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-
- ## Introduction
-
- [Rspamd](https://rspamd.com) is an advanced spam filtering system that allows evaluation of messages by a number of
- rules including regular expressions, statistical analysis and custom services
- such as URL black lists. Each message is analysed by Rspamd and given a `spam score`.
-
- According to this spam score and the user's settings Rspamd recommends an action for
- the MTA to apply to the message, for example, to pass, reject or add a header.
- Rspamd is designed to process hundreds of messages per second simultaneously and has a number of
- features available.
-
- You can watch the following [introduction video](https://www.youtube.com/watch?v=_fl9i-az_Q0) from the [FOSDEM-2016](http://fosdem.org) where I describe the main features of Rspamd and explain why Rspamd runs so fast.
-
- Rspamd is [packaged](https://rspamd.com/downloads.html) for the major Linux distributions and is also available via [FreeBSD ports](https://freshports.org/mail/rspamd) and NetBSD [pkgsrc](https://pkgsrc.org).
-
- ## Spam filtering features
-
- Rspamd distribution contains a number of mail processing features, including such techniques as:
-
- * **Regular expressions filtering** - allows basic processing of messages, their textual parts, MIME headers and
- SMTP data received by MTA against a set of expressions that includes both normal regular expressions and
- message processing functions. Rspamd expressions are the powerful tool that allows to filter messages based on
- some pre-defined rules. This feature is similar to regular expressions in SpamAssassin spam filter.
-
-
- * **SPF module** that allows to validate a message's sender against the policy defined in the DNS record of sender's domain. You can read
- about SPF policies [here](http://www.openspf.org/). A number of mail systems includes SPF support, such as `Gmail` or `Yahoo Mail`.
-
-
- * **DKIM module** validates a message cryptographic signature against a public key placed in the DNS record of sender's domain. Like SPF,
- this technique is widely spread and allows to validate that a message is sent from that specific domain.
-
-
- * **DNS black lists** allows to estimate reputation of sender's IP address or network. Rspamd uses a number of DNS lists including such lists as
- `SORBS` or `Spamhaus`. However, Rspamd doesn't trust any specific DNS list and use a conjunction of estimations instead that allows to
- avoid mistakes and false positives. Rspamd also uses positive and grey DNS lists for checking for trusted senders.
-
-
- * **URL black lists** are rather similar to DNS black lists but uses URLs in a message to make an estimation of sender's reputation.
- This technique is very useful for finding malicious or phished domains and filter such mail.
-
-
- * **Statistics** - Rspamd uses Bayesian classifier based on five-grams of input. This means that the input is estimated not based on individual
- words, but all input is organized in chains that are further estimated by Bayesian classifier. This approach allows to achieve better results than
- traditionally used monograms (or words literally speaking), that is described in details in the following [paper](http://osbf-lua.luaforge.net/papers/osbf-eddc.pdf).
-
-
- * **Fuzzy hashes** - for checking of malicious mail patterns Rspamd uses so called `fuzzy hashes`. Unlike normal hashes, these structures are targeted to hide
- small differences between text patterns allowing to find similar messages quickly. Rspamd has internal storage of such hashes and allows to block mass spam sendings
- quickly based on user's feedback that specifies messages reputation. Moreover, this allows to feed Rspamd with data from [`honeypots`](http://en.wikipedia.org/wiki/Honeypot_(computing)#Spam_versions)
- without polluting the statistical module.
-
- Rspamd uses the conjunction of different techniques to make the final decision about a message. This allows to improve the overall quality of filtering and reduce the number of
- false positives (e.g. when a innocent message is badly classified as a spam one). I have tried to simplify Rspamd usage by adding the following elements:
-
- * **Web interface** - Rspamd is shipped with the fully functional Ajax-based web interface that allows to observe Rspamd statistic, to configure rules, weights and lists, to scan
- and learn messages and to view the history of scans. The interface is self-hosted, requires zero configuration and follows the recent web applications standards. You don't need a
- web server or applications server to run WebUI - you just need to run Rspamd itself and a web browser.
-
- * **Integration with MTA** - Rspamd can work with the most popular mail transfer systems, such as Postfix, Exim or Sendmail. For Postfix and Sendmail, there is an [`Rmilter` project](https://github.com/vstakhov/rmilter),
- whilst for Exim there are several solutions to work with Rspamd. Should you require MTA integration then please consult with the [integration guide](https://rspamd.com/doc/integration.html).
-
- * **Extensive Lua API** - Rspamd ships with hundreds of [Lua functions](https://rspamd.com/doc/lua) that are available to write own rules for efficient and targeted spam filtering.
