ProxyKit

⚠️ ProxyKit is obsolescent. See announcement. Migrate to YARP.

A toolkit to create code-first HTTP Reverse Proxies hosted in ASP.NET Core as middleware. Thisallows focused code-first proxies that can be embedded in existing ASP.NET Coreapplications or deployed as a standalone server. Deployable anywhere ASP.NETCore is deployable such as Windows, Linux, Containers and Serverless (withcaveats).

Having built proxies many times before, I felt it is time to make a package. Forkedfrom ASP.NET labs, it has been heavily modified with a differentAPI, to facilitate a wider variety of proxying scenarios (i.e. routing based ona JWT claim) and interception of the proxy requests / responses forcustomization of headers and (optionally) request / response bodies. It alsouses HttpClientFactory internally that will mitigate against DNS cachingissues making it suitable for microservice / container environments.

• 1. Quick Start
  • 1.1. Install
  • 1.2. Forward HTTP Requests
  • 1.3. Forward WebSocket Requests
• 2. Core Features
  • 2.1. Customising the upstream HTTP request
  • 2.2. Customising the upstream response
  • 2.3. X-Forwarded Headers
    • 2.3.1. Client Sent X-Forwarded-Headers
    • 2.3.2. Adding X-Forwarded-* Headers
    • 2.3.3. Copying X-Forwarded headers
  • 2.4. Configuring ProxyKit's HttpClient
  • 2.5. Error handling
  • 2.6. Testing
  • 2.7. Load Balancing
    • 2.7.1. Weighted Round Robin
  • 2.8. Typed Handlers
• 3. Recipes
• 4. Making upstream servers reverse proxy friendly
• 5. Performance considerations
• 6. Note about serverless
• 7. Comparison with Ocelot
• 8. How to build
• 9. Contributing / Feedback / Questions
• 10. Articles, blogs and other external links

1. Quick Start

1.1. Install

ProxyKit is a NetStandard2.0 package. Install into your ASP.NET Core project:

dotnet add package ProxyKit

1.2. Forward HTTP Requests

In your Startup, add the proxy service:

public void ConfigureServices(IServiceCollection services)
{
    ...
    services.AddProxy();
    ...
}

Forward HTTP requests to upstream-server:5001:

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context => context
        .ForwardTo("http://upstream-server:5001/")
        .AddXForwardedHeaders()
        .Send());
}

What is happening here?

1. context.ForwardTo(upstreamHost) is an extension method onHttpContext that creates and initializes an HttpRequestMessage withthe original request headers copied over, yielding a ForwardContext.
2. AddXForwardedHeaders adds X-Forwarded-For, X-Forwarded-Host,X-Forwarded-Proto and X-Forwarded-PathBase headers to the upstreamrequest.
3. Send Sends the forward request to the upstream server and returns anHttpResponseMessage.
4. The proxy middleware then takes the response and applies it toHttpContext.Response.

Note: RunProxy is terminal - anything added to the pipeline afterRunProxy will never be executed.

1.3. Forward WebSocket Requests

Forward WebSocket requests to upstream-server:5002:

public void Configure(IApplicationBuilder app)
{
    app.UseWebSockets();
    app.UseWebSocketProxy(
        context => new Uri("ws://upstream-host:80/"),
        options => options.AddXForwardedHeaders());
}

What is happening here?

1. app.UseWebSockets() must first be added otherwise websocket requests willnever be handled by ProxyKit.
2. The first parameter must return the URI of the upstream host with a schemeof ws://.
3. The second parameter options allows you to do some customisation of theinitial upstream requests such as adding some headers.

2. Core Features

2.1. Customising the upstream HTTP request

One can modify the upstream request headers prior to sending them to suitcustomisation needs. ProxyKit doesn't add, remove, nor modify any headers bydefault; one must opt in any behaviours explicitly.

