decoders.rst

Decoders Syntax

The decoders extract the information from the received events. When an event is received, the decoders separate the information in blocks to prepare them for their subsequent analysis.

Options

There are many options to configure the decoders:

• decoder
• parent
• accumulate
• program_name
• prematch
• regex
• order
• fts
• ftscomment
• plugin_decoder
• use_own_name
• json_null_field
• var

decoder

The attributes listed below define a decoder.

Attribute	Description
name	The name of the decoder
type	The type of the decoder

Example:

Set name and type of decoder to ossec:

<decoder name="ossec" type ="ossec">
  ...
</decoder>

parent

It is used to link a subordinate codeblock to his parent.

Default Value	n/a
Allowed values	Any decoder name

Example:

Assign the decoder which father it belongs:

<decoder name="decoder_junior">
  <parent>decoder_father</parent>
  ...
</decoder>

accumulate

Allow Wazuh to track events over multiple log messages based on a decoded id.

Note

Requires a regex populating the id field.

Example of use

<accumulate />

program_name

It defines the name of the program with which the decoder is associated.

Default Value	n/a
Allowed values	Any sregex expression

Example:

Define that the decoder is related with the syslogd process:

<decoder name="syslogd_decoder">
  <program_name>syslogd</program_name>
  ...
</decoder>

prematch

It attempts to find a match within the log for the string defined.

Default Value	n/a
Allowed values	Any sregex expression

The attribute below is optional, it allows to discard some of the content of the entry.

Attribute	Value
offset	after_regex

regex

Regular expressions or regex are sequences of characters that define a pattern. Decoders use them to find words or other patterns into the rules.

An example is this regex that matches any numeral:

<regex> [+-]?(\d+(\.\d+)?|\.\d+)([eE][+-]?\d+)? </regex>

Default Value	n/a
Allowed values	Any regex expression

The attribute below is optional, it allows to discard some of the content of the entry.

Attribute	Value
offset	after_regex after_parent after_prematch

Example:

Show when an user executed the sudo command for the first time:

<decoder name="sudo-fields">
  <parent>sudo</parent>
  <prematch>\s</prematch>
  <regex>^\s*(\S+)\s*:</regex>
  <order>srcuser</order>
  <fts>name,srcuser,location</fts>
  <ftscomment>First time user executed the sudo command</ftscomment>
</decoder>

order

It defines what the parenthesis groups contain and the order in which they were received.

Default Value	n/a
Static fields	srcuser	Extracts the source username
	dstuser	Extracts the destination (target) username
	user	An alias to dstuser (only one of the two can be used)
	srcip	Source ip
	dstip	Destination ip
	srcport	Source port
	dstport	Destination port
	protocol	Protocol
	id	Event id
	url	Url of the event
	action	Event action (deny, drop, accept, etc)
	status	Event status (success, failure, etc)
	extra_data	Any extra data
Dynamic fields	Any string not included in the previous list

fts

It is used to designate a decoder as one in which the first time it matches the administrator would like to be alerted.

Default Value	n/a
Allowed values	location	Where the log came from
	srcuser	Extracts the source username
	dstuser	Extracts the destination (target) username
	user	An alias to dstuser (only one of the two can be used)
	srcip	Source ip
	dstip	Destination ip
	srcport	Source port
	dstport	Destination port
	protocol	Protocol
	id	Event id
	url	Url of the event
	action	Event action (deny, drop, accept, etc)
	status	Event status (success, failure, etc)
	extra_data	Any extra data

Example:

The following decoder will extract the user who generated the alert and the location from where it comes:

</decoder>
  <fts>srcuser, location</fts>
  ...
</decoder>

The decoder will consider this option if the decoded event triggers a rule that uses if_fts.

ftscomment

It adds a comment to a decoder when <fts> tag is used.

Default Value	n/a
Allowed values	Any string

plugin_decoder

Use a specific plugin decoder to decode the incoming fields. It is useful for particular cases where it would be tricky to extract the fields by using regexes.

Default Value	n/a
Allowed values	PF_Decoder
	SymantecWS_Decoder
	SonicWall_Decoder
	OSSECAlert_Decoder
	JSON_Decoder

The attribute below is optional, it allows to start the decode process after a particular point of the log.

Attribute	Value
offset	after_parent after_prematch

An example of its use is described at the :doc:`JSON decoder <../json-decoder>` section.

use_own_name

Allows to set the name of the child decoder from the name attribute instead of using the name of the parent decoder.

Default Value	n/a
Allowed values	true

json_null_field

Specify how to treat the NULL fields coming from the JSON events. Only for the JSON decoder.

Default Value	string
Allowed values	string (It shows the NULL value as string)
	discard (It discard NULL fields and doesn't store them into the alert)
	empty (It shows the NULL field as an empty field)

var

Defines a variable that may be used in any place of the same file.

Attribute	Value
name	Name for the variable.

Example:

<var name="header">myprog</var>
<var name="offset">after_parent</var>
<var name="type">syscall</var>

<decoder name="syscall">
  <prematch>^$header</prematch>
</decoder>

<decoder name="syscall-child">
  <parent>syscall</parent>
  <prematch offset="$offset">^: $type </prematch>
  <regex offset="after_prematch">(\S+)</regex>
  <order>syscall</order>
</decoder>