NGINX Rate Limiting for Unsecured Apps

Some applications don’t properly support IP blackholing in the case of failed login attempts.  There’s a few ways to handle that, but one nice way is to make use of nginx in the front of the application to apply rate limiting.

I’m considering using nginx as a reverse proxy for your application here as out of scope for this article.  It’s a good idea to get used to using it to front your applications and control access to them.

Rate Limiting in NGINX

We’ll be making use of the ngx_http_limit_req module.  Simply put, you create a zone using limit_req_zone, then define allowed locations that will use the zone using limit_req.

The mental abstraction you can use for the zone is a bucket.  The zone definition describes a data table which will hold IP addresses (in this case), and how many requests they’ve made.  The requests (which are water in the bucket in this analogy) flow out a ‘hole’ in the bucket at a fixed rate.  Therefore, if requests come in faster than the rate, they will ‘fill’ the bucket.

The ‘size’ of the bucket is determined by the parameters you’ve set on limit_req for the allowed burst size.  So a large burst size enables a lot of requests to be made in a time period that exceeds the recharge rate, but it’ll fill the bucket up eventually.  They then slowly recharge at the described rate.

IMPORTANT – If you do not use the nodelay option in limit_req, what happens is that nginx delays incoming requests to force them to match the rate – irrespective of bursts.  In this article, we’ll use nodelay, because we want to flat out return errors when the burst size is exceeded.

Configuring Rate Limiting

In the http context of your nginx.conf, insert a zone definition like this;

limit_req_zone $binary_remote_addr zone=myzone:10m rate=1r/m;

This defines a new zone named myzone which will be populated with the binary forms of remote addresses of clients of size 10Mb.  This will hold a large number of addresses, so it should be fine.  It will recharge limits at a rate of one per minute (which is very slow, but this is intentional, as you’ll see).

Then, let’s assume your app has a login page that you know is at /app/login, and the rest of the app is under /.  You could write some locations like this;

location = /app/login {
    limit_req zone=myzone burst=10 nodelay;

    # whatever you do to get nginx to forward to your app here

location / {
    # whatever you do to get nginx to forward to your app here

That way, calls to /app/login will be rate limited, but the rest of your app will not.

In the above example, calls to /app/login from a single IP will be rate limited such that they can make a burst of 10 calls without limits, but then are limited to an average rate of one per minute.

For something that’s a login page, this should be sufficient to allow legitimate logins (and likely with a mistyped password or two), but it’ll put a big tarpit on dictionary attacks and the like.

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