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Working with HTTP rate-limiting

Overview

All HTTP servers enforce a collection of HTTP rate-limiting to reduce abuse and achieve a high reliability. This system is built on a fork of mod_evasive, which implements an interval-based bean counter, in other words it begins counting URI requests for a given duration once the first request is received.

There are two classes of URI requests, pages in total and same page requests. Exceeding either threshold will result in an automatic 10 minute ban. Repeating the process three times in 24 hours results in an automatic 7 day ban for HTTP ports, 80 (HTTP) and 443 (HTTPS).

Blocked clients are returned a 403 status code (Forbidden).

Pages in total

Pages in total (PIT) log all URL requests from an IP address in a window discussed below. If an IP address exceeds that number of requests within the window, it will be blocked automatically. If a page is image heavy as verified by webpagetest.org, consider consolidating images into sprites or inlining small assets to bypass accessory HTTP requests.

Same page

Same page requests are more stringent and affect requests to the same URI. This is designed to filter out brute-force attacks. If you poll a page repeatedly, such as autocomplete with a keydown event, add a collection threshold via setTimeout that will only poll after the typist has given a momentary repose to collect thought. For instance, a simple jQuery implementation:

$("#input").on('keydown', function() {
    var timer;
    timer = setTimeout(function() {
        cancelTimeout(timer);
        // cancel other async events
        // do autocomplete AJAX callback
    }, 250 /** 250 milliseconds */);
});

This assumes that the person will type at least 4 characters per second. Words per minute is standardized to 5 characters, this it works out to be 48 WPM. You can evaluate for yourself what 48 WPM is. To avoid triggering the same-page block, without a delay (via setTimeout), one would need to type of 96 WPM with an autocomplete AJAX callback. Feasible, but unlikely.

Blocking criteria

The following thresholds are in place to filter bot from human.

Same page: 4 pages in 1 second
Pages in total: 150 pages in 3 seconds

Three blocks in 24 hours results in a seven day ban. Once a ban is in place, the only way to proceed forward is to open a ticket to remove the ban.

Updated on July 8, 2019

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