libvmod-queryfilter Overview

This is a simple VMOD for Varnish Cache which provides query string filtering. It accepts a whitelist of parameter names and returns the request URL with only the provided parameters and their values.

Sort and Filter Functionality

The order of query string parameters is typically not considered, so an effective caching strategy involves sorting the querystring parameters so that identical name/value pairs that are in a different order for a given URL do not create multiple cache records.

Libvmod-queryfilter does not require separate sort and filter calls because it works by traversing the provided parameters and filtering as it goes. The output contains parameter names in the same order that they were provided to the VMOD, so by passing in parameter names in a consistent manner (e.g., alphabetical order), a resulting filtered request URL will be unique for its combination of parameter names and values.


Rewrite the request URL so that the query string only contains parameter name/value pairs for the "id" and "q" parameters:

import queryfilter;
set req.url = queryfilter.filterparams(req.url, "id,q");


Libvmod-queryfilter attempts to be 100% C99 conformant. You should, therefore, be able to build it without issue on most major compilers. However, it has only been thorougly tested with the following compilers and targets:

  • gcc-4.4.7 (CentOS release 6.5 x86_64)
  • gcc-4.9.2p1 (x86_64-apple-darwin13.4.0)
  • clang-600.0.57 (x86_64-apple-darwin13.4.0)


Before anything else is done, your source directory has to be initialized:



This vmod must be compiled against a pre-built Varnish Cache 3.x source tree. The path to the Varnish Cache source tree is specified via the VARNISHSRC variable at configure time, e.g.:

./configure VARNISHSRC=path/to/varnish-3.0.6 && make check

Additional configuration variables and options can be found by invoking configure --help.

Query Arrays

By default, libvmod-queryfilter assumes query parameters are individual name/value pairs (e.g. a=1&b=2...). Optional support for arrays in query parameters (e.g. a[]=1&a[]=2... - see this Stackoverflow Question or Issue #2) can be enabled by passing the --enable-query-arrays at configure time. With this option enabled, array parameters will be preserved - in order - in the output URI.

NOTE: At present, query arrays are a compile-time option. There are plans to make this behavior run-time configurable in the next major-level release.

Check Targets

Libvmod-queryfilter provides a set of simple unit tests driven by varnishtest. They can be executed as part of the build process by invoking make check.


There is only support for the Varnish 3 API at the moment, though support for the 4 series API is planned.


Copyright © 2014,2015 The New York Times Company. Licensed under the Apache 2.0 License. See LICENSE for more information.

Used in production
Varnish version supported: 
Commercial support: