Module Name:    mmrfc5424addhmac

Author:Rainer Gerhards <>

Available since: 7.5.6


This module adds a hmac to RFC5424 structured data if not already present. This is a custom module and uses openssl as requested by the sponsor. This works exclusively for RFC5424 formatted messages; all others are ignored.

If both mmpstrucdata and mmrfc5424addhmac are to be used, the recommended calling sequence is

  1. mmrfc5424addhmac
  2. mmpstrucdata

with that sequence, the generated hash will become available for mmpstrucdata.

Module Configuration Parameters:

Currently none.

Action Confguration Parameters:

  • key The “key” (string) to be used to generate the hmac.
  • hashfunction An openssl hash function name for the function to be used. This is passed on to openssl, so see the openssl list of supported function names.
  • sd_id The RFC5424 structured data ID to be used by this module. This is the SD-ID that will be added. Note that nothing is added if this SD-ID is already present.

Verification method

rsyslog does not contain any tools to verify a log file (this was not part of the custom project). So you need to write your own verifier.

When writing the verifier, keep in mind that the log file contains messages with the hash SD-ID included. For obvious reasons, this SD-ID was not present when the hash was created. So before the actual verification is done, this SD-ID must be removed, and the remaining (original) message be verified. Also, it is important to note that the output template must write the exact same message format that was received. Otherwise, a verification failure will obviously occur - and must so, because the message content actually was altered.

So in a more formal description, verification of a message m can be done as follows:

  1. let m’ be m with the configured SD-ID removed (everything between []). Otherwise, m’ must be an exact duplicate of m.
  2. call openssl’s HMAC function as follows: HMAC(hashfunction, key, len(key), m', len(m'), hash, &hashlen); Where hashfunction and key are the configured values and hash is an output buffer for the hash.
  3. let h be the extracted hash value obtained from m within the relevant SD-ID. Be sure to convert the hex string back to the actual byte values.
  4. now compare hash and h under consideration of the sizes. If these values match the verification succeeds, otherwise the message was modified.

If you neeed help implementing a verifier function or want to sponsor development of a verification tool, please simply email for a quote.

See Also

Caveats/Known Bugs:

  • none

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