Tag Archives: c#

Gzip Encoding an HTTP POST Request Body

I was wondering how difficult it was to Gzip-compress the body of an HTTP POST request (or any HTTP request with a body, that is), for large request bodies. While the .Net HttpClient has supported compression of response bodies for a while, it appears that to this day there is no out-of-the-box support for encoding the body of a request. Setting aside for now that the server may not natively support Gzip-compressed request bodies, let’s look at what we need to do to support this on the client side.

Enter HttpMessageHandler

The HttpMessageHandler abstract base class and its derived classes are used by the HttpClient class to asynchronously send HTTP requests and receive the response from the server. But since we don’t actually want to send the message ourselves – just massage the body and headers a little bit before sending – we’ll derive a new class GzipCompressingHandler from DelegatingHandler so we can delegate sending (and receiving) to another handler and just focus on the transformation of the content. So here’s what that looks like.

public sealed class GzipCompressingHandler : DelegatingHandler
{
    public GzipCompressingHandler(HttpMessageHandler innerHandler)
    {
        if (null == innerHandler)
        {
            throw new ArgumentNullException("innerHandler");
        }

        InnerHandler = innerHandler;
    }

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        HttpContent content = request.Content;

        if (request.Method == HttpMethod.Post)
        {
            // Wrap the original HttpContent in our custom GzipContent class.
            // If you want to compress only certain content, make the decision here!
            request.Content = new GzipContent(request.Content);
        }

        return base.SendAsync(request, cancellationToken);
    }
}

As you can see, all we’re doing is just wrapping the original HttpContent in our GzipContent class. So let’s get right to that.

Gzip-compressed HttpContent: GzipContent

We’re almost there, all we need to do is actually compressing the content and modify the request headers to indicate the new content encoding.

internal sealed class GzipContent : HttpContent
{
    private readonly HttpContent content;

    public GzipContent(HttpContent content)
    {
        this.content = content;

        // Keep the original content's headers ...
        foreach (KeyValuePair<string, IEnumerable<string>> header in content.Headers)
        {
            Headers.TryAddWithoutValidation(header.Key, header.Value);
        }

        // ... and let the server know we've Gzip-compressed the body of this request.
        Headers.ContentEncoding.Add("gzip");
    }

    protected override async Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        // Open a GZipStream that writes to the specified output stream.
        using (GZipStream gzip = new GZipStream(stream, CompressionMode.Compress, true))
        {
            // Copy all the input content to the GZip stream.
            await content.CopyToAsync(gzip);
        }
    }

    protected override bool TryComputeLength(out long length)
    {
        length = -1;
        return false;
    }
}

Easy, right? Of course you could add other supported compression algorithms, using more or less the same code (or even adding some abstraction for different compression algorithms), but this is basically all that’s required.

Summary

Using the HttpMessageHandler and its associated classes makes it extremely easy to apply transformations to all (or a well-defined subset) of HTTP requests you’re sending. In this case, we’re applying Gzip-compression to the bodies of all outgoing POST requests, but the logic to decide when to compress can be as customized as you want; you could even apply Gzip-compression only if the requested URI ends with “.gzip” or for certain content types.

Dynamic AES Key Exchange Through RSA Encryption

I wanted to prototype encrypted communication channel between a client and a server. Now of course there are HTTPS and other TLS channels that work quite well, but what I have in mind is supposed to be used to transfer rather sensitive data. So how can I establish a secure channel through an HTTP/HTTPS channel?

  1. Have the server generate an RSA key pair and send the public key to the client.
  2. Have the client generate an AES key, encrypt it with the received public key, and send the encrypted key to the server.
  3. Let the server decrypt the AES key.
  4. Both the client and the server are now in possession of the same AES key and can therefore communicate securely.

Of course, the generated AES key should only be used for the communication with the one client which sent it, so some sort of secure key management on the server (also regarding the RSA key pair) is vital. Also, the AES key could periodically be updated (i.e. a new key generated). At the very least, every message sent back and forth encrypted with AES will have to use a separate IV — but naturally that IV could be part of the transmitted message. So let’s get a very basic REST API-based implementation going.

