Working with Cookies

Friday Jan 24th 2003 by Adrian Bacaianu

Send cookies to a client's browser from an ISAPI extension. An easy way to retrieve and visualize those cookies is also provided.

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Environment: ASP.NET, MFCC++, ISAPI


This article presents a way to send some cookies to a client's browser from an ISAPI extension. An easy way to retrieve and visualize those cookies is also provided.

What is a cookie? A cookie is a small packet of information used to store persistent state information on the user's computer. Cookies are the means by which, under HTTP protocol, a server or a script can maintain state information on the client's workstation.

The Cookie header is included with any HTTP requests that have a cookie and whose domain and path match the request.

HTTP cookies provide the server with a mechanism to store and retrieve state information on the client application's system. This mechanism allows Web-based applications the ability to store information about selected items, user preferences, registration information, and other information that can be retrieved later.

Cookie-Related Headers

There are two HTTP headers, Set-Cookie and Cookie, that are related to cookies. Set-Cookie header is sent by the server in response to an HTTP request, which is used to create a cookie on the user's system. Cookie header is included by the client application with an HTTP request sent to a server if there is a cookie that has a matching domain and path.

Set-Cookie Header

The Set-Cookie response header uses the following format:

Set-Cookie: <name>=<value>[; <name>=<value>]...
[; expires=<date>][; domain=<domain_name>]
[; path=<some_path>][; secure]

One or more string sequences (separated by semicolons) that follow the pattern <name>=<value> must be included in the Set-Cookie response header. The server can use these string sequences to store data on the client's system.

The expiration date is set by using the format =<date>, where <date> is the expiration date in Greenwich Mean Time (GMT). If the expiration date is not set, the cookie expires after the Internet session ends. Otherwise, the cookie is persisted in the cache until the expiration date. The date must follow the format DAY, DD-MMM-YYYY HH:MM:SS GMT, where DAY is the day of the week (Sun, Mon, Tue, Wed, Thu, Fri, Sat), DD is the day in the month (such as 01 for the first day of the month), MMM is the three-letter abbreviation for the month (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec), YYYY is the year, HH is the hour value in military time (22 would be 10:00 P.M., for example), MM is the minute value, and SS is the second value.

Specifying the domain name, using the pattern domain=<domain_name>, is optional for persistent cookies and is used to indicate the end of the domain for which the cookie is valid. Session cookies that specify a domain are rejected. If the specified domain name ending matches the request, the cookie tries to match the path to determine whether the cookie should be sent. For example, if the domain name ending is, requests to and would be checked to see whether the specified pattern matches the request. The domain name must have at least two or three periods in it to prevent cookies from being set for widely used domain name endings, such as .com, .edu, and Allowable domain names would be similar to,, and Only hosts within the specified domain can set a cookie for a domain.

Setting the path, using the pattern path=<some_path>, is optional and can be used to specify a subset of the Uniform Resource Locators (URLs) for which the cookie is valid. If a path is specified, the cookie is considered valid for any requests that match that path. For example, if the specified path is /example, requests with the paths /examplecode and /example/code.htm would match. If no path is specified, the path is assumed to be the path of the resource associated with the Set-Cookie header.

The cookie can also be marked as secure, which specifies that the cookie can be sent only to HTTPS servers.

Cookie Header

The Cookie header is included with any HTTP requests that have a cookie whose domain and path match the request. The Cookie header has the following format:

Cookie: <name>=<value> [;<name>=<value>]...

One or more string sequences, using the format <name>=<value>, contain the information that was set in the cookie.

Sample Cookie Header

"Set-Cookie:Test=test_value; expires=Sat, 01-Jan-2000
                             00:00:00 GMT; path=/;"

Storing Cookies

Netscape Navigator stores cookies in a different fashion than Internet Explorer, though it captures the same material from the server. Instead of storing each cookie in its own file, as Internet Explorer does (see for yourself, as your cookies are typically stored in a directory named "Temporary Internet Files" in your Windows system folder),

Netscape Navigator stores every cookie inside a single file called cookies.txt. (If you're using Nav3, cookies.txt is typically in the C:\Program Files\Netscape\Nav3 directory; Nav4 stores its cookies according to the registered computer user file—in my case, C:\Program Files\Netscape\Users\user.)


The Default method loads a simple form on the browser; the user is able to put what cookie name/value pair he wants on it. All jobs are done in the GetCookie method. To be able to write the cookies' values on the client machine, you must inform the IIS server not to write its own headers:

   //Turn off sending header by MFC.
   pCtxt->m_bSendHeaders = FALSE;

The headers that will contain the cookies will be written on the client computer by our program, using the ServerSupportFunction HTTP function:

   // Send our own headers.
   if (!pCtxt->ServerSupportFunction
      (LPDWORD ) szHeaders))
      ISAPITRACE1 ("ServerSupportFunction failed: %d\n",
                   GetLastError());    // Report an error.

The parameters' values from the HTML form will be received into the GetCookie method using the POST HTTP data transfer, and written after that on the client.

  //add the user input cookie
  bstrToken = L"DATA";         //attention, the entry name in
                               //vector collection is "DATA"
  index     = vecServerCtx.Find(bstrToken);
  if (index > -1)
  { map = vecServerCtx[index].GetValueAsMap();
                               //get a map list from the POST
                               //data values
      bstrFormCookieName  = map[L"FormCookieName"];
                               //get the current parameter name
      bstrFormCookieValue = map[L"FormCookieValue"];
                               //get the current parameter value

      wsprintf(szHeader, "Set-Cookie: %s=%s\r\n",
      strcat(szHeaders, szHeader);
                               //add that cookie to the header list

For example considerations, another two cookies are written in the GetCookie method, one simple and one with time expiration.

All the time on the browser will appear the actual client cookie values from the server context vector collection,

Click here for a larger image.

the cookies separate, individual values from a cookies map collection,

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and the POST data received from the form. Again, the map collection is used to access the individual parameters' values.

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Finally, the HTML forms are written again to the browser, giving the user the possibility to input/modify the cookie.

//output again the form to input another cookie
   dwLen = strForm.GetLength();
   pCtxt->WriteClient( (void*)(LPCTSTR)strForm, &dwLen, 0);

This article also provides a function to make a server redirect.


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