used to change or delete the value of a header. Function parameters:
pfc: Filter context for this request from the pfc passed to the HttpFilterProc.
lpszName: Pointer to the name of the header to change or delete.
lpszValue: Pointer to the string to change the header to, or a pointer to '\0' to delete the header.
BOOL (WINAPI * AddHeader) (
struct _HTTP_FILTER_CONTEXT * pfc,
LPSTR lpszName,
LPSTR lpszValue
);
Pointer to a function to add a header. Function parameters:
pfc: Filter context for this request from the pfc passed to the HttpFilterProc.
lpszName: Pointer to the name of the header to change or delete.
lpszValue: Pointer to the string to change the header to, or a pointer to '\0' to delete the header.
Remarks
This structure is pointed to by the pvNotification in the HttpFilterProc when NotificationType is SF_NOTIFY_PREPROC_HEADERS, when the server is about to process the client headers.
See Also
typedef struct _HTTP_FILTER_AUTHENT
{
CHAR * pszUser;
DWORD cbUserBuff;
CHAR * pszPassword;
DWORD cbPasswordBuff;
} HTTP_FILTER_AUTHENT, *PHTTP_FILTER_AUTHENT;
Members
pszUser [IN/OUT]
Pointer to a string containing the username for this request. An empty string indicates an anonymous user.
cbUserBuff [IN]
Size of the buffer pointed to by pszUser. This is guaranteed to be at least SF_MAX_USERNAME.
pszPassword [IN/OUT]
Pointer to a string containing the password for this request.
cbPasswordBuff [IN]
Size of the buffer pointed to by pszPassword. This is guaranteed to be at least SF_MAX_PASSWORD.
Remarks
This structure is pointed to by the pvNotification in the HttpFilterProc when NotificationType is SF_NOTIFY_AUTHENTICATION, when the server is about to authenticate the client. This can be used to implement a different authentication scheme.
See Also
typedef struct _HTTP_FILTER_URL_MAP
{
const CHAR * pszURL;
CHAR * pszPhysicalPath;
DWORD cbPathBuff;
} HTTP_FILTER_URL_MAP, *PHTTP_FILTER_URL_MAP;
Members
pszURL [IN]
Pointer to the URL that is being mapped to a physical path.
pszPhysicalPath [IN/OUT]
Pointer to the buffer where the physical path is stored.
cbPathBuff [IN]
Size of the buffer pointed to by pszPhysicalPath.
Remarks
This structure is pointed to by the pvNotification in the HttpFilterProc when NotificationType is SF_NOTIFY_URL_MAP, when the server is about to map the specified URL to a physical path. Filters can modify the physical path in place.
See Also
typedef struct _HTTP_FILTER_LOG
{
const CHAR * pszClientHostName;
const CHAR * pszClientUserName;
const CHAR * pszServerName;
const CHAR * pszOperation;
const CHAR * pszTarget;
const CHAR * pszParameters;
DWORD dwHttpStatus;
DWORD dwWin32Status;
} HTTP_FILTER_LOG, *PHTTP_FILTER_LOG;
Members
pszClientHostName [IN/OUT]
Client's host name.
pszClientUserName [IN/OUT]
Client's user name.
pszServerName [IN/OUT]
Name of the server the client is connected to.
pszOperation [IN/OUT]
HTTP command.
pszTarget [IN/OUT]
Target of the HTTP command.
pszParameters [IN/OUT]
Parameters passed to the HTTP command.
dwHttpStatus [IN/OUT]
HTTP return status.
dwWin32Status [IN/OUT]
Win32 error code.
Remarks
This structure is pointed to by the pvNotification in the HttpFilterProc when NotificationType is SF_NOTIFY_LOG, when the server is about to log information to the server log file. The strings cannot be changed, but pointers can be replaced. If string pointers are changed, the memory they point to must have been allocated by the AllocMem callback function in the HTTP_FILTER_CONTEXT structure.
Every filter is contained in a separate DLL with two common entry points: GetFilterVersion and HttpFilterProc. When the DLL is loaded, GetFilterVersion is called, which lets the filter know the version of the server and allows the filter to tell the server the version of the filter and the events that the filter is interested in. After this, the server will call the filter's HttpFilterProc entry point with appropriate notifications. Note that filters should only register for notifications that the filter needs to see--some filter notifications are very expensive in terms of CPU resources and I/O throughput and can have a dramatic effect on the speed and scalability of the Microsoft Internet Information Server.
BOOL
WINAPI
GetFilterVersion(
HTTP_FILTER_VERSION * pVer
);
DWORD
WINAPI
HttpFilterProc(
HTTP_FILTER_CONTEXT * pfc,
DWORD NotificationType,
VOID * pvNotification
);