How can I modify a POST request using a custom IHttpModule and an HttpRequest filter?

Question

Overview

I want to be able to modify request parameters and content to 3rd party web services (ArcGIS Server). This will be used to create a security layer that exists between any client application and the server application.

I think that I have found a solution but I am current having some difficulties in the implementation.

Potential Solution: Modify Request with a Custom Request Filter

For the solution I implemented a custom request filter loosely based on the sample shown on MSDN. I have 'enhanced' the code so that I can search and replace the necessary content using regular expressions. This involves:

1. Converting the content (stored in a byte array) into a string.
2. Searching the string and performing any necessary modifications.
3. Converting the modified string into a byte array and writing it to the buffer.

An example is shown below:

public override int Read(byte[] buffer, int offset, int count)
{
    int bytesRead = _stream.Read(buffer, offset, count);

    string orgContent = Encoding.UTF8.GetString(buffer, offset, bytesRead);
    string orgContentDecoded = HttpUtility.UrlDecode(orgContent);

    string layersPattern = @"&layers=(show|hide|include|exclude):([0-9]+,?)+";
    Regex layersRegex = new Regex(layersPattern, RegexOptions.IgnoreCase);

    string[] permittedLayers = new string[] { "0" , "1" };
    string replacementLayers = "&layers=show:" + String.Join(",", permittedLayers);
    string newContentDecoded = layersRegex.Replace(orgContentDecoded, replacementLayers);

    string newContent =  newContentDecoded.Replace(",", "%2C").Replace(":", "%3A");

    byte[] newBuffer = Encoding.UTF8.GetBytes(newContent);
    int newByteCountLength = Encoding.UTF8.GetByteCount(newContent);

    Encoding.UTF8.GetBytes(newContent, 0, Encoding.UTF8.GetByteCount(newContent), buffer, 0);

    return bytesRead;
}

This seems to work well so long as the modified content length is not different than the original content length. For instance, if I replace a 1 with a 2 everything works. However, if I replace a 1 with a 10 (thereby increasing the message size by 1) then I receive an error from ArcGIS Server that the format is unsupported.

This has brought two concerns to my attention:

1. The current implementation does not handle chunked requests. That is, if the request sie is large enough Read may be called multiple times for a single request. How should chunking be handled in this scenario?
2. What is the root cause of the error message? Is the problem related to the content length being different than the stream length? How do I correctly modify the content so that changing its length is not an issue?

Any thoughts?

Answer 1

The answer to part two of this question is to return the modified content size, not the size of the original stream. Behold!

// return bytesRead;
return newByteCountLength;