Fuzzy check module

This module is intended to check messages for specific fuzzy patterns stored in fuzzy storage workers. At the same time, this module is responsible for teaching fuzzy storage with message patterns.

Fuzzy patterns

Rspamd uses the shingles algorithm to perform a fuzzy match of messages. This algorithm is probabilistic and uses word chains as patterns (in the shingles algorithm), and thus filter spam or ham messages. The shingles algorithm is described in the following research paper. We use 3-grams (trigrams) for this algorithm and a set of hash functions: siphash, mumhash and others. Currently, rspamd uses 32 hashes for shingles.

Attachments and images are not currently matched against fuzzy hashes, but they are checked by way of blake2 digests using strict match.

Module outline

# local.d/fuzzy_check.conf
fuzzy_check
{
	max_errors = ...; //int: Maximum number of upstream errors; affects error rate threshold
	min_bytes = ...; //int: Minimum number of *bytes* to check a non-text part
	min_height = ...; //int: Minimum pixel height of embedded images to check using fuzzy storage
	min_length = ...; //int: Minimum number of *words* to check a text part
	min_width = ...; //int: Minimum pixel width of embedded images to check using fuzzy storage
	retransmits = ...; //int: Maximum number of retransmissions for a single request
	revive_time = ...; //float: Time (seconds?) to lapse before re-resolving faulty upstream
	symbol = "default symbol"; //string: Default symbol for rule (if no flags defined or matched)
	text_multiplier = ...; //float: Multiplier for bytes limit when checking for text parts
	timeout = ...; //time: Timeout to wait for a reply from a fuzzy server, e.g. 1s, 2m, 5h
	whitelist = "..."; //string: Whitelisted IPs map

	rule { //Fuzzy check rule
		algorithm = "..."; //string: rule hashing algo
		encryption_key = "..."; //string: Base32 value for the protocol encryption public key
		fuzzy_key = "..."; //string: Base32 value for the hashing key (for private storages)
		fuzzy_map = { //object: Map of SYMBOL -> data for flags configuration
			max_score = ... ; //int: Maximum score for this flag
			flag = ... ; //int: Flag number (ordinal)
		}; 
		fuzzy_shingles_key = "..."; //string: Base32 value for the shingles hashing key (for private storages)
		headers = "..."; //array: Headers that are used to make a separate hash
		learn_condition = "..."; //string: Lua script that returns boolean function to check whether this task should be considered when training fuzzy storage
		max_score = ...; //int: Max value for fuzzy hash when weight of symbol is exactly 1.0 (if value is higher, then the score is still 1.0)
		mime_types = "..."; //array: Set of mime types (in form type/subtype, or type/*, or *) to check with fuzzy
		min_bytes = ...; //int: Override module default min bytes for this rule
		read_only = ...; //boolean: If true then never try to train this fuzzy storage
		servers = "..."; //string: List of servers to check (or train)
		short_text_direct_hash = ...; //boolean: Use direct hash for short texts
		skip_hashes = "..."; //string: Whitelisted hashes map
		skip_unknown = ...; //boolean: If true then ignores unknown flags and does not add the default fuzzy symbol
		symbol = "..."; //string: Default symbol for rule (if no flags defined or matched)
	}
}

Module configuration

The fuzzy_check module has several global options, including:

  • min_bytes: minimum length of attachments and images in bytes to check them in fuzzy storage
  • min_height: minimum pixel height of images to be checked
  • min_length: minimum length of text parts in words to perform fuzzy check (default - check all text parts)
  • min_width: minimum pixel width of images to be checked
  • retransmits: maximum retransmissions before giving up
  • symbol: default symbol to insert (if no flags match)
  • timeout: timeout to wait for a reply, e.g. 1s, 2m, 5h
  • whitelist: IPs in this list bypass all fuzzy checks

e.g.

# local.d/fuzzy_check.conf
# the following are defaults in 1.9.4
fuzzy_check {
    min_bytes = 1k; # Since small parts and small attachments cause too many FP
    timeout = 2s;
    retransmits = 1;
    ...
    rule {...}
}

A fuzzy rule is defined as a set of rule definitions. Each rule must have a servers list to check or train (teach), and a set of flags and optional parameters.

The servers parameter can have the round-robin parameter to alternately try each entry, e.g. using a list:

        servers = "round-robin:fuzzy1.rspamd.com:11335,fuzzy2.rspamd.com:11335";

Available keywords for use in the servers parameter include:

  • hash:: use a stable hashing algorithm to distribute values
  • master-slave:: always prefer upstream with a higher priority unless it is unavailable
  • round-robin:: balance upstreams one by one, by selecting according to their weight
  • random:: choose each entry in a list randomly
  • sequential:: use each entry in a list sequentially

Usable parameters include:

  • algorithm: rule hashing algo; one of: fasthash (or just fast), mumhash, siphash (or old) or xxhash
  • encryption_key: Base32 value public key to access this rule
  • fuzzy_map: Map of SYMBOL -> data for flags configuration
  • fuzzy_key: Base32 value for the hashing key (for private storages).
  • learn_condition: An Lua script that returns a boolean function to check whether this task should be considered when training fuzzy storage
  • max_score: float value: score threshold for this rule’s activation/trigger
  • mime_types: array or list of acceptable mime-type regexs for this rule. Can be: ["*"] to match anything
  • read_only: set to no to enable training, set to yes for no training
  • servers: list of servers to check or train
  • short_text_direct_hash: whether to check the exact hash match for short texts where fuzzy algorithm is not applicable.
  • skip_unknown: whether or not to ignore unmatched content; if true or yes then ignore unknown flags and does not add the default fuzzy symbol
  • symbol: the default symbol applied for a rule.

Here is an example rule:

# local.d/fuzzy_check.conf
...
rule "FUZZY_CUSTOM" {
  # List of servers. Can be an array or multi-value item
  servers = "127.0.0.1:11335";

  # List of additional mime types to be checked in this fuzzy ("*" for any)
  mime_types = ["application/*", "*/octet-stream"];

  # Maximum global score for all maps combined
  max_score = 20.0;

  # Ignore flags that are not listed in maps for this rule
  skip_unknown = yes;

  # If this value is false (i.e. no), then allow learning for this fuzzy rule
  read_only = no;

  # Fast hash type
  algorithm = "mumhash";
}
...

Each rule can have several fuzzy_map values, ordered by an ordinal flag value. A single fuzzy storage can contain both good and bad hashes that should have different symbols, and thus, different weights. Multiple fuzzy_maps are defined inside fuzzy rules as follows:

# local.d/fuzzy_check.conf
rule "FUZZY_LOCAL" {
...
fuzzy_map = {
  FUZZY_DENIED {
    # Maximum weight for this list
    max_score = 20.0;
    # Flag value
    flag = 1
  }
  FUZZY_PROB {
    max_score = 10.0;
    flag = 2
  }
  FUZZY_WHITE {
    max_score = 2.0;
    flag = 3
  }
}
...
}

From the above we can infer that email messages accruing a max_score above 20.0 will receive the FUZZY_DENIED mapping, and thus be categorised as spam.

The meaning of max_score can be rather unclear. First of all, all hashes in fuzzy storage have individual weights. For example, if we have a hash A and 100 users marked it as a spam hash, then it will have a weight of 100 * single_vote_weight. Therefore, if a single_vote_weight is 1 then the final weight will be 100. max_score is the weight that is required for the rule to add its symbol to the maximum score 1.0 (that will then be multiplied by the metric value’s weight). For example, if the weight of the hash is 100 and the max_score is set to 20, then the rule will be added with the weight of 1. If max_score is set to 200, then the rule will be added with the weight likely 0.2 (calculated via hyperbolic tangent).

In the following configuration:

metric {
	name = "default";
	...
	symbol {
		name = "FUZZY_DENIED";
		weight = "10.0";
	}
	...
}
fuzzy_check {
	rule {
	...
	fuzzy_map = {
		FUZZY_DENIED {
			# Maximum weight for this list
			max_score = 20.0;
			# Flag value
			flag = 1
        }
        ...
    }
}

If a hash has value 10, then a symbol FUZZY_DENIED with weight of 2.0 will be added. If a hash has value 100500, then FUZZY_DENIED will have weight 10.0.

Training fuzzy_check

Module fuzzy_check can also learn from messages. You can use rspamc command or connect to the controller worker using HTTP protocol. For learning, you must check the following settings:

  1. Controller worker should be accessible by rspamc or HTTP (check bind_socket)
  2. Controller should allow privilleged commands for this client (check enable_password or allow_ip settings)
  3. Controller should have fuzzy_check module configured to the servers specified
  4. You should know fuzzy_key and fuzzy_shingles_key to operate with this storage
  5. Your fuzzy_check module should have fuzzy_map configured to the flags used by server
  6. Your fuzzy_check rule must have read_only option turned off (read_only = false)
  7. Your fuzzy_storage worker should allow updates from the controller’s host (allow_update option)
  8. Your controller should be able to communicate with fuzzy storage by means of the UDP protocol

If all these conditions are met, then you can teach rspamd messages with rspamc:

rspamc -w <weight> -f <flag> fuzzy_add ...

or delete hashes:

rspamc -f <flag> fuzzy_del ...

you can also delete a hash that you find in the log output:

rspamc -f <flag> fuzzy_delhash <hash-id>

On learning, rspamd sends commands to all servers inside a specific rule. On check, rspamd selects a server in a round-robin manner.

Usage of the feeds provided by rspamd.com

If you use rspamd.com feeds (enabled by default) you need to qualify free usage policy or you would be blocked from using this service. There is a special symbol called FUZZY_BLOCKED that means that you violate these terms and are no longer permitted to use this service. This symbol has no weight and it should not affect any mail processing operations.