Generate RSA key-pair on the Server

[...]

public sealed class SessionKey
{
    public Guid Id;
    public byte[] SymmetricKey;
    public RSAParameters PublicKey;
    public RSAParameters PrivateKey;
}

[...]

private Dictionary<Guid, SessionKey> sessionKeys;

[...]

public RSAParameters Generate(Guid sessionId)
{
    // NOTE: Make the key size configurable.
    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048))
    {
        SessionKey s = new SessionKey()
        {
            Id = sessionId,
            PublicKey = rsa.ExportParameters(false /* no private key info */),
            PrivateKey = rsa.ExportParameters(true /* with private key info */),
            SymmetricKey = null, // To be generated by the client.
        };

        sessionKeys.Add(id, s);

        return s.PublicKey;
    }
}

[...]

This key generation can then be used to generate a new RSA key pair whenever a new client connects and requests secure communication. Of course, make sure you send the public key back to the client, and not the private key — else there’s no point in encrypting in the first place.

Generate an AES key on the Client

[...]

// Get the Public Key from the Server
RSAParameters publicKey = GetFromServer(...);

// Holds the current session's key.
byte[] MySessionKey;

// Send encrypted session key to Server.
SendToServer(GenerateAndEncryptSessionKey(publicKey));

[...]

private byte[] GenerateAndEncryptSessionKey(RSAParameters publicKey)
{
    using (Aes aes = Aes.Create())
    {
        aes.KeySize = aes.LegalKeySizes[0].MaxSize;
        // Setting the KeySize generates a new key, but if you're paranoid, you can call aes.GenerateKey() again.

        MySessionKey = aes.Key;
    }

    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
    {
        rsa.ImportParameters(publicKey);

        return rsa.Encrypt(MySessionKey, true /* use OAEP padding */);
    }
}

[...]

As you can see, we just take the public key we got from the server to set up the RSA provider and then encrypt the generated AES key using that public key. Once the client sends the encrypted key to the server, they both share the same secret and can securely communicate with each other.

Decrypt AES Key on the Server

[...]

public void SetSymmetricKey(Guid id, byte[] encryptedKey)
{
    SessionKey session = sessionKeys[id];

    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
    {
        rsa.ImportParameters(session.PrivateKey);

        session.SymmetricKey = rsa.Decrypt(encryptedKey, true /* use use OAEP padding */);
    }
}

[...]

Since we already have the private key for this session, we can just use it to decrypt the AES key we got from the client. Again, making sure that the stored symmetric key is safe, is key to security.

Encrypt / Decrypt

Encrypting and decrypting can now be done the same way on both sides (since we’re using a symmetric-key algorithm). So here’s what that looks like.

[...]

public byte[] EncryptData(byte[] key, string data)
{
    using (Aes aes = Aes.Create())
    {
        byte[] result;

        aes.Key = key;
        aes.GenerateIV();

        using (ICryptoTransform encryptor = aes.CreateEncryptor())
        {
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter writer = new StreamWriter(cs))
                    {
                        writer.Write(data);
                    }
                }

                byte[] encrypted = ms.ToArray();
                result = new byte[aes.BlockSize / 8 + encrypted.Length];

                // Result is built as: IV (plain text) + Encrypted(data)
                Array.Copy(aes.IV, result, aes.BlockSize / 8);
                Array.Copy(encrypted, 0, result, aes.BlockSize / 8, encrypted.Length);

                return result;
            }
        }
    }
}

public string Decrypt(byte[] key, byte[] data)
{
    using (Aes aes = Aes.Create())
    {
        aes.Key = key;

        // Extract the IV from the data first.
        byte[] iv = new byte[aes.BlockSize / 8];
        Array.Copy(data, iv, iv.Length);
        aes.IV = iv;

        // The remainder of the data is the encrypted data we care about.
        byte[] encryptedData = new byte[data.Length - iv.Length];
        Array.Copy(data, iv.Length, encryptedData, 0, encryptedData.Length);

        using (ICryptoTransform decryptor = aes.CreateDecryptor())
        {
            using (MemoryStream ms = new MemoryStream(encryptedData))
            {
                using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader reader = new StreamReader(cs))
                    {
                        return reader.ReadToEnd();
                    }
                }
            }
        }
    }
}

[...]

As you can see, each time we encrypt something we generate a new IV, which we send at the beginning of the data to the other side. The other side then extracts the IV first and uses it to initialize AES.

