IP Address Anonimization Module (mmanon)

Module Name:    mmanon

Author: Rainer Gerhards <>

Available since: 7.3.7


The mmanon module permits to anonymize IP addresses. It is a message modification module that actually changes the IP address inside the message, so after calling mmanon, the original message can no longer be obtained. Note that anonymization will break digital signatures on the message, if they exist.

How are IP-Addresses defined?

We assume that an IP address consists of four octets in dotted notation, where each of the octets has a value between 0 and 255, inclusively.


Module Configuration Parameters:

Currently none.


Action Confguration Parameters:

  • mode - default "rewrite"
    There exists the "simple" and "rewrite" mode. In simple mode, only octets as whole can be anonymized and the length of the message is never changed. This means that when the last three octets of the address are anonymized, the result will be This means that the length of the original octets is still visible and may be used to draw some privacy-evasive conclusions. This mode is slightly faster than "overwrite" mode, and this may matter in high throughput environments.
    The default "rewrite" mode will do full anonymization of any number of bits and it will also normlize the address, so that no information about the original IP address is available. So in the above example, would be anonymized to
  • ipv4.bits - default 16
    This set the number of bits that should be anonymized (bits are from the right, so lower bits are anonymized first). This setting permits to save network information while still anonymizing user-specific data. The more bits you discard, the better the anonymization obviously is. The default of 16 bits reflects what German data privacy rules consider as being sufficinetly anonymized. We assume, this can also be used as a rough but conservative guideline for other countries.
    Note: when in simple mode, only bits on a byte boundary can be specified. As such, any value other than 8, 16, 24 or 32 is invalid. If an invalid value is given, it is rounded to the next byte boundary (so we favor stronger anonymization in that case). For example, a bit value of 12 will become 16 in simple mode (an error message is also emitted).
  • replacementChar - default "x"
    In simple mode, this sets the character that the to-be-anonymized part of the IP address is to be overwritten with. In rewrite mode, this parameter is not permitted, as in this case we need not necessarily rewrite full octets. As such, the anonymized part is always zero-filled and replacementChar is of no use. If it is specified, an error message is emitted and the parameter ignored.

See Also

Caveats/Known Bugs:

  • only IPv4 is supported


In this snippet, we write one file without anonymization and another one with the message anonymized. Note that once mmanon has run, access to the original message is no longer possible (execept if stored in user variables before anonymization).

This next snippet is almost identical to the first one, but here we anonymize the full IPv4 address. Note that by modifying the number of bits, you can anonymize different parts of the address. Keep in mind that in simple mode (used here), the bit values must match IP address bytes, so for IPv4 only the values 8, 16, 24 and 32 are valid. Also, in this example the replacement is done via asterisks instead of lower-case "x"-letters. Also keep in mind that "replacementChar" can only be set in simple mode.

The next snippet is also based on the first one, but anonimzes an "odd" number of bits, 12. The value of 12 is used by some folks as a compromise between keeping privacy and still permiting to gain some more in-depth insight from log files. Note that anonymizing 12 bits may be insufficient to fulfill legal requirements (if such exist).

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This documentation is part of the rsyslog project.
Copyright © 2008-2013 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 3 or higher.