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a power-magnet
Module: mod_magnet
.. contents:: Table of Contents
:Version: lighttpd 1.4.12 or higher
:Packages: lua >= 5.1
mod_magnet is a module to control the request handling in lighty.
.. note::
Keep in mind that the magnet is executed in the core of lighty. EVERY long-running operation is blocking
ALL connections in the server. You are warned. For time-consuming or blocking scripts use mod_fastcgi and friends.
For performance reasons mod_magnet caches the compiled script. For each script-run the script itself is checked for
freshness and recompile if neccesary.
mod_magnet needs a lighty which is compiled with the lua-support ( --with-lua). Lua 5.1 or higher are required by
the module. Use "--with-lua=lua5.1" to install on Debian and friends. ::
server.modules = ( ..., "mod_magnet", ... )
mod_magnet can attract a request in several stages in the request-handling.
* either at the same level as mod_rewrite, before any parsing of the URL is done
* or at a later stage, when the doc-root is known and the physical-path is already setup
It depends on the purpose of the script which stage you want to intercept. Usually you want to use
the 2nd stage where the physical-path which relates to your request is known. At this level you
can run checks against lighty.env["physical.path"].
magnet.attract-raw-url-to = ( ... )
magnet.attract-physical-path-to = ( ... )
You can define multiple scripts when separated by a semicolon. The scripts are executed in the specified
order. If one of them a returning a status-code, the following scripts will not be executed.
Most of the interaction between between mod_magnet and lighty is done through tables. Tables in lua are hashes (Perl), dictionaries (Java), arrays (PHP), ...
Lighttpd has its internal variables which are exported as read/write to the magnet.
If "" is requested this results in a request like ::
GET /search.php?q=lighty HTTP/1.1
When you are using ``attract-raw-url-to`` you can access the following variables:
* parts of the request-line
* lighty.env["request.uri"] = "/search.php?q=lighty"
* HTTP request-headers
* lighty.request["Host"] = ""
Later in the request-handling, the URL is splitted, cleaned up and turned into a physical path name:
* parts of the URI
* lighty.env["uri.path"] = "/search.php"
* lighty.env["uri.path-raw"] = "/search.php"
* lighty.env["uri.scheme"] = "http"
* lighty.env["uri.authority"] = ""
* lighty.env["uri.query"] = "q=lighty"
* filenames, pathnames
* lighty.env["physical.path"] = "/my-docroot/search.php"
* lighty.env["physical.rel-path"] = "/search.php"
* lighty.env["physical.doc-root"] = "/my-docroot"
All of them are readable, not all of the are writable (or don't have an effect if you write to them).
As a start, you might want to use those variables for writing: ::
-- 1. simple rewriting is done via the request.uri
lighty.env["request.uri"] = ...
return lighty.RESTART_REQUEST
-- 2. changing the physical-path
lighty.env["physical.path"] = ...
-- 3. changing the query-string
lighty.env["uri.query"] = ...
Response Headers
If you want to set a response header for your request, you can add a field to the lighty.header[] table: ::
lighty.header["Content-Type"] = "text/html"
Sending Content
You can generate your own content and send it out to the clients. ::
lighty.content = { "<pre>", { filename = "/etc/passwd" }, "</pre>" }
lighty.header["Content-Type"] = "text/html"
return 200
The lighty.content[] table is executed when the script is finished. The elements of the array are processed left to right and the elements can either be a string or a table. Strings are included AS IS into the output of the request.
* Strings
* are included as is
* Tables
* filename = "<absolute-path>" is required
* offset = <number> [default: 0]
* length = <number> [default: size of the file - offset]
Internally lighty will use the sendfile() call to send out the static files at full speed.
Status Codes
You might have seen it already in other examples: In case you are handling the request completly in the magnet you
can return your own status-codes. Examples are: Redirected, Input Validation, ... ::
if (lighty.env["uri.scheme"] == "http") then
lighty.header["Location"] = "https://" .. lighty.env["uri.authority"] .. lighty.env["request.uri"]
return 302
You every number above and equal to 100 is taken as final status code and finishes the request. No other modules are
executed after this return.
A special return-code is lighty.RESTART_REQUEST (currently equal to 99) which is usually used in combination with
changing the request.uri in a rewrite. It restarts the splitting of the request-uri again.
If you return nothing (or nil) the request-handling just continues.
To easy debugging we overloaded the print()-function in lua and redirect the output of print() to the error-log. ::
print("Host: " .. lighty.request["Host"])
print("Request-URI: " .. lighty.env["request.uri"])
Sending text-files as HTML
This is a bit simplistic, but it illustrates the idea: Take a text-file and cover it in a <pre> tag.
Config-file ::
magnet.attract-physical-path-to = server.docroot + "/readme.lua"
readme.lua ::
lighty.content = { "<pre>", { filename = "/README" }, "</pre>" }
lighty.header["Content-Type"] = "text/html"
return 200
Maintainance pages
Your side might be on maintainance from time to time. Instead of shutting down the server confusing all
users, you can just send a maintainance page.
