The latest pull request to syslog-ng adds a really useful feature: the flip-parser(): https://github.com/syslog-ng/syslog-ng/pull/3971
It allows you to flip the message text, reverse it, or both. As I also reported a couple of minor problems related to UTF-8 character handling, this PR most likely will not be merged today. However, you can compile it yourself, or if you use openSUSE Tumbleweed, use my packages from the openSUSE Build Service.
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