REST APIs?

Using all this through REST APIs is trivial: All you really need to make sure is that the client sends the session GUID (or whatever you use to identify a session) with every encrypted message, either through the URL, parameters or headers. Of course it is vital to guarantee that a client cannot get access to another client’s session (e.g. to provide a new session key), but through ordinary (secure) authentication that should easily be doable.

Next Steps

As far as encryption is concerned, this should already do the trick. You may want to add signatures to the encrypted messages too, to make sure that the encrypted blocks have not been tampered with. In addition, the AES key exchange could be repeated periodically (maybe even after every exchanged message).

A Basic Framework for Using Performance Counters with .Net Applications

In my last post I promised to tell you more about the performance counters. So here we are: today I’m going to tell you about how to use the performance counter infrastructure offered by Windows in a .net application. On top of this I’m providing you with a basic framework that you can use to author performance counters through XML and then get the code to read/write the counters generated for you at build time. So let’s get started.

Some Basics

Before I go off to the code, here’s a little overview on performance counters in Windows. I’m sure if you’re reading this, you already know about Perfmon.exe, a nice little tool to look at various performance aspects of Windows machines. When plottig performance counters, you’re typically adding the counters you’re interested in. These counters are grouped in categories, for instance Processor or PhsyicalDisk. Each category can contain multiple counters, for instance Disk Read Bytes/sec or Disk Write Bytes/sec. And finally, each counter can have multiple instances, for instance in multi-processor machines, you’ll find one instance of the % Idle Time counter per processor.

What tools like Perfmon.exe do is to grab the values of the counters you selected every second (by default, but that can typically be changed) and record/plot the values. Your job here is to build performance counters that measure certain aspects of your application so these tools can help you in analyzing your performance. And that’s why you’re here, right?

Step 1: The XML Schema

In order to know what we talk about in the XML I announced, let me start with the XML schema. I use this mainly to make sure that what I have in the XML declaring the perf counters makes some sense and is legal input. And of course because Visual Studio tells you that something’s wrong when the schema is not adhered to.

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="urn:Cymbeline.Diagnostics.PerfCounters"
           xmlns="urn:Cymbeline.Diagnostics.PerfCounters"
           xmlns:tns="urn:Cymbeline.Diagnostics.PerfCounters"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           attributeFormDefault="unqualified"
           elementFormDefault="qualified">
  <xs:element name="PerfCounters">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Category" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Counter" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:attribute name="Name" type="xs:string" use="required" />
                  <xs:attribute name="Symbol" type="tns:Symbol" use="required" />
                  <xs:attribute name="Type" use="required">
                    <xs:simpleType>
                      <xs:restriction base="xs:string">
                        <xs:enumeration value="NumberOfItems32" />
                        <xs:enumeration value="RateOfCountsPerSecond32" />
                        <xs:enumeration value="RawFraction" />
                      </xs:restriction>
                    </xs:simpleType>
                  </xs:attribute>
                  <xs:attribute name="Help" type="xs:string" use="required" />
                </xs:complexType>
              </xs:element>
            </xs:sequence>
            <xs:attribute name="Name" type="xs:string" use="required" />
            <xs:attribute name="Symbol" type="tns:Symbol" use="required" />
            <xs:attribute name="Help" type="xs:string" use="required" />
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:simpleType name="Symbol">
    <xs:restriction base="xs:token">
      <xs:pattern value="[a-zA-Z_][\w_]*" />
    </xs:restriction>
  </xs:simpleType>
</xs:schema>

On lines 21 to 23 you find some enumeration values. These map to the values defined in the PerformanceCounterType Enumeration. This is also where you can add support for more performance counter types when you need it. The other elements and attributes are used to describe the category for the performance counters and the performance counters themselves, including the help text that will show up in Perfmon.exe and also including the symbol that you can use in the code to access the counter.

Step 2: Declaring the Performance Counters

Now let’s use the schema we built above. To give you an idea of the context, I’m providing here some of the XML I use to build the perf counters for my SMTP server.