In this example we will add a X-Correlation-ID header if the incoming request does not bear one:

public const string XCorrelationId = "X-Correlation-ID";

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context =>
    {
        var forwardContext = context.ForwardTo("http://upstream-server:5001/");
        if (!forwardContext.UpstreamRequest.Headers.Contains(XCorrelationId))
        {
            forwardContext.UpstreamRequest.Headers.Add(XCorrelationId, Guid.NewGuid().ToString());
        }
        return forwardContext.Send();
    });
}

This can be encapsulated as an extension method:

public static class CorrelationIdExtensions
{
    public const string XCorrelationId = "X-Correlation-ID";
    
    public static ForwardContext ApplyCorrelationId(this ForwardContext forwardContext)
    {
        if (!forwardContext.UpstreamRequest.Headers.Contains(XCorrelationId))
        {
            forwardContext.UpstreamRequest.Headers.Add(XCorrelationId, Guid.NewGuid().ToString());
        }
        return forwardContext;
    }
}

... making the proxy code a little nicer to read:

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context => context
        .ForwardTo("http://upstream-server:5001/")
        .ApplyCorrelationId()
        .Send());
}

2.2. Customising the upstream response

The response from an upstream server can be modified before it is sent to theclient. In this example we are removing a header:

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(async context =>
    {
        var response = await context
            .ForwardTo("http://localhost:5001/")
            .Send();

        response.Headers.Remove("MachineID");

        return response;
    });
}

2.3. X-Forwarded Headers

2.3.1. Client Sent X-Forwarded-Headers

⚠️ To mitigate against spoofing attacks and misconfiguration ProxyKitdoes not copy X-Forward-* headers from the incoming request to the upstreamrequest by default. Copying them requires opting in; see 2.3.3 CopyingX-Forwarded headers below.

2.3.2. Adding X-Forwarded-* Headers

Many applications will need to know what their "outside" host / URL is in orderto generate correct values. This is achieved using X-Forwarded-* andForwarded headers. ProxyKit supports applying X-Forward-* headers out of thebox (applying Forwarded headers support is on backlog). At the time of writing,Forwarded is not supportedin ASP.NET Core.

To add X-Forwarded-* headers to the request to the upstream server:

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context => context
        .ForwardTo("http://upstream-server:5001/")
        .AddXForwardedHeaders()
        .Send());
}

This will add X-Forwarded-For, X-Forwarded-Host and X-Forwarded-Protoheaders to the upstream request using values from HttpContext. If the proxymiddleware is hosted on a path and a PathBase exists on the request, then anX-Forwarded-PathBase is also added.

2.3.3. Copying X-Forwarded headers

Chaining proxies is a common pattern in more complex setups. In this case, ifthe proxy is an "internal" proxy, you will want to copy the "X-Forwarded-*"headers from previous proxy. To do so, use CopyXForwardedHeaders():

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context => context
        .ForwardTo("http://upstream-server:5001/")
        .CopyXForwardedHeaders()
        .Send());
}

You may optionally also add the "internal" proxy details to the X-Forwarded-*header values by combining CopyXForwardedHeaders() andAddXForwardedHeaders() (note the order is important):

public void Configure(IApplicationBuilder app)
{
    app.RunProxy(context => context
        .ForwardTo("http://upstream-server:5001/")
        .CopyXForwardedHeaders()
        .AddXForwardedHeaders()
        .Send());
}

2.4. Configuring ProxyKit's HttpClient

When adding the Proxy to your application's service collection, there is anopportunity to configure the internal HttpClient. AsHttpClientFactoryis used, its builder is exposed for you to configure:

services.AddProxy(httpClientBuilder => /* configure http client builder */);

Below are two examples of what you might want to do:

1. Configure the HTTP Client's timeout to 5 seconds:

   services.AddProxy(httpClientBuilder =>
       httpClientBuilder.ConfigureHttpClient =
           client => client.Timeout = TimeSpan.FromSeconds(5));

2. Configure the primary HttpMessageHandler. This is typically used in testingto inject a test handler (see Testing below).

   services.AddProxy(httpClientBuilder =>
       httpClientBuilder.ConfigurePrimaryHttpMessageHandler = 
           () => _testMessageHandler);

2.5. Error handling

When HttpClient throws, the following logic applies:

1. When upstream server is not reachable, then 503 ServiceUnavailable is returned.
2. When upstream server is slow and client timeouts, then 504 GatewayTimeout isreturned.

