Some samples about usage of Lua with HAProxy

It's not easy to understand usage of Lua in HAProxy. The best way is a cpy paste of samples. This page will collect some samples of basic usages.

Note that this tuto is not:

  • HAProxy configuraiton tutorial. The reader must known HAProxy basics
  • Lua programation tutorial. The reader must known Lua basics

Useful links



Before start

In the following examples, I use the function print_r(). This function is usefull for displaying complex Lua structures. It is used for debugging. You can found this function in the page Lua scripts. The samples.lua file starts with an include (require()) of the print_r script.


For testing the following samples, we always use HAProxy in debug mode. The global section of the configuration file is like the following. This is not a production ready configuration !

	lua-load samples.lua
	stats socket /tmp/haproxy.sock mode 644 level admin
	tune.ssl.default-dh-param 2048

	timeout client 1m
	timeout server 1m

The configuration file is named haproxy.cfg, all the Lua samples which could be copy, need to be pasted in the file samples.lua. The command line for starting sample is like the following. We assume a start from the directory containing the two configuration file (for default path reasons).

haproxy -d -f haproxy.cfg

Most of tests are performed whith Curl.

The documentation links points to the HAProxy version 1.7 documentation.

Note about sample-fetches wrappers and Lua Notation: txn.f is an array of wrapper functions. The object like members of an array doesn't supports the "." which is a reserved character. So all the sample fetches are replaced by a "_". All the sample-fetches wrapper for the HAProxy version 1.7 are:

» Display / hide sample-fetches wrapper names

» Display / hide converters wrapper names

With Lua, a function is a variable. For convenience, the executed function can be declared inline as anonymous functions or referenced as named function. So the two sample below have the same behavior.

-- Named function
function a(arg)

-- Inline function

I will use the inline notation. Bellow the first sample commented. Note that all the Lua registered function are mapped in the HAProxy configuration with a prefix "lua.".

Using HAProxy sample fetches inside Lua

This first example is absolutely useless. It just shows how creating a Lua sample-fetch which return the same content that an embedded sample-fetch. By the way, it shows how using embedded sample fetches in Lua.

First, HAProxy provides some functions and class. Some of these ones are availaible from the start of HAProxy. Where the lua file is read, it is executed, so the Lua file is executed during the start of HAProxy.

New all the functions which will be call by HAProxy (called "hook") needs to be registered. For declaring a Lua sample-fetch we will use the function core.register_fetches()

  • core.register_fetches(): Function (doc): Register Lua sample-fetch. register_fetches takes a function as argument. This function will be called according with the HAProxy
  • txn: Class TXN (doc): Provides functions available with the transaction.
  • txn.f: Class Fetches (doc): Provides an access to all the haproxy sample fetches. There are called sample-fetches wrappers.

-- This register new sample-fetch called "lua.sample1"
core.register_fetches("sample1", function(txn)

	-- By default, all varibles are global. specify the local
	-- keyword to avoid bug. The following line store the 
	-- content of the sample-fetch "path" in the variable
	-- "path".
	local path = txn.f:path()

	-- Print out the content of path variable. The data
	-- id display on the HAProxy standard output.

	-- Return the value of the path as the sample-fetch
	-- result.
	return txn.f:path()

We can use this new sample-fetch like the embedded sample-fetch"path":

listen sample1
	mode http
	bind *:10010
	acl use-lua lua.sample1 -m beg /use-lua
	acl use-php lua.sample1 -m beg /use-php
	http-request redirect location /lua if use-lua
	http-request redirect location /php if use-php
	http-request redirect location /other

Start HAProxy and test with curl:

~$ curl -s -i | grep Location
Location: /lua

~$ curl -s -i | grep Location
Location: /php

~$ curl -s -i | grep Location
Location: /other

Simplify complex conditions

In some cases, configurations have complex conditions based on sample-fetches. These kind of conditions are not easy to maintain because there are not easily understandable. We are quickly lost with a lot of ACL. A prod likely example is choosing redirect to https or no according with some conditions. The conditions are prod or preprod, already ssl or no, protected page or no. We want to force https for the payment page, except for the preprod, except if the request is already SSL and except if the DEBUG cookie is set.

We want to redirect to http if the request is ssl and if the preprod is required or if not a payment page is required and if the exception cookie is not set.

These condition are a little bit complex with classic ACL. The Lua can help us. The foolowing table resule input name, their definition and the testing method:

Condition nameDescriptionTest with Curl
is_payment_pageThe payment page is requestedcurl
is_sslThe request is performed using SSLcurl -k
is_preprodThe preprod is requiredcurl -H 'Host:'
is_cookie_exceptionA DEBUG cookie is setcurl -H 'Cookie: DEBUG=1'

Below the truth table resuming actions:

0100HTTP redirect
0101HTTP redirect
0110HTTP redirect
0111HTTP redirect
1000HTTPS redirect
1101HTTP redirect
1110HTTP redirect
1111HTTP redirect

Below the Lua code performing conditions. The code is split in two parts. The first part extract inputs, and the second part perform conditions based on inputs.

-- This function returns all condition in an array.
function get_variables(txn)

	-- This array will contains conditions
	local cond = {}

	-- True if the path starts with "/payment/"
	cond['is_payment_page'] = string.match(txn.sf:path(), '^/payment/') ~= nil

	-- True if the front connection is SSL
	cond['is_ssl'] = txn.f:ssl_fc() == 1

	-- True if the domain name asked is preprod
	cond['is_preprod'] = txn.f:req_fhdr('host') == ''

	-- True if the cookie 'DEBUG' is set
	cond['is_cookie_exception'] = txn.f:req_cook_cnt('DEBUG') >= 1

	-- Display extracted conditions
-- print_r(cond)
	return cond

-- This sample fetch return 1 if we need HTTPS redirect
core.register_fetches("sample2_1", function(txn)

	-- Get input conditions
	local cond = get_variables(txn)

	-- Return result according with conditions value and policy.
	if cond['is_ssl']              then return 0 end
	if cond['is_cookie_exception'] then return 0 end
	if cond['is_preprod']          then return 0 end
	if cond['is_payment_page']     then return 1 end
	return 0

-- This sample fetch returns 1 if we need HTTP redirect
core.register_fetches("sample2_2", function(txn)

	-- Get input conditions
	local cond = get_variables(txn)

	-- Return result according with conditions value and policy.
	if not cond['is_ssl']          then return 0 end
	if cond['is_cookie_exception'] then return 1 end
	if cond['is_preprod']          then return 1 end
	if not cond['is_payment_page'] then return 1 end
	return 0

And the HAProxy corresponding code:

listen sample2
	mode http
	bind *:10020
	bind *:10021 ssl crt crt
	http-request redirect location /to-https if { lua.sample2_1 1 }
	http-request redirect location /to-http  if { lua.sample2_2 1 }
	http-request redirect location /forward