<?xml version="1.0" encoding="utf-8" ?>
<PerfCounters xmlns="urn:Cymbeline.Diagnostics.PerfCounters">
  <Category Name="CymbeMail" Symbol="CymbeMail" Help="CymbeMail SMTP Server v1">
    <Counter Name="# Total Connections"
         Symbol="TotalConnections"
         Type="NumberOfItems32"
         Help="The total number of connections since the server was started." />
    <Counter Name="# Total Connections Refused"
         Symbol="TotalRefusedConnections"
         Type="NumberOfItems32"
         Help="The total number of connections refused since the server was started." />
    <Counter Name="# Total Connections with Errors"
         Symbol="TotalErroneousConnections"
         Type="NumberOfItems32"
         Help="The total number of connections which reported errors since the server was started." />
    <Counter Name="# Active Connections"
         Symbol="ActiveConnections"
         Type="NumberOfItems32"
         Help="The number of currently active connections." />
    <Counter Name="# Authorization Records"
         Symbol="AuthorizationRecords"
         Type="NumberOfItems32"
         Help="The number of authorization records currently kept in the server." />
    <Counter Name="# Connections/sec"
         Symbol="ConnectionsPerSec"
         Type="RateOfCountsPerSecond32"
         Help="The number of connections per second." />
    <Counter Name="# Refused Connections/sec"
         Symbol="RefusedConnectionsPerSec"
         Type="RateOfCountsPerSecond32"
         Help="The number of connections refused per second." />
  </Category>
</PerfCounters>

As you can see, I create seven perf counters, most of them ordinary counters that count the number of occurrences of a certain event (like when a client makes a connection to the server). I actually also use the NumberOfItems32 type to count the number of active connections — I basically increment the counter when a connection was established and decrement it when the connection is terminated. And I have a couple of counters which count the number of occurrences per second (the RateOfCountsPerSecond32 counter type). The good thing about these counters is that you don’t need to provide your own counter base (check out MSDN for more info on this, starting with the above mentioned PerformanceCounterType Enumeration).

Step 3: Building the XSLT to Generate Code

We already have our perf counters declared, so let’s generate some code from that XML. Using XSL stylesheets makes this very easy: We’ll use the XML as input and get c# code as output that we can simply include into our project. I won’t show the full XSLT here (it’s about 240 lines).

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:msxsl="urn:schemas-microsoft-com:xslt"
        xmlns:tns="urn:Cymbeline.Diagnostics.PerfCounters"
        exclude-result-prefixes="msxsl">
  <xsl:output method="text" indent="no"/>

  <xsl:param name="TargetNamespace" />
  <xsl:param name="TargetClassName" select="'PerfCounters'" />
  <xsl:param name="AccessModifier" select="'public'" />

  <xsl:template match="tns:PerfCounters">
    <xsl:if test="$TargetNamespace=''">
      <xsl:message terminate="yes">
        The Parameter 'TargetNamespace' is undefined.
      </xsl:message>
    </xsl:if>
    <!-- ... -->
  </xsl:template>

  <xsl:template match="tns:Counter" mode="Counter">
    <xsl:text>
      private static PerformanceCounter _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>;

      </xsl:text><xsl:value-of select="$AccessModifier"/><xsl:text> static PerformanceCounter </xsl:text><xsl:value-of select="@Symbol"/><xsl:text>
      {
        get
        {
          MakeSureCountersAreInitialized();
          return _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>;
        }
      }
</xsl:text>
  </xsl:template>

  <xsl:template match="tns:Counter[@Type='RawFraction']" mode="Counter">
    <xsl:text>
      private static PerformanceCounter _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>;

      </xsl:text><xsl:value-of select="$AccessModifier"/><xsl:text> static PerformanceCounter </xsl:text><xsl:value-of select="@Symbol"/><xsl:text>
      {
        get
        {
          MakeSureCountersAreInitialized();
          return _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>;
        }
      }

      private static PerformanceCounter _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>Base;

      </xsl:text><xsl:value-of select="$AccessModifier"/><xsl:text> static PerformanceCounter </xsl:text><xsl:value-of select="@Symbol"/><xsl:text>Base
      {
        get
        {
          MakeSureCountersAreInitialized();
          return _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>Base;
        }
      }
</xsl:text>
  </xsl:template>