Config-file ::
magnet.attract-physical-path-to = server.docroot + "/maintainance.lua"
maintainance.lua ::
require "lfs"
if (nil == lfs.attributes(lighty.env["physical.doc-root"] .. "/maintainance.html")) then
lighty.content = ( lighty.env["physical.doc-root"] .. "/maintainance.html" )
lighty.header["Content-Type"] = "text/html"
return 200
Config-file ::
magnet.attract-physical-path-to = server.docroot + "/flv-streaming.lua"
if (lighty.env["uri.query"]) then
-- split the query-string
get = {}
for k, v in string.gmatch(lighty.env["uri.query"], "(%w+)=(%w+)") do
get[k] = v
if (get["start"]) then
-- missing: check if start is numeric and positive
-- send te FLV header + a seek into the file
lighty.content = { "FLV\x1\x1\0\0\0\x9\0\0\0\x9",
{ filename = lighty.env["physical.path"], offset = get["start"] } }
lighty.header["Content-Type"] = "video/x-flv"
return 200
selecting a random file from a directory
Say, you want to send a random file (ad-content) from a directory.
To simplify the code and to improve the performance we define:
* all images have the same format (e.g. image/png)
* all images use increasing numbers starting from 1
* a special index-file names the highest number
Config ::
server.modules += ( "mod_magnet" )
magnet.attract-physical-path-to = "random.lua"
random.lua ::
dir = lighty.env["physical.path"]
f = assert( .. "/index", "r"))
maxndx = f:read("*all")
ndx = math.random(maxndx)
lighty.content = { { filename = dir .. "/" .. ndx }}
lighty.header["Content-Type"] = "image/png"
return 200
denying illegal character sequences in the URL
Instead of implementing mod_security, you might just want to apply filters on the content
and deny special sequences that look like SQL injection.
A common injection is using UNION to extend a query with another SELECT query.
if (string.find(lighty.env["request.uri"], "UNION%s")) then
return 400
Traffic Quotas
If you only allow your virtual hosts a certain amount for traffic each month and want to
disable them if the traffic is reached, perhaps this helps: ::
host_blacklist = { [""] = 0 }
if (host_blacklist[lighty.request["Host"]]) then
return 404
Just add the hosts you want to blacklist into the blacklist table in the shown way.
Complex rewrites
If you want to implement caching on your document-root and only want to regenerate
content if the requested file doesn't exist, you can attract the physical.path: ::
magnet.attract-physical-path-to = ( server.document-root + "/rewrite.lua" )
rewrite.lua ::
require "lfs"
attr = lfs.attributes(lighty.env["physical.path"])
if (not attr) then
-- we couldn't stat() the file for some reason
-- let the backend generate it
lighty.env["uri.path"] = "/dispatch.fcgi"
lighty.env["physical.rel-path"] = lighty.env["uri.path"]
lighty.env["physical.path"] = lighty.env["physical.doc-root"] .. lighty.env["physical.rel-path"]
We are requiring the lua-module 'lfs' (
I had to compile lfs myself for lua-5.1 which required a minor patch as compat-5.1 is not needed::
$ wget
$ wget
$ gzip -cd luafilesystem-1.2.tar.gz | tar xf -
$ cd luafilesystem-1.2
$ patch -ls -p1 < ../luafilesystem-1.2-lua51.diff
$ make install
It will install into /usr/lib/lua/5.1/ which is where lua expects the extensions on my system.
SuSE and Gentoo are known to have their own lfs packages and don't require a compile.
... or how to store data globally in the script-context:
Each script has its own script-context. When the script is started it only contains the lua-functions
and the special lighty.* name-space. If you want to save data between script runs, you can use the global-script
if (nil == _G["usertrack"]) then
_G["usertrack"] = {}
if (nil == _G["usertrack"][lighty.request["Cookie"]]) then
_G["usertrack"][lighty.request["Cookie"]] = _G["usertrack"][lighty.request["Cookie"]] + 1
print _G["usertrack"][lighty.request["Cookie"]]
The global-context is per script. If you update the script without restarting the server, the context will still be maintained.
mod_status support a global statistics page and mod_magnet allows to add and update values in the status page:
Config ::
status.statistics-url = "/server-counters"
magnet.attract-raw-url-to = server.docroot + "/counter.lua"
counter.lua ::
lighty.status["core.connections"] = lighty.status["core.connections"] + 1
core.connections: 7
fastcgi.backend.php-foo.0.connected: 0
fastcgi.backend.php-foo.0.died: 0
fastcgi.backend.php-foo.0.disabled: 0
fastcgi.backend.php-foo.0.load: 0
fastcgi.backend.php-foo.0.overloaded: 0
fastcgi.backend.php-foo.1.connected: 0
fastcgi.backend.php-foo.1.died: 0
fastcgi.backend.php-foo.1.disabled: 0
fastcgi.backend.php-foo.1.load: 0
fastcgi.backend.php-foo.1.overloaded: 0
fastcgi.backend.php-foo.load: 0
Porting mod_cml scripts
mod_cml got replaced by mod_magnet.
A CACHE_HIT in mod_cml::
output_include = { "file1", "file2" }
return CACHE_HIT
content = { { filename = "/path/to/file1" }, { filename = "/path/to/file2"} }
return 200
while a CACHE_MISS like (CML) ::
trigger_handler = "/index.php"
becomes (magnet) ::
lighty.env["request.uri"] = "/index.php"
return lighty.RESTART_REQUEST