Use this documentation with care! It describes the outdated version 7, which was actively developed around 2014 and is considered dead by the rsyslog team.

This documentation reflects the latest update of the v7-stable branch. It describes the 7.6.8 version, which was never released. As such, it contains some content that does not apply to any released version.

To obtain the doc that properly matches your installed v7 version, obtain the doc set from your distro. Each version of rsyslog contained the version that exactly matches it.

As general advise, it is strongly suggested to upgrade to the current version supported by the rsyslog project. The current version can always be found on the right-hand side info box on the rsyslog web site.

Note that there is only limited rsyslog community support available for the outdated v7 version (officially we do not support it at all, but we usually are able to answer simple questions). If you need to stick with v7, it probably is best to ask your distribution for support.


Fields Extraction Module (mmfields)

Module Name:    mmfields

Author:Rainer Gerhards <>

Available since: 7.5.1


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 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 primiary 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 prequisites.

Module Configuration Parameters:

Currently none.

Action Confguration Parameters:

  • separator - separatorChar (default ‘,’) 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 - path (default “!”) This parameters specifies into which json path the extracted fields shall be written. The default is to use the json root object itself.

Caveats/Known Bugs:

  • Currently none.


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=”omfields”) action(type=”omfile” file=”/path/to/logfile” template=”ftpl”)

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=”omfields” separator=”:” jsonRoot=”!mmfields”) action(type=”omfile” file=”/path/to/logfile” template=”ftpl”)

[rsyslog.conf overview] [manual index] [rsyslog site]

This documentation is part of the rsyslog project. Copyright © 2013 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 3 or higher.

Previous topic


Next topic

Log Message Normalization Module

This Page