+# Rspamd lua API
+
+Rspamd lua api is a core part of rspamd functionality. Lua is used for writing rules and plugins in rspamd. There are several objects and libraries that simplify classifying of mail.
+
+## Using lua API from rules
+
+Many lua rules are shipped with rspamd. They can be included to rspamd by using tag **lua** in rspamd.conf:
+
+~~~nginx
+lua = "$CONFDIR/lua/rspamd.lua"
+~~~
+
+### Global configuration tables
+
+While load of this file rspamd defines two global variables:
+- *config* - a global table of modules configuration. Here is a sample of usage of this table:
+
+~~~lua
+-- Init empty module configuration
+config['module'] = {}
+
+-- Rewrite module configuration
+config['regexp'] = {
+ RULE_NAME = '/some_re/'
+}
+
+-- Insert by index
+config['regexp']['RULE_NAME2'] = '/more_re/'
+~~~
+
+- *metrics* - a global table of metrics definitions. This variable is a table that is indexed by metric name and provide ability to set up symbols' properties:
+
+~~~lua
+
+metrics['default'] = {
+-- Set weight and description
+ SYMBOL = { weight = 9.0, description = 'description'},
+-- Just set weight
+ SYMBOL2 = 9.0,
+}
+-- Add symbol definition
+metrics['default']['SYMBOL3'] = { weight = 1, description = 'description' }
+~~~
+
+* *classifiers* - a table of classifiers pre-filters. Pre-filter must be a function that accepts 4 parameters: `classifier`, `task`, `is_learn` and `is_spam`. Pre-filter must return a table of statfiles to be checked or learned for this message or nil if all suitable statfiles must be learned or checked. Here is an example of language detection for classification:
+
+~~~lua
+
+-- Detect language of message and selects appropriate statfiles for it
+
+classifiers['bayes'] = function(classifier, task, is_learn, is_spam)
+ -- Subfunction for detection of message's language
+ local detect_language = function(task)
+ local parts = task:get_text_parts()
+ for _,p in ipairs(parts) do
+ local l = p:get_language()
+ if l then
+ return l
+ end
+ end
+ return nil
+ end
+
+ -- Main procedure
+ language = detect_language(task)
+ if language then
+ -- Find statfiles with specified language
+ local selected = {}
+ for _,st in pairs(classifier:get_statfiles()) do
+ local st_l = st:get_param('language')
+ if st_l and st_l == language then
+ -- Insert statfile with specified language
+ table.insert(selected, st)
+ end
+ end
+ if table.maxn(selected) > 1 then
+ return selected
+ end
+ else
+ -- Language not detected
+ local selected = {}
+ for _,st in ipairs(classifier:get_statfiles()) do
+ local st_l = st:get_param('language')
+ -- Insert only statfiles without language
+ if not st_l then
+ table.insert(selected, st)
+ end
+ end
+ if table.maxn(selected) > 1 then
+ return selected
+ end
+ end
+
+ return nil
+end
+~~~
+
+### Writing complex rules
+So by using these two tables it is possible to configure rules and metrics. Also note that it is possible to use any lua functions and rspamd libraries:
+
+~~~lua
+-- Declare variable that contains rule definition
+local rulebody = string.format('%s & !%s', '/re1/', '/re2')
+-- Set global table element
+config['regexp']['test_rule'] = rulebody
+-- Write message to log
+rspamd_logger.info('Loaded test rule: ' .. rulebody)
+~~~
+
+Also it is possible to declare functions and use `closures` when defining rspamd rules:
+
+~~~lua
+local reconf = config['regexp']
+reconf['R_EMPTY_IMAGE'] = function (task)
+ -- Get text parts from message
+ parts = task:get_text_parts()
+ -- Iterate through all text parts
+ if parts then
+ for _,part in ipairs(parts) do
+ -- Find empty parts
+ if part:is_empty() then
+ -- Get all images
+ images = task:get_images()
+ if images then
+ -- We have images and empty part, insert symbol
+ return true
+ end
+ return false
+ end
+ end
+ end
+ return false
+end
+-- Here is a sample of using other function inside rule
+local function check_headers_tab(task, header_name)
+ -- Extract raw headers from message
+ local raw_headers = task:get_raw_header(header_name)
+ -- Make match of headers, that are separated with tabs, not spaces
+ if raw_headers then
+ for _,rh in ipairs(raw_headers) do
+ if rh['tab_separated'] then
+ -- We have header value separated by tab symbol
+ return true
+ end
+ end
+ end
+ return false
+end
+
+reconf['HEADER_TAB_FROM_WHITELISTED'] = function(task) return check_headers_tab(task, "From") end
+reconf['HEADER_TAB_TO_WHITELISTED'] = function(task) return check_headers_tab(task, "To") end
+reconf['HEADER_TAB_DATE_WHITELISTED'] = function(task) return check_headers_tab(task, "Date") end
+~~~
+
+Using lua in rules provides many abilities to write complex mail filtering rules.
