rsyslog features a new message modification module, that will check for the SD ID in RFC5424 messages and append a HMAC hash to the structured data part of the message. Please note, that even if the module works on all messages, only RFC5424 messages will be processed.
Before starting, you need a private Enterprise Number from IANA so you can use the module to add the HMAC hash to the message.
For the functionality you need the module "mmrfc5424addhmac". This is currently available only in the git master branch and will be first released in the next devel release 7.5.4 and for stable in 7.6.0.
When doing the configure, please do not forget to enable this module:
./configure --prefix=/usr --enable-imtcp --enable-mmrfc5424addhmac
Now for the configuration:
action(type="mmrfc5424addhmac" key="yourenterprisekey" hashFunction="sha256" sd_id="id@32473")
template(name="addhmac" type="string" string="<%PRI%>1 %TIMESTAMP:::date-rfc3339% %HOSTNAME% %APP-NAME% %PROCID% %MSGID% %STRUCTURED-DATA% %msg%\n")
action(type="omfile" file="/var/log/logfile" template="addhmac")
This is a relatively simple configuration. We load the modules imtcp and mmrfc5424addhmac. We will receive all RFC5424 messages through our tcp input. The really interesting part are the actions. Since we need to modify the messages, we need an action with the message modification module. As parameters for the action we define a key that will be used to create the hash, choose a hash function (basically all hash functions from openssl work) and define our SD ID which consists of a name, "@" and the ID received from IANA.
The message will be parsed for the ID, if it exists, a hash will be generated and appended to the structured data of the message.
Now we need to do something with these messages. The template above is a RFC5424 representation and gives out accordingly formatted messages. As follow-up action we will write all messages into a file. Alternatively, you could also forward them to another host or write them into a database.