Fields Extraction Module (mmfields) — rsyslog 8.2412.0 documentation

# Fields Extraction Module (mmfields)

Module Name: | mmfields
Author: | Rainer Gerhards <rgerhards@adiscon.com>
Available since: | 7.5.1

## Purpose

The mmfield module permits to extract fields. It is an alternate to using the property replacer field extraction capabilities. In contrast to the property replacer, all fields are extracted as once and stored inside the structured data part (more precisely: they become Lumberjack [JSON] properties).

Using this module is of special advantage if a field-based log format is to be processed, like for example CEF and either a large number of fields is needed or a specific field is used multiple times inside filters. In these scenarios, mmfields potentially offers better performance than the property replacer of the RainerScript field extraction method. The reason is that mmfields extracts all fields as one big sweep, whereas the other methods extract fields individually, which requires multiple passes through the same data. On the other hand, adding field content to the rsyslog property dictionary also has some overhead, so for high-performance use cases it is suggested to do some performance testing before finally deciding which method to use. This is most important if only a smaller subset of the fields is actually needed.

In any case, mmfields provides a very handy and easy to use way to parse structured data into a it’s individual data items. Again, a primary use case was support for CEF (Common Event Format), which is made extremely easy to do with this module.

This module is implemented via the action interface. Thus it can be conditionally used depending on some prerequisites.

## Configuration Parameters

Note

Parameter names are case-insensitive.

### Action Parameters

#### separator

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
char | , | no | none

This is the character used to separate fields. Currently, only a single character is permitted, while the RainerScript method permits to specify multi-character separator strings. For CEF, this is not required. If there is actual need to support multi-character separator strings, support can relatively easy be added. It is suggested to request it on the rsyslog mailing list, together with the use case - we intend to add functionality only if there is a real use case behind the request (in the past we too-often implemented things that actually never got used). The fields are named fnbr, where nbr is the field number starting with one and being incremented for each field.

#### jsonRoot

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
string | ! | no | none

This parameters specifies into which json path the extracted fields shall be written. The default is to use the json root object itself.

## Examples

### Parsing messages and writing them to file

This is a very simple use case where each message is parsed. The default separator character of comma is being used.

    module(load="mmfields")
    template(name="ftpl"
             type="string"
             string="%$!%\\n")
    action(type="mmfields")
    action(type="omfile"
           file="/path/to/logfile"
           template="ftpl")

### Writing into a specific json path

The following sample is similar to the previous one, but this time the colon is used as separator and data is written into the “$!mmfields” json path.

    module(load="mmfields")
    template(name="ftpl"
             type="string"
             string="%$!%\\n")
    action(type="mmfields"
           separator=":"
           jsonRoot="!mmfields")
    action(type="omfile"
           file="/path/to/logfile"
           template="ftpl")

See also

Help with configuring/using `Rsyslog`:

- Mailing list - best route for general questions

- GitHub: rsyslog source project - detailed questions, reporting issues that are believed to be bugs with `Rsyslog`

See also

Contributing to `Rsyslog`:

- Source project: rsyslog project README.

- Documentation: rsyslog-doc project README

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.