Not all exception scenarios and variations are caught, which may result in aInternalServerError being returned to your clients. Please create an issue ifa scenario is missing.

2.6. Testing

As ProxyKit is a standard ASP.NET Core middleware, it can be tested using thestandard in-memory TestServer mechanism.

Often you will want to test ProxyKit with your application and perhaps test thebehaviour of your application when load balanced with two or more instances asindicated below.

                               +----------+
                               |"Outside" |
                               |HttpClient|
                               +-----+----+
                                     |
                                     |
                                     |
                         +-----------+---------+
    +-------------------->RoutingMessageHandler|
    |                    +-----------+---------+
    |                                |
    |                                |
    |           +--------------------+-------------------------+
    |           |                    |                         |
+---+-----------v----+      +--------v---------+     +---------v--------+
|Proxy TestServer    |      |Host1 TestServer  |     |Host2 TestServer  |
|with Routing Handler|      |HttpMessageHandler|     |HttpMessageHandler|
+--------------------+      +------------------+     +------------------+

RoutingMessageHandler is an HttpMessageHandler that will route requeststo specific hosts based on the origin it is configured with. For ProxyKitto forward requests (in memory) to the upstream hosts, it needs to be configuredto use the RoutingMessageHandler as its primary HttpMessageHandler.

Full example can been viewed in Recipe 6.

2.7. Load Balancing

Load balancing is a mechanism to decide which upstream server to forward therequest to. Out of the box, ProxyKit currently supports one type ofload balancing - Weighted Round Robin. Other types are planned.

2.7.1. Weighted Round Robin

Round Robin simply distributes requests as they arrive to the next host in adistribution list. With optional weighting, more requests are sent to the host withthe greater weight.

public void Configure(IApplicationBuilder app)
{
    var roundRobin = new RoundRobin
    {
        new UpstreamHost("http://localhost:5001/", weight: 1),
        new UpstreamHost("http://localhost:5002/", weight: 2)
    };

    app.RunProxy(
        async context =>
        {
            var host = roundRobin.Next();

            return await context
                .ForwardTo(host)
                .Send();
        });
}

2.8. Typed Handlers

New in version 2.1.0

Instead of specifying a delegate, it is possible to use a typed handler. Thereason you may want to do this is when you want to better leverage dependencyinjection.

Typed handlers must implement IProxyHandler that has a single method with samesignature as HandleProxyRequest. In this example our typed handler has adependency on an imaginary service to lookup hosts:

public class MyTypedHandler : IProxyHandler
{
    private IUpstreamHostLookup _upstreamHostLookup;

    public MyTypeHandler(IUpstreamHostLookup upstreamHostLookup)
    {
        _upstreamHostLookup = upstreamHostLookup;
    }

    public Task<HttpResponseMessage> HandleProxyRequest(HttpContext context)
    {
        var upstreamHost = _upstreamHostLookup.Find(context);
        return context
            .ForwardTo(upstreamHost)
            .AddXForwardedHeaders()
            .Send();
    }
}

We then need to register our typed handler service:

public void ConfigureServices(IServiceCollection services)
{
    ...
    services.AddSingleton<MyTypedHandler>();
    ...
}

When adding the proxy to the pipeline, use the generic form:

public void ConfigureServices(IServiceCollection services)
{
    ...
    appInner.RunProxy<MyTypedHandler>());
    ...
}

3. Recipes

Recipes have moved to own repo.

