Express Slow Down

Basic rate-limiting middleware for Express that slows down responses rather than blocking them outright. Use to limit repeated requests to public APIs and/or endpoints such as password reset.

Plays nice with Express Rate Limit

Note: this module does not share state with other processes/servers by default. This module was extracted from Express Rate Limit 2.x and can work with it's stores:

Stores

Memory Store (default, built-in) - stores hits in-memory in the Node.js process. Does not share state with other servers or processes.
Redis Store
Memcached Store

Note: when using express-slow-down and express-rate-limit with an external store, you'll need to create two instances of the store and provide different prefixes so that they don't double-count requests.

Install

$ npm install --save express-slow-down

Usage

For an API-only server where the rules should be applied to all requests:

const slowDown = require("express-slow-down");

app.enable("trust proxy"); // only if you're behind a reverse proxy (Heroku, Bluemix, AWS if you use an ELB, custom Nginx setup, etc)

const speedLimiter = slowDown({
  windowMs: 15 * 60 * 1000, // 15 minutes
  delayAfter: 100, // allow 100 requests per 15 minutes, then...
  delayMs: 500 // begin adding 500ms of delay per request above 100:
  // request # 101 is delayed by  500ms
  // request # 102 is delayed by 1000ms
  // request # 103 is delayed by 1500ms
  // etc.
});

//  apply to all requests
app.use(speedLimiter);

For a "regular" web server (e.g. anything that uses express.static()), where the rate-limiter should only apply to certain requests:

const slowDown = require("express-slow-down");

app.enable("trust proxy"); // only if you're behind a reverse proxy (Heroku, Bluemix, AWS if you use an ELB, custom Nginx setup, etc)

const resetPasswordSpeedLimiter = slowDown({
  windowMs: 15 * 60 * 1000, // 15 minutes
  delayAfter: 5, // allow 5 requests to go at full-speed, then...
  delayMs: 100 // 6th request has a 100ms delay, 7th has a 200ms delay, 8th gets 300ms, etc.
});

// only apply to POST requests to /reset-password/
app.post("/reset-password/", resetPasswordSpeedLimiter, function(req, res) {
  // handle /reset-password/ request here...
});

`req.slowDown`

A req.slowDown property is added to all requests with the following fields:

limit: The options.delayAfter value (defaults to 1)
current: The number of requests in the current window
remaining: The number of requests remaining before rate-limiting begins
resetTime: When the window will reset and current will return to 0, and remaining will return to limit (in milliseconds since epoch - compare to Date.now()). Note: this field depends on store support. It will be undefined if the store does not provide the value.
delay: Amount of delay imposed on current request (milliseconds)

Configuration

windowMs: milliseconds - how long to keep records of requests in memory. Defaults to 60000 (1 minute).
delayAfter: max number of connections during windowMs before starting to delay responses. Number or function that returns a number. Defaults to 1. Set to 0 to disable delaying.
delayMs: milliseconds - how long to delay the response, multiplied by (number of recent hits - delayAfter). Defaults to 1000 (1 second). Set to 0 to disable delaying.

maxDelayMs: milliseconds - maximum value for delayMs after many consecutive attempts, that is, after the n-th request, the delay will be always maxDelayMs. Important when your application is running behind a load balancer or reverse proxy that has a request timeout. Defaults to Infinity.

// Example

// Given:
{
    delayAfter: 1,
    delayMs: 1000,
    maxDelayMs: 20000,
}

// Results will be:
// 1st request - no delay
// 2nd request - 1000ms delay
// 3rd request - 2000ms delay
// 4th request - 3000ms delay
// ...
// 20th request - 19000ms delay
// 21st request - 20000ms delay
// 22nd request - 20000ms delay
// 23rd request - 20000ms delay
// 24th request - 20000ms delay <-- will not increase past 20000ms
// ...

skipFailedRequests: when true failed requests (response status >= 400) won't be counted. Defaults to false.
skipSuccessfulRequests: when true successful requests (response status < 400) won't be counted. Defaults to false.
keyGenerator: Function used to generate keys. By default user IP address (req.ip) is used. Defaults:
```
function (req /*, res*/) {
    return req.ip;
}
```
skip: Function used to skip requests. Returning true from the function will skip limiting for that request. Defaults:
```
function (/*req, res*/) {
    return false;
}
```
onLimitReached: Function to listen the first time the limit is reached within windowMs. Defaults:
```
function (req, res, options) {
/* empty */
}
```
store: The storage to use when persisting rate limit attempts. By default, the MemoryStore is used.
- Note: when using express-slow-down and express-rate-limit with an external store, you'll need to create two instances of the store and provide different prefixes so that they don't double-count requests.
headers: Add X-SlowDown-Limit, X-SlowDown-Remaining, and if the store supports it, X-SlowDown-Reset headers to all responses. Modeled after the equivalent headers in express-rate-limit. Default: false