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vmod_vsthrottle

Varnish Throttling Module

Author: Dag Haavi Finstad
Date: 2023-10-13
Version: 1.0.2
Manual section:3

SYNOPSIS

import vsthrottle;

DESCRIPTION

A Varnish vmod for rate-limiting traffic on a single Varnish server. Offers a simple interface for throttling traffic on a per-key basis to a specific request rate.

Keys can be specified from any VCL string, e.g. based on client.ip, a specific cookie value, an API token, etc.

The request rate is specified as the number of requests permitted over a period. To keep things simple, this is passed as two separate parameters, 'limit' and 'period'.

This VMOD implements a token-bucket algorithm. State associated with the token bucket for each key is stored in-memory using BSD's red-black tree implementation.

Memory usage is around 100 bytes per key tracked.

FUNCTIONS

is_denied

Prototype
is_denied(STRING key, INT limit, DURATION period)
Arguments

key: A unique identifier to define what is being throttled - more examples below

limit: How many requests in the specified period

period: The time period

Return value
BOOL
Description
Can be used to rate limit the traffic for a specific key to a maximum of 'limit' requests per 'period' time. A token bucket is uniquely identified by the triplet of its key, limit and period, so using the same key multiple places with different rules will create multiple token buckets.
Example
sub vcl_recv {
        if (vsthrottle.is_denied(client.identity, 15, 10s)) {
                # Client has exceeded 15 reqs per 10s
                return (synth(429, "Too Many Requests"));
        }

        # ...
}

INSTALLATION

The source tree is based on autotools to configure the building, and does also have the necessary bits in place to do functional unit tests using the varnishtest tool.

This VMOD is written for Varnish Cache 4.1.

Pre-installation configuration:

./autogen.sh
./configure

If you have installed Varnish to a non-standard directory, call autogen.sh and configure with PKG_CONFIG_PATH pointing to the appropriate path. For example, when varnishd configure was called with --prefix=$PREFIX, use

PKG_CONFIG_PATH=${PREFIX}/lib/pkgconfig export PKG_CONFIG_PATH

Make and install the vmod:

make           # builds the vmod
make install   # installs your vmod in `VMODDIR`
make check     # runs the unit tests in ``src/tests/*.vtc``

The libvmod-vsthrottle vmod will now be available in your VMODDIR and can be copied to other systems as required.

USAGE

In your VCL you can now use this vmod along the following lines:

import vsthrottle;

sub vcl_recv {
        if (vsthrottle.is_denied(client.identity, 15, 10s)) {
                # Client has exceeded 15 reqs per 10s
                return (synth(429, "Too Many Requests"));
        }
}