  <xsl:template match="tns:Counter" mode="InitCounter">
    <xsl:text>
        _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text> = new PerformanceCounter(
          CategoryName,
          "</xsl:text><xsl:value-of select="@Name"/><xsl:text>",
          false);
</xsl:text>
  </xsl:template>

  <xsl:template match="tns:Counter[@Type='RawFraction']" mode="InitCounter">
    <xsl:text>
        _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text> = new PerformanceCounter(
          CategoryName,
          "</xsl:text><xsl:value-of select="@Name"/><xsl:text>",
          false);

        _</xsl:text><xsl:value-of select="@Symbol"/><xsl:text>Base = new PerformanceCounter(
          CategoryName,
          "</xsl:text><xsl:value-of select="@Name"/><xsl:text> Base",
          false);
</xsl:text>
  </xsl:template>

  <xsl:template match="tns:Counter" mode="CreateCounter">
      <xsl:text>
      {
        CounterCreationData counter = new CounterCreationData();

        counter.CounterName = "</xsl:text><xsl:value-of select="@Name" /><xsl:text>";
        counter.CounterHelp = "</xsl:text><xsl:value-of select="@Help" /><xsl:text>";
        counter.CounterType = PerformanceCounterType.</xsl:text><xsl:value-of select="@Type" /><xsl:text>;

        counters.Add(counter);
      }
</xsl:text>
  </xsl:template>

  <xsl:template match="tns:Counter[@Type='RawFraction']" mode="CreateCounter">
      <xsl:text>
      {
        CounterCreationData counter = new CounterCreationData();

        counter.CounterName = "</xsl:text><xsl:value-of select="@Name" /><xsl:text>";
        counter.CounterHelp = "</xsl:text><xsl:value-of select="@Help" /><xsl:text>";
        counter.CounterType = PerformanceCounterType.</xsl:text><xsl:value-of select="@Type" /><xsl:text>;

        CounterCreationData counterBase = new CounterCreationData();

        counterBase.CounterName = "</xsl:text><xsl:value-of select="@Name" /><xsl:text> Base";
        counterBase.CounterHelp = "</xsl:text><xsl:value-of select="@Help" /><xsl:text>";
        counterBase.CounterType = PerformanceCounterType.RawBase;

        counters.AddRange(new CounterCreationData[]{counter, counterBase});
      }
</xsl:text>
  </xsl:template>

  <!-- ... -->
</xsl:stylesheet>

On lines 9 to 11 you’ll find the declaration for the parameters which you can use to change the namespace, the class name and the access modifiers (public vs internal really) for the generated classes.

Starting on lines 22 and 37 respectively, you find different templates for general counters and the RawFraction type counters. I use this to automatically generate the base counter so you won’t have to worry about it. You can do similar things for other counters which need a base counter. Then, starting on lines 63 and 72 respecctively, I initialize the counters, with specialization again for the RawFraction counter types. And starting on lines 86 and 100 respectively you’ll find the code to create and set the CounterCreationData which is used to register perf counters.

You can find the full code in the ZIP file linked to at the end of this post. With this we basically have all the tools we need to actually generate the code as part of the build. We just need to bring the pieces together.

Step 4: Bringing it Together aka Updating the Project

Well yes, I have silently assumed that you have a c# project (.csproj or any other project that is supported by MSBuild.exe) that you’re working on to update. All we need to do in the .csproj is to add the files we authored, then run the XSLT transformation and add the output .cs file to the list of source code files as well. And here’s how you can do that.

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build">
  <!-- ... -->
  <ItemGroup>
    <PerfCounterXml Include="Diag\PerfCounters.xml">
      <OutputCs>Diag\PerfCounters.Generated.cs</OutputCs>
      <Parameters>
        <Parameter Name="TargetNamespace" Value="$(RootNamespace).Diag" />
      </Parameters>
      <SubType>Designer</SubType>
    </PerfCounterXml>
    <PerfCounterXslt Include="Diag\PerfCounters.xslt" />
    <Compile Include="Diag\PerfCounters.Generated.cs">
      <PerfCounters>true</PerfCounters>
      <AutoGen>true</AutoGen>
      <DependentUpon>PerfCounters.xml</DependentUpon>
    </Compile>
  </ItemGroup>
  <ItemGroup>
    <None Include="Diag\PerfCounters.xsd">
      <SubType>Designer</SubType>
    </None>
  </ItemGroup>
  <!-- XSL Transform for Perf Counters -->
  <Target Name="GeneratePerfCounters" BeforeTargets="BeforeBuild"
      Inputs="@(PerfCounterXml);@(PerfCounterXslt)"
      Outputs="@(PerfCounterXml->'%(OutputCs)')">
    <XslTransformation XmlInputPaths="@(PerfCounterXml)"
      XslInputPath="@(PerfCounterXslt)"
      OutputPaths="@(PerfCounterXml->'%(OutputCs)')"
      Parameters="@(PerfCounterXml->'%(Parameters)')" />
  </Target>
</Project>