-
- * **Dynamic tables** - Rspamd allows to specify bulk lists as `dynamic maps` that are checked in runtime with updating data when they are changed. Rspamd supports file, HTTP and HTTPS maps.
-
- ## Performance
-
- Rspamd is designed to be fast. The core of Rspamd is written in `C` and uses an event-driven model that allows to process multiple messages simultaneously and without blocking.
- Moreover, a set of techniques is used in Rspamd to process messages faster:
-
- * **Finite state machines processing** - Rspamd uses specialized finite state machines for the performance critical tasks to process input faster than a set of regular expressions.
- Of course, it is possible to implement these machines by ordinary `Perl regular expressions` but then they won't be compact or human-readable. On the contrary, Rspamd optimizes
- such actions as headers processing, received elements extraction, protocol operations by building the concrete automata for an assigned task.
-
- * **Expressions optimizer** - allows to optimize expressions by execution of `likely false` or `likely true` expressions in order in the branches. That allows to reduce number of
- expensive expressions calls when scanning a message.
-
- * **Symbols optimizer** - Rspamd tries to check first the rules that are frequent or inexpensive in terms of time or CPU resources which allows to block spam before processing of
- expensive rules (rules with negative weights are always checked before other ones).
-
- * **Event driven model** - Rspamd is designed not to block anywhere in the code and knowing that a spam check requires a lot of network operations, Rspamd can process many messages
- simultaneously increasing the efficiency of shared DNS caches and other system resources. Moreover, event-driven system normally scales automatically and you won't need to do any
- tuning in the most of cases.
-
- * **Hyperscan regular expressions engine** - Rspamd utilizes [Hyperscan](https://01.org/hyperscan) engine to match multiple regular expressions at the same time. You can watch the following [presentation](https://highsecure.ru/rspamd-hyperscan.pdf) where the main benefits of Hyperscan are described.
-
- * **Clever choice of data structures** - Rspamd tries to use the optimal data structure for each task. For example, it uses very efficient suffix tries for fast matching of a text
- against a set of multiple patterns. Or it uses radix bit trie for storing IP addresses information that provides O(1) access time complexity.
-
- ## Extensions
-
- Besides its `C` core, Rspamd provides an extensive [Lua](http://lua.org) API to access almost all the features available directly from `C`. Lua is an extremely easy
- to learn programming language though it is powerful enough to implement complex mail filters. In fact Rspamd has a significant amount of code written completely in Lua such as
- DNS blacklists checks, user's settings or different maps implementation. You can also write your own filters and rules in Lua adopting Rspamd functionality to your needs.
- Furthermore, Lua programs are very fast and their performance is rather [close](http://attractivechaos.github.io/plb/) to pure `C`. However, you should mention that for the most
- of performance critical tasks you usually use the Rspamd core functionality than Lua code. Anyway, you can also use `LuaJIT` with Rspamd if your goal is maximum performance.
- From the Lua API you can do the following tasks:
-
- * **Reading the configuration parameters** - Lua code has the full access to the parsed configuration knobs and you can easily modify your plugins behaviour by means of the main
- Rspamd configuration.
-
- * **Registering custom filters** - it is more than simple to add your own filters to Rspamd: just add new index to the global variable `rspamd_config`:
-
- ~~~lua
- rspamd_config.MYFILTER = function(task)
- -- Do something
- end
- ~~~
-
- * **Full access to the content of messages** - you can access text parts, headers, SMTP data and so on and so forth by using of `task` object. The full list of methods could be found
- [here](https://rspamd.com/doc/lua/task.html).
-
-
- * **Pre- and post- filters** - you can register callbacks that are called before or after messages processing to make results more precise or to make some early decision,
- for example, to implement a rate limit.
-
- * **Registering functions for Rspamd** - you can write your own functions in Lua to extend Rspamd internal expression functions.
-
- * **Managing statistics** - Lua scripts can define a set of statistical files to be scanned or learned for a specific message allowing to create more complex
- statistical systems, e.g. based on an input language. Moreover, you can even learn Rspamd statistic from Lua scripts.
-
- * **Standalone Lua applications** - you can even write your own worker based on Rspamd core and performing some asynchronous logic in Lua. Of course, you can use the
- all features from Rspamd core, including such features as non-blocking IO, HTTP client and server, non-blocking Redis client, asynchronous DNS, UCL configuration and so on
- and so forth.
-
- * **API documentation** - Rspamd Lua API has an [extensive documentation](https://rspamd.com/doc/lua) where you can find examples, references and the guide about how to extend
- Rspamd with Lua.
-
-
- ## References
-
- * Home site: <https://rspamd.com>
- * Development: <https://github.com/vstakhov/rspamd>
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