4. Making upstream servers reverse proxy friendly

Applications that are deployed behind a reverse proxy typically need to besomewhat aware of that so they can generate correct URLs and paths whenresponding to a browser. That is, they look at X-Forward-* / Forwardedheaders and use their values.

In ASP.NET Core, this means using the ForwardedHeaders middleware in yourapplication. Please refer to the documentationfor correct usage (and note the security advisory!).

Note: the Forwarded Headers middleware does not supportX-Forwarded-PathBase. This means if you proxy http://example.com/foo/ tohttp://upstream-host/ the /foo/ part is lost and absolute URLs cannot begenerated unless you configure your application's PathBase directly.

Related issues and discussions:

• https://github.com/aspnet/AspNetCore/issues/5978
• https://github.com/aspnet/AspNetCore/issues/5898

To support PathBase dynamically in your application with X-Forwarded-PathBase,examine the header early in your pipeline and set the PathBase accordingly:

var options = new ForwardedHeadersOptions
{
   ...
};
app.UseForwardedHeaders(options);
app.Use((context, next) => 
{
    if (context.Request.Headers.TryGetValue("X-Forwarded-PathBase", out var pathBases))
    {
        context.Request.PathBase = pathBases.First();
    }
    return next();
});

Alternatively you can use ProxyKit's UseXForwardedHeaders extension thatperforms the same as the above (including calling UseForwardedHeaders):

var options = new ForwardedHeadersOptions
{
   ...
};
app.UseXForwardedHeaders(options);

5. Performance considerations

According to TechEmpower's Web Framework Benchmarks, ASP.NET Core is up therewith the fastest for plaintext.As ProxyKit simply captures headers and async copies request and response bodystreams, it will be fast enough for most scenarios.

If absolute raw throughput is a concern for you, thenconsider nginx or alternatives. For me being able to create flexible proxiesusing C# is a reasonable tradeoff for the (small) performance cost. Note thatwhat your specific proxy (and its specific configuration) does will impact performanceso you should measure for yourself in your context.

On Windows, ProxyKit is ~3x faster than nginx. However, nginx has clearlydocumented that it has knownperformance issues on Windows. Sinceone wouldn't be running production nginx on Windows, this comparison isacademic.

Memory wise, ProxyKit maintained a steady ~20MB of RAM after processing millionsof requests for simple forwarding. Again, it depends on what your proxy does soyou should analyse and measure yourself.

6. Note about serverless

Whilst it is possible to run full ASP.NET Core web application in AWSLambda and Azure Functions it should be noted that Serverless systems aremessage based and not stream based. Incoming and outgoing HTTP request messageswill be buffered and potentially encoded as Base64 if binary (so larger). Thismeans ProxyKit should only be used for API (json) proxying in production onServerless. (Though proxying other payloads is fine for dev / exploration /quick'n'dirty purposes.)

7. Comparison with Ocelot

Ocelot is an API Gateway that also runs on ASP.NET Core. A key differencebetween API Gateways and general Reverse Proxies is that the former tend to bemessage based whereas a reverse proxy is stream based. That is, an APIGateway will typically buffer every request and response message to be ableto perform transformations. This is fine for an API Gateway but not suitable fora general reverse proxy performance wise nor for responses that arechunked-encoded. See Not Supported Ocelot docs.

Combining ProxyKit with Ocelot would give some nice options for a variety ofscenarios.

8. How to build

Requirements: .NET Core SDK 2.2.100 or later.

On Windows:

.\build.cmd

On Linux:

./build.sh

9. Contributing / Feedback / Questions

Any ideas for features, bugs or questions, please create an issue. Pull requestsgratefully accepted but please create an issue for discussion first.

I can be reached on twitter at @randompunter

10. Articles, blogs and other external links

• An alternative way to secure SPAs (with ASP.NET Core, OpenID Connect, OAuth 2.0 and ProxyKit)

_{logo is distribute by ChangHoon Baek from the Noun Project.}