+
+## Writing lua plugins
+
+Plugins are more complex filters than ordinary rules. Plugins can have their own configuration parameters and multiple callbacks. Plugins can make DNS requests, read from rspamd maps and insert custom results.
+
+### Structure of the typical plugin
+
+Each rspamd plugin has a common structure:
+
+- Registering configuration parameters
+- Reading configuration parameters and set up callbacks
+- Callbacks that are called by rspamd during message processing
+
+Here is a simple plugin example:
+
+~~~lua
+local config_param = 'default'
+
+function sample_callback(task)
+end
+
+-- Registration
+-- Check API version
+if type(rspamd_config.get_api_version) ~= 'nil' then
+ if rspamd_config:get_api_version() >= 1 then
+ rspamd_config:register_module_option('maillist', 'symbol', 'string')
+ end
+end
+
+-- Reading configuration
+-- Get all options for this plugin
+local opts = rspamd_config:get_all_opt('sample')
+if opts then
+ if opts['config'] then
+ config_param = opts['config']
+ -- Register callback
+ rspamd_config:register_symbol('some_symbol', 1.0, 'sample_callback')
+ end
+end
+~~~
+
+This plugin uses global variable *rspamd_config* to extract configuration options. Then it registers function `sample_callback` that will be called for processing symbol `some_symbol`.
+
+### Using DNS requests inside plugins
+
+It is often required to make DNS requests for messages checks. Here is an example of making asynchronous DNS request from rspamd lua plugin:
+
+~~~lua
+-- Function-callback of rspamd rule
+function symbol_cb(task)
+ -- Task is now local variable
+ local function dns_cb(resolver, to_resolve, results, err, str)
+ -- Increase total count of dns requests
+ task:inc_dns_req()
+ if results then
+ task:insert_result('symbol', 1, str)
+ end
+ end
+ -- Resolve 'example.com' using primitives from the task passed
+ task:get_resolver():resolve_a(task:get_session(), task:get_mempool(),
+ 'example.com', dns_cb, 'sample string')
+end
+~~~
+
+### Using maps from lua plugin
+
+Maps can hold dynamically loaded data like lists or ip trees. It is possible to use 2 types of maps: **radix_tree** that stores ip addresses and **hash_map** that stores plain strings (domains usually). Here is a sample of using maps from lua API:
+
+~~~lua
+-- Add two maps in configuration section
+local hash_map = rspamd_config:add_hash_map ('file:///path/to/file')
+local radix_tree = rspamd_config:add_radix_map ('http://somehost.com/test.dat')
+
+function sample_symbol_cb(task)
+ -- Check whether hash map contains from address of message
+ if hash_map:get_key(task:get_from()) then
+ -- Check whether radix map contains client's ip
+ if radix_map:get_key(task:get_from_ip_num()) then
+ ...
+ end
+ end
+end
+~~~
+
+## Conclusions
+
+Lua plugins is a powerful tool for creating complex filters that can access practically all features of rspamd. Lua plugins can be used for writing custom tests that can be configured from XML, can use maps and make DNS requests. Rspamd is shipped with a couple of lua plugins that can be a good example for writing own plugins.
+
+## References
+
+- [Rspamd lua API reference](reference.md)
+- [Lua manual](http://www.lua.org/manual/5.2/)
+- [Programming in lua](http://www.lua.org/pil/)
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