Rate limiting is about restricting the number of requests to your application. It's usually applied within a specific time window or based on other criteria.

It's helpful for a few reasons:

• Improves security
• Guards against DDoS attacks
• Prevents overloading of application servers
• Reduces costs by preventing unnecessary resource consumption

.NET 7 shipped with a built-in rate limiter, but you need to know how to implement it correctly. Or you could grind your system to a halt - and we don't want that.

In this week's newsletter, I'll teach you:

• How to rate limit users by IP address
• How to rate limit users by their identity
• How to apply rate limiting on the reverse proxy

So let's dive in.

Built-In Rate Limiting In .NET 7

Starting with .NET 7, we have access to built-in rate limiting middleware in the Microsoft.AspNetCore.RateLimiting namespace. The API is straightforward, and you can create a rate limit policy with a few lines of code.

We can use one of the four rate limiting algorithms :

• Fixed window
• Sliding window
• Token bucket
• Concurrency

Here's how to define a rate limit policy by calling the AddTokenBucketLimiter method:

builder.Services.AddRateLimiter(rateLimiterOptions =>
{
    options.RejectionStatusCode = StatusCodes.Status429TooManyRequests;

    rateLimiterOptions.AddTokenBucketLimiter("token", options =>
    {
        options.TokenLimit = 1000;
        options.ReplenishmentPeriod = TimeSpan.FromHours(1);
        options.TokensPerPeriod = 700;
        options.AutoReplenishment = true;
    });
});

Now you can reference the token rate limit policy on your endpoint or controller.

You also have to add the RateLimitingMiddleware to the request pipeline:

app.UseRateLimiter();

You can learn more about rate limiting in .NET 7 here, so I won't go deeper into the fundamentals.

Rate Limiting Users By IP Address

The approach I just showed you has a problem - the rate limit policy is global and applies to all users.

Most of the time, you don't want to do this. Rate limiting should be granular and apply to individual users.

Luckily, you can achieve this by creating a RateLimitPartition.

The RateLimitPartition has two components:

• Partition key
• Rate limiter policy

Here's how to define a rate limiter with a fixed window policy, and the partition key is the user's IP address.

builder.Services.AddRateLimiter(options =>
{
    options.AddPolicy("fixed-by-ip", httpContext =>
        RateLimitPartition.GetFixedWindowLimiter(
            partitionKey: httpContext.Connection.RemoteIpAddress?.ToString(),
            factory: _ => new FixedWindowRateLimiterOptions
            {
                PermitLimit = 10,
                Window = TimeSpan.FromMinutes(1)
            }));
});

Rate limiting by IP address can be a good layer of security for unauthenticated users. You don't know who is accessing your system and can't apply more granular rate limiting. This can help protect your system from malicious users trying to perform a DDoS attack.

You can also create chained limiters using the CreateChained API. It allows you to pass in multiple PartitionedRateLimiter, which are combined into one PartitionedRateLimiter. The chained limiter runs all the input limiters in sequence (one by one).

If your application is running behind a reverse proxy, you need to make sure not to rate limit the proxy IP address. Reverse proxies usually forward the original IP address with the X-Forwarded-For header. So you can use it as the partition key:

builder.Services.AddRateLimiter(options =>
{
    options.AddPolicy("fixed-by-ip", httpContext =>
        RateLimitPartition.GetFixedWindowLimiter(
            httpContext.Request.Headers["X-Forwarded-For"].ToString(),
            factory: _ => new FixedWindowRateLimiterOptions
            {
                PermitLimit = 10,
                Window = TimeSpan.FromMinutes(1)
            }));
});

Rate Limiting Users By Identity

If you require users to authenticate with your API, you can determine who the current is. Then you can use the user's identity as the partition key for a RateLimitPartition.

Here's how you would create such a rate limit policy:

builder.Services.AddRateLimiter(options =>
{
    options.AddPolicy("fixed-by-user", httpContext =>
        RateLimitPartition.GetFixedWindowLimiter(
            partitionKey: httpContext.User.Identity?.Name?.ToString(),
            factory: _ => new FixedWindowRateLimiterOptions
            {
                PermitLimit = 10,
                Window = TimeSpan.FromMinutes(1)
            }));
});

I'm using the User.Identity value on the HttpContext to get the current user's Name claim. This usually corresponds to the sub claim inside a JWT - which is the user identifier.

Applying Rate Limting On The Reverse Proxy

In a robust implementation, you want to rate limit on the reverse proxy level before the request hits your API. And if you have a distributed system, this is a requirement. Otherwise, your system wouldn't function correctly.

There are many reverse proxy implementations to choose from.

YARP is a reverse proxy with excellent .NET integration. Not surprising since it was written in C#. You can learn more about building an API Gateway with YARP here.

To implement rate limiting on the reverse proxy with YARP you need to:

• Define a rate limit policy (covered in previous examples)
• Configure the RateLimiterPolicy for the route in YARP settings

"products-route": {
  "ClusterId": "products-cluster",
  "RateLimiterPolicy": "sixty-per-minute-fixed",
  "Match": {
    "Path": "/products/{**catch-all}"
  },
  "Transforms": [
    { "PathPattern": "{**catch-all}" }
  ]
}

The built-in rate limiter middleware uses an in-memory store to track the number of requests. If you want to run your reverse proxy in a high-availability setup, you will need to use a distributed cache. A nice option to look into is using a Redis backplane for rate limiting.

Closing Thoughts

With the PartitionedRateLimiter you can easily create granular rate limit policies.

The two common approaches are:

• Rate limiting by IP address
• Rate limiting by the user identifier

I was really excited to see the .NET team ship rate limiting. But, the current implementation has its shortcomings. The main issue is that it only works in memory. For a distributed solution, you need to implement something yourself or use an external library.

You can use the YARP reverse proxy to build robust and scalable distributed systems. And it only takes a few lines of code to add rate limiting on the reverse proxy level. I'm using it extensively in my systems.

Thanks for reading.

And stay awesome!

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