Fix invalid UTF-8 Sequences (mmutf8fix)

Module Name: mmutf8fix

Author: Rainer Gerhards <>

Available since: 7.5.4


The mmutf8fix module permits to fix invalid UTF-8 sequences. Most often, such invalid sequences result from syslog sources sending in non-UTF character sets, e.g. ISO 8859. As syslog does not have a way to convey the character set information, these sequences are not properly handled. While they are typically uncritical with plain text files, they can cause big headache with database sources as well as systems like ElasticSearch.

The module supports different “fixing” modes and fixes. The current implementation will always replace invalid bytes with a single US ASCII character. Additional replacement modes will probably be added in the future, depending on user demand. In the longer term it could also be evolved into an any-charset-to-UTF8 converter. But first let’s see if it really gets into widespread enough use.

Proper Usage:

Some notes are due for proper use of this module. This is a message modification module utilizing the action interface, which means you call it like an action. This gives great flexibility on the question on when and how to call this module. Note that once it has been called, it actually modifies the message. The original message is then no longer available. However, this does not change any properties set, used or extracted before the modification is done.

One potential use case is to normalize all messages. This is done by simply calling mmutf8fix right in front of all other actions.

If only a specific source (or set of sources) is known to cause problems, mmutf8fix can be conditionally called only on messages from them. This also offers performance benefits. If such multiple sources exists, it probably is a good idea to define different listeners for their incoming traffic, bind them to specific ruleset and call mmutf8fix as first action in this ruleset.

Module Configuration Parameters:

Note: parameter names are case-insensitive.

Currently none.

Action Confguration Parameters:

Note: parameter names are case-insensitive.

  • mode - utf-8/controlcharacters

    This sets the basic detection mode. In utf-8 mode (the default), proper UTF-8 encoding is checked and bytes which are not proper UTF-8 sequences are acted on. If a proper multi-byte start sequence byte is detected but any of the following bytes is invalid, the whole sequence is replaced by the replacement method. This mode is most useful with non-US-ASCII character sets, which validly includes multibyte sequences. Note that in this mode control characters are NOT being replaced, because they are valid UTF-8. In controlcharacters mode, all bytes which do not represent a printable US-ASCII character (codes 32 to 126) are replaced. Note that this also mangles valid UTF-8 multi-byte sequences, as these are (deliberately) outside of that character range. This mode is most useful if it is known that no characters outside of the US-ASCII alphabet need to be processed.

  • replacementChar - default “ “ (space), a single character

    This is the character that invalid sequences are replaced by. Currently, it MUST be a printable US-ASCII character.

Caveats/Known Bugs:

  • overlong UTF-8 encodings are currently not detected in utf-8 mode.


In this snippet, we write one file without fixing UTF-8 and another one with the message fixed. Note that once mmutf8fix has run, access to the original message is no longer possible.

module(load="mmutf8fix") action(type="omfile"
file="/path/to/non-fixed.log") action(type="mmutf8fix")
action(type="omfile" file="/path/to/fixed.log")

In this sample, we fix only message originating from host

module(load="mmutf8fix") if $fromhost-ip == "" then
action(type="mmutf8fix") # all other actions here...

This is mostly the same as the previous sample, but uses “controlcharacters” processing mode.

module(load="mmutf8fix") if $fromhost-ip == "" then
action(type="mmutf8fix" mode="controlcharacters") # all other actions here...

See also

Help with configuring/using Rsyslog:

See also

Contributing to Rsyslog:

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