Add lines 4 to 32 to the end of the project file and make sure the paths you’re using point to the places where you actually stored the files. On line 8 you can see that I’m passing the target namespace parameter (called out in step 3) by concatenating the assemblies default root namespace and “.Diag”. This way you can actually put this stuff into a .targets file that you can include in all of the projects which take advantage of this framework.

Starting on line 25 I defined the actual target which runs the XslTransformation task that comes with .net 4.0. I also declared the inputs as being the XML file and the XSLT file — this way, while experimenting with the XSLT, the code gets regenerated also if the XML file hasn’t changed, but the XSLT has.

Using the Generated Classes

So now you have pretty much all the code you need to use these performance counters of yours. All that’s left is actually writing the code which

  1. Installs the Performance Counters
  2. Updates the Performance Counters
  3. Uninstalls the Performance Counters

#1 and #3 would probably be used by your installer and #2 by your application at runtime. Incrementing and decrementing the counters is as easy as

PerfCounters.CymbeMail.ActiveConnections.Increment();
//...
PerfCounters.CymbeMail.ActiveConnections.Decrement();

Where PerfCounters is the name of the generated class which contains all the generated perf counters, CymbeMail is the name of the class for the category (the Symbol attribute, remember?) and ActiveConnections is of course the symbol name for the counter we’re modifying. Isn’t this simple?

As for setting things up (or removing them), the relevant piece of code gets generated for you, too. So all you need to do is actually call it, potentially from a small application that you run — as I mentioned before — from your installer.

// Setup the Performace Counters
PerfCounters.Setup();

// And for the Uninstaller:
// Remove the Performance Counters
PerfCounters.Remove();

Of course you’ll have to make sure that the version of the generated code that’s run by the uninstaller is the same version as the installer did run — else you may end up with stale categories / counters on the machine. But then again, if you’re installing through an MSI package, you get that for free … when done properly.

Summary

In just a few steps I have shown how you can build a framework to use performance counters in .Net applications. On top of it’s simplicity, it’s also quite easy to modify the framework and adapt it to your own needs or extend it to allow more performance counter types.

And finally, as promised, here’s the ZIP file which contains the relevant pieces. Please forgive me for not adding the .csproj file — that actually contains other relevant data that I didn’t want to share. Instead I added the generated c# code file for reference.

PerfCounters.zip (4.08kb)

Windows Authentication through Forms

Let’s assume you have a web site which is exposed to the internet (i.e. to the public) but the site itself works with windows accounts internally. If you have problems visualizing this scenario, think about a web mail interface for a mail server system which offers mail services for user accounts in active directory. Users inside your corporate network can use integrated Windows authentication to access this site, so they don’t really have a problem. But when they want to access the interface from outside the corporate network, or from a machine/device which doesn’t understand integrated Windows authentication, you’ll realize that it doesn’t just work like that.

So you’ll want to do authentication based on the credentials the user enters in a simple form on a web page. The challenge now is that the rest of the site (i.e. apart from the one form which does the initial authentication) will likely still work with the HttpContext.Current.User property to determine which user is currently authenticated and provide actions and data based on that identity, because you don’t want to re-implement the entire logic. The good news is, you can do that! The bad news is, you don’t just get it for free. But that’s why you came here, and that’s why I’ll try to help you with this.

