PRB: CGI Applications Hang Under IIS 4.0 With POST

• Microsoft Internet Information Server version 4.0

SYMPTOMS

Common gateway interface (CGI) applications that worked correctly under Microsoft Internet Information Server (IIS) 3.0 with the HTTP POST method appear to hang under IIS 4.0. Using CGI with the HTTP GET method still works correctly.

CAUSE

A typical scenario is that you upgrade your Microsoft Internet Information Server 3.0 to 4.0 and suddenly all or some of your CGI applications appear to hang when called with a POST method.

The most likely reason for this is that the CGI in question was not written correctly. Under certain conditions, an incorrectly written CGI can work properly with IIS 3.0 and earlier, but will appear to break with IIS 4.0. The reason for this is that IIS 3.0 and earlier was more tolerant of incorrectly written CGI applications. Browsers like Internet Explorer and Netscape Navigator will append a Carriage Return + Line Feed (CRLF) pair to the end of any posted data. IIS 3.0 and earlier will pass this extra CRLF pair to the CGI application, so that CGIs written depend on this additional CRLF (which is against CGI and HTTP specifications) to work with IIS 3.0 and earlier.

However, any CGI application running under IIS 4.0 reading from standard input and waiting for a CRLF character to indicate when the Posted data is complete will hang until a timeout occurs (or the user aborts the session). This is because IIS 4.0 will no longer pass on the additional CRLF pair to the CGI application.

The following CGI code sample is typical of this problem:

   int main(int argc, char* argv[])
   {
      char   szBuf[1024];
      gets(szBuf);
      // Do something with szBuf...
      return 0;
   } 

RESOLUTION

The correct solution is to rewrite the CGI application so it will adhere to the CGI and HTTP specifications. Typically, there are two issues to deal with:

1. Reading from standard input only the amount of data specified by CONTENT_LENGTH variable.



2. Ensuring the standard input is read in binary mode and not text mode.

   int main(int argc, char* argv[])
   {
      char*   pCL = getenv("CONTENT_LENGTH");
      if(pCL != NULL)
      {
         int   nCL = atoi(pCL);
         if(nCL > 0)
         {
            // WARNING:  Error checking removed for clarity...
            printf("Content-Type: text/html\r\n\r\n");
            _setmode( _fileno( stdin ), _O_BINARY );   
            char* szBuf = new char[nCL + 1];
            fread(szBuf + nRead, sizeof(char), nCL-nRead, stdin);
            szBuf[nCL] = '\0';
            // Do something with szBuf... 
            delete [] szBuf;
         }
      }
      return 0;
   } 

STATUS

This behavior is by design.

MORE INFORMATION

From the HTTP 1.1 Specification (RFC2068):

NOTE: Certain buggy HTTP/1.0 client implementations generate an extra CRLF's after a POST request. To restate what is explicitly forbidden by the BNF, an HTTP/1.1 client must not preface or follow a request with an extra CRLF.

Which implies that server applications cannot rely on client's sending an extra CRLF.

You should also be aware that similar problems could result in scripts as well as executables. While some script interpreters may internally buffer up the posted data before executing the actual script resulting in correct script behavior, the scripts should adhere to the correct behavior.

REFERENCES

The third-party contact information included in this article is provided to help you find the technical support you need. This contact information is subject to change without notice. Microsoft in no way guarantees the accuracy of this third-party contact information.


For more information on the Common Gateway Interface 1.1 Specification see the following website at:

http://hoohoo.ncsa.uiuc.edu/cgi/interface.html

ftp://ftp.isi.edu/in-notes/rfc2068.txt

Additional query words: kbDSupport

Keywords          : kbDSupport kbIIS 
Version           : winnt:4.0
Platform          : winnt 
Issue type        : kbprb 

Last Reviewed: August 9, 1999