Opportunistic device detection for Nginx and/or Varnish using memcached and DeviceAtlas
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This package contains code to do device detection in Nginx and Varnish on a best-effort basis.

It relies on an external service called DeviceAtlas that provides an API to identify device features the User-Agent: header that browsers including in HTTP requests.


How it works

User-Agent: strings in incoming HTTP requests are hashed (to provide fixed length keys) and looked up against a local memcached instance. If the key is found in memcached, a new X-Device: header is set on the request object in Nginx/Varnish with the value returned from memcached.

If the key was not found in memcached, the User-Agent: string is stored in memcached under a numbered key of the form ua-<sequence number>. The name of the key is constructed by first incrementing the value of a dedicated counter key called ua-idx. This allows an external process to later retrieve a list of User-Agent: strings for which lookups against memcached failed.

Backend requirements

The presence of the X-Device: header in incoming HTTP requests allows backend services to return different content depending on the value of the header.

Because requests for a single URL may yield different responses depending on the contents of the X-Device: header, Varnish (or other caching proxies) needs a way to distinguish each variant in its cache. This can be achieved by including a Vary: header in HTTP responses.

To make Varnish differentiate cache entries by both the URL and the request header X-Device:, set the HTTP response header Vary: to:

Vary: X-Device



On Debian/Ubuntu, install memcached:

$ sudo apt-get install memcached

With the default options in /etc/memcached.conf, memcached will listen on


Nginx needs to be built with the (non-official) LUA module from https://github.com/openresty/lua-nginx-module.

On Debian/Ubuntu systems the package nginx-extras provides a version of Nginx built with support for LUA:

$ sudo apt-get install nginx-extras

Nginx configuration

This repository contains a number of files to support doing lookups against an Nginx shared dictionary (optional) and memcached:

  • nginx/lua/DeviceDetection.lua (wrapper which does device detection against Nginx/memcached)
  • nginx/lua/memcached.lua (LUA package to talk to memcached)
  • nginx/lua/nginx-device-lookup-init.lua (sample code needed in the http{} block)
  • nginx/lua/nginx-device-lookup-rewrite.lua (sample code for a server{} block)

Copy the .lua files to a directory available to Nginx, for instance /etc/nginx/lua.

The device lookup code needs to be configured in the http {} block of Nginx. Create /etc/nginx/conf.d/devicedetection.conf and put the following lines in it:

# Assumning this file lives inside /etc/nginx/conf.d and is being parsed
# in the context of the http {} block:
lua_package_path '/etc/nginx/lua/?.lua;;';

# User-Agent<->device type cache shared among Nginx workers
# Not needed if you're OK with queries against memcached for each
# request.  In that case, do not set the 'dict' key below.
lua_shared_dict ua_type 10m;

# Initialize the device detection instance
init_by_lua '
	local memcached = require("memcached")
	local DeviceDetection = require("DeviceDetection")

	-- Global variable
	g_dd = DeviceDetection:new(memcached, {
		dict = "ua_type",       -- (not required but recommended)
		mc_host = "",  -- (default)
		mc_port = 11211,        -- (default)
		mc_socket = nil,        -- (could be: unix:/run/memcached.sock)
		mc_timeout = 200        -- (connect timeout, default 200ms)

For each server {} block where you need device detection, add the following line:

# Set a request header called X-Device:
rewrite_by_lua_file '/etc/nginx/lua/nginx-device-lookup-rewrite.lua';

This will tag each request with a X-Device: header.

NOTE: You will probably want to edit the default X-Device: header set when a lookup against memcached fails because of a not-yet-known User-Agent: string. It's currently set to touch which may or may not match your requirements. The value should be chosen from what the external script may map User-Agent: strings to (see mapPropertiesToDeviceType() in php/deviceatlas-resolve-unknown-ua.php).


The required VCL code to lookup and store data in memcached is available here:

  • varnish/vcl_recv-deviceatlas.vcl

You'll want to include this in your vcl_recv {} block in your VCL file.

Varnish modules

The following extra modules are required to talk to memcached and compress User-Agent: strings into fixed length values. You'll want to include this code block at the top of your VCL file:

 * Digest and memcached - used for DeviceAtlas
 * https://github.com/varnish/libvmod-digest
 * https://github.com/sodabrew/libvmod-memcached
import digest;
import memcached;

These modules are non-standard and need to be be compiled by hand. For more information on how to do this, see their respective READMEs on GitHub:

Update service

The code required to keep data in memcached fresh is available here:

  • php/DeviceAtlas.class.php
  • php/deviceatlas-resolve-unknown-ua.php

The first file, DeviceAtlas.class.php, contains code to talk to DeviceAtlas' service.

The second file, deviceatlas-resolve-unknown-ua.php, retrieves User-Agent: strings from memcached, queries DeviceAtlas for devices features, interprets the results and updates memcached keys corresponding to the User-Agent: strings.

Thus, this script needs to be called regularly, preferably via cron:

# Update memcached on
* * * * php /usr/local/bin/deviceatlas-resolve-unknown-ua.php LICENSE-KEY-HERE

The script deviceatlas-resolve-unknown-ua.php takes one or more addresses to memcached servers on its command line. To update multiple servers, use something like:

$ php /usr/local/bin/deviceatlas-resolve-unknown-ua.php LICENSE-KEY-HERE varnish1.example.com varnish2.example.com

The DeviceAtlas license key needs to be supplied as the first argument to the script.

NOTE: You will probably want to modify mapPropertiesToDeviceType() in php/deviceatlas-resolve-unknown-ua.php and decide for yourself what you want to map different User-Agent: strings to. Currently the script use four variants:

  • mobile (dumb feature phone)
  • touch (smartphones with touch screens)
  • tablet (tablet devices)
  • desktop (everything else)

PHP packages required (Debian/Ubuntu)

The following packages needs to be installed to be able to run the update service. CA certificates are required to talk to DeviceAtlas over https (to avoid leaking visitors' User-Agent: strings over plaintext internet).

$ sudo apt-get install php5-cli php5-curl php5-memcache ca-certificates