Let’s first arrange a few things on the web site facing the public — you don’t necessarily need to change this on your internal site. First, the login page will need to be accessible for anyone, i.e. anonymous access must be turned on on the site. Second, from ASP.net’s point of view, all the pages which require the user to be authenticated should be in/under the same directory. If you don’t want to do this, you’ll simply have to add a element for all pages requiring authentication in your web.config or do the opposite and add all pages which don’t require authentication in the same way. With the directories however, the basic structure can be as simple as

WebSiteRoot/
+--Default.aspx        Could redirect to /ActualSite/Default.aspx
+--ActualSite/
   +--Default.aspx     The actual home page with all its logic
+--Auth/
   +--Login.aspx       The login page

So all the logic you had in the site’s root directly would be under ActualSite or whatever name you chose.

Updating Web.config

Now for the above scenario with different directories, the web.config in the site’s root could look as follows. Please note that for the pages under ActualSite we are simply disabling anonymous access, while for everything else, we allow it. Also, in the authentication element we’re setting the mode to None because we’re not going to use any predefined authentication mechanism as is.

<configuration>
    <location path="ActualSite">
        <system.web>
            <authorization>
                <deny users="?"/>
            </authorization>
        </system.web>
    </location>

    <system.web>
        <authentication mode="None">
            <forms loginUrl="~/Auth/Login.aspx" defaultUrl="~/ActualSite/Default.aspx" />
        </authentication>

        <authorization>
            <allow users="*" />
        </authorization>
    </system.web>
</configuration>

But of course that’s not all yet. You’ll see that if now you wanted to look at the actual site, you’ll get a 401 because you’re not authenticated. So let’s take a look at that.

Validating credentials

Next, let’s create the login page which the user will use to enter his credentials. Fortunately, ASP.net offers the Login control which does almost everything we need. So add that one to your login page plus add an event handler for the OnAuthenticate event of that control. Alternatively, you can also derive your own control from the Login control and override the OnAuthenticate method. This event handler is where we’ll do our custom logic to check that the credentials really map to an existing Windows user. Below’s the code which does that.

protected override void OnAuthenticate(AuthenticateEventArgs args)
{
    string[] parts = UserName.Split('\\');
    string password = Password;
    string username;
    string domain = null;

    args.Authenticated = false;

    if (parts.Length == 1)
    {
        username = parts[0];
    }
    else if (parts.Length == 2)
    {
        domain = parts[0];
        username = parts[1];
    }
    else
    {
        return;
    }

    if (WebAuthenticationModule.AuthenticateUser(username, domain, password))
    {
        string userData = String.Format("{0}\n{1}\n{2}",
            username, domain, password);

        FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(
            2                           /* version */,
            username,
            DateTime.Now                /* issueDate */,
            DateTime.Now.AddMinutes(30) /* expiration */,
            true                        /* isPersistent */,
            userData,
            FormsAuthentication.FormsCookiePath);

        HttpCookie ticketCookie = new HttpCookie(
            FormsAuthentication.FormsCookieName,
            FormsAuthentication.Encrypt(ticket));

        Context.Response.Cookies.Add(ticketCookie);

        Context.Response.Redirect(
            FormsAuthentication.GetRedirectUrl(username, false), false);
    }
}

The AuthenticateUser method from WebAuthenticationModule is a wrapper on the LogonUser function from the Win32 API which will return true if the user could be logged on. So if the credentials are valid, we’re going to pass them in to the FormsAuthenticationTicket in the UserData property so later on, we’ll be able to use them again. At least, we don’t want the consumers of the site have to enter credentials for every request their making, right? Also, we’re encrypting the entire ticket because we’re going to send it over the wire. The Encrypt method from the FormsAuthentication class does this. However, you’ll have to make sure that the protection attribute of the forms element in the web.config is set to All which actually is the default (but it can be inherited, so watch out!).

What you see is that we’re heavily using the functionality offered by the FormsAuthentication class and related classes to handle the tickets, encryption, settings, etc. This is not mandatory but it helps a lot. Plus it’s better anyway than coming with your own ticketing and encryption mechanisms; unless you have a degree in maths and/or cryptography, chances are that that’s not so secure as you think it is.

Authenticating users

Then, we need to authenticate the user for all the requests he makes after providing the credentials. Thus, we need to add some custom logic to the AuthenticateRequest event of the HttpApplication. There’s multiple ways to do that:

  • Add a file called global.asax to your site’s root
  • Create and register a new HttpModule by implementing the System.Web.IHttpModule interface and adding an entry in the httpModules section in your root’s web.config

Personally, I like the approach with the custom module more, but adding this stuff to the global.asax can be done a little bit faster. In either case, make sure you can handle the AuthenticateRequest event. My code proposal for the handler is given below. I’m intentionally omitting most error handling code here.

private static void OnAuthenticateRequest(object sender, EventArgs args)
{
    HttpApplication application = sender as HttpApplication;

    HttpContext context = application.Context;
    HttpRequest request = context.Request;
    HttpResponse response = context.Response;

    if (!request.IsAuthenticated &amp;&amp;
        !context.SkipAuthorization)
    {
        if (request.CurrentExecutionFilePath.Equals(FormsAuthentication.LoginUrl,
                                                    StringComparison.OrdinalIgnoreCase) ||
            request.CurrentExecutionFilePath.EndsWith(".axd"))
        {
            context.SkipAuthorization = true;
        }
        else
        {
            HttpCookie cookie = request.Cookies.Get(FormsAuthentication.FormsCookieName);
            FormsAuthenticationTicket ticket = FormsAuthentication.Decrypt(cookie.Value);

            if (!ticket.Expired)
            {
                IntPtr hToken = LogonUserFromTicket(ticket);

                WindowsIdentity identity = new WindowsIdentity(
                    hToken, "Win/Forms", WindowsAccountType.Normal, true);

                context.User = new WindowsPrincipal(identity);

                if (FormsAuthentication.SlidingExpiration)
                {
                    ticket = FormsAuthentication.RenewTicketIfOld(ticket);
                    cookie.Value = FormsAuthentication.Encrypt(ticket);
                    response.Cookies.Set(cookie);
                }

                return;
            }

            FormsAuthentication.RedirectToLoginPage();
        }
    }
}

So let’s go through the important things here. First we check (lines 9/10) if the request is already authenticated or authorization is to be skipped completely. If either of those is true, we’re out of the picture already. Else it’s our job to do the authentication. Lines 12-17 are used to prevent authentication on the login page as well as the *.axd handlers, which are typically used to return resources like scripts for ASP.net components. Lines 20-42 do the actual authentication: we first retrieve the cookie from the previous credential validation and decrypt it to get the ticket. If the ticket has not expired, we log the user on (the call to LogonUserFromTicket is again only a wrapper for the LogonUser function from the Win32 API; it uses the data from the UserData property of the ticket) to get the logon token which we’ll pass in to the WindowsIdentity constructor to get the WindowsIdentity object all the code in ActualSite will use to determine which user is making the request. Then of course we need to update the context with the new identity. If sliding expiration is turned on in the web.config, we renew the ticket. And if the ticket has expired, of course we don’t authenticate the user but instead we redirect him to the login page.

Cleanup

Finally, we still have a tiny problem here. We have used the LogonUser function, but according to its documentation, we should call the CloseHandle function from the Win32 API. For the method AuthenticateUser I have already done that; there we don’t need the token/handle anymore when we have verified the credentials. But what about the authentication we’re doing in AuthenticateRequest? We’re setting the newly created WindowsIdentity (which is here used to represent the user token) to the current HttpContext because we’ll need it there to ultimately handle the request. But once the request handler is done, we don’t need it anymore. Luckily, there’s also an event for this purpose. It’s the last event that there is and it’s called EndRequest. So let’s add the following code in the handler for that event.

private static void OnEndRequest(object sender, EventArgs args)
{
    HttpApplication application = sender as HttpApplication;

    HttpContext context = application.Context;

    if (null != context.User)
    {
        WindowsIdentity identity = context.User.Identity as WindowsIdentity;

        if (null != identity)
        {
            NativeAuth.CloseHandle(identity.Token);
        }
    }
}

Basically all it does, if the request was given a WindowsIdentity, it’ll call the CloseHandle function from the Win32 API on that identity’s token handle. That should do the trick and we shouldn’t leak handles anymore.

Summary

I have shown here how you can make use of the forms authentication mechanisms which come with ASP.net to do Windows authentication behind the scenes. This can be very useful in cases when not all users have access to systems which know how to do Windows authentication.

Please note that I do not claim that this solution is going to work for every challenge you may be facing. The solution shown here is neither claimed to be complete nor suitable for every web application.