Connect middleware that helps maintain a high quality of service during heavy traffic. The basic idea is to identify bad actors and not penalize legitimate traffic more than necessary until proper mitigation can be activated.

While this library provides some basic HTTP (Layer 7) flood attack protection, it does NOT remove the need for proper multi-layered DDoS defenses.

It's recommended to monitor for 5xx errors and alarm if threshold exceeded -- otherwise you may face an attack and not know about it.

Using Connect or Express?

const
	connect = require("connect"),
	http = require("http");
const { ConnectQOS } = require("connect-qos");

var app = connect()
	.use(new ConnectQOS().getMiddleware())
	.use(function(req, res) {
		res.end("Hello World!");
	});

http.createServer(app).listen(8392);

Real coders don't use middleware? We've got you covered too...

const http = require("http");
const { ConnectQOS } = require("connect-qos");

var qos = new ConnectQOS();
var qosMiddleware = qos.getMiddleware();
http.createServer(function(req, res) {
	qosMiddleware(req, res, function() {
		res.end("Hello World!");
	});
}).listen(8392);

• beforeThrottle(qosInstance, req, reason) - If a function is provided it will be invoked prior to throttling a request in case a decision is desired. Only if the function explicitly returns false will the throttle request be denied, not resulting in a 503 status.
• destroySocket (default: true) - If denying bad actor also destroy the socket to prevent reuse.

Users may also invoke methods isBadHost(host) or isBadIp(ip) on the qos instance to check the status of a given host or IP address. These methods will return true or false indicating whether the host or ip is currently considered to be a bad actor. This can be done for TLS/SNI to provide additional layer 5 mitigations.

1. Identify potential bad actors
2. Respond with 503 (BUSY) during heavy traffic for bad actors only
3. Very light weight, no complex algorithms

For you tweakers out there, here's some levers to pull:

• minLag (default: 70) - Lag time in milliseconds before throttling kicks in. Default should typically suffice unless you support cpu-intensive operations.
• maxLag (default: 300) - The highest lag threshold which will block the greatest amount of traffic determined by maxBadHostThreshold or maxBadIpThreshold.
• minHostRate (default: 20) - Minimum rate if lag is >= maxLag. Disable rate limiting by setting to 0.
• maxHostRate (default: 40) - Maximum rate if lag is <= minLag.
• maxHostRatio (default: 0) - If a given host receives the specified threshold (0.1 = 10%) a hostViolation will be returned. This prevents a single host from accounting for excessive traffic and is an effective method for combating very large attacks.
• minIpRate (default: 0) - Minimum rate if lag is >= maxLag. Disable rate limiting by setting to 0.
• maxIpRate (default: 0) - Maximum rate if lag is <= minLag.
• maxIpRateHostViolation (default: 0) - Maximum rate if target host is currently exceeding the configured maxHostRatio. This can be used to increase IP throttling if a particular host is being targeted by a large number of IPs. Requests hitting this max rate will receive hostViolation while requests below the rate threshold but hitting the target host will not be flagged.
• errorStatusCode (default: 503) - The HTTP status code to return if the request has been throttled.
• historySize (default: 200) - The LRU history size to use in tracking bad actors. Hosts and IPs both get their own dedicated LRU.
• maxAge (default: 10000) - Time (in ms) before history is purged. 10 seconds is generally more than adequate to capture an accurate hit rate.
• hostWhitelist Set<string>(['localhost']) - If provided will never flag hosts as bad actors.
• ipWhitelist Set<string>([]) - If provided will never flag IPs as bad actors.
• httpBehindProxy (default: false) - x-forwarded-for header only supported if this option is set to true.
• httpsBehindProxy (default: false) - x-forwarded-for header only supported if this option is set to true.

With quality of service being the entire purpose of this library needless to say performance is a critical influence in every decision. You can expect connect-qos to never be a bottleneck of any kind. Local tests on modest laptop easily exceeds 3.5K req/sec on a hello world http server. See for yourself with npm run bench.