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is it possible to intercept syslog messages and evaluate (in special cases change) them before they are written into /var/log/?

At the moment i m using inotify to monitor the log file and write the evaluated log into a new file.

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Yikes. Why? What are you actually trying to accomplish here? Either fix the application that is generating the logs or use something like logstash to collect and post-process your logs. –  EEAA Jun 11 '13 at 4:36

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up vote 2 down vote accepted

Yes, it is possible. You can do it with syslog-ng and filters if you're creative.

No, I'm not going to tell you specifically how to do it, because it's a TERRIBLE idea.
As a general rule you do not alter the data you're logging. You log raw data, and post-process it prior to displaying it if necessary. Altering log data destroys the integrity of the logging process - it's the equivalent to planting evidence in a criminal case.

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...or (in a untrusted environment) not telling the whole world, when i did something why and from where ... sometimes ln -s /dev/null can't be used. and thank you for the syslog-ng hint. –  inselberg Jun 11 '13 at 23:51
    
If you're in an untrusted environment you probably don't have access to the log stream. (Conversely, if you have control of the logging you should have control of everything else too - you usually need to be root to pick the program that handles the logs.) The point of logging is for the sysadmin (you) to know when someone else is doing something they shouldn't be. If YOU are doing something you shouldn't be, knock it off! :-) –  voretaq7 Jun 12 '13 at 0:56
    
i think my definition of "untrusted" is different to yours: not located next to me, has an internet connection, unencrypted and it doesn't matter if i m "root" and pay for it. is this yet a philosophic approach about "trust"? :D –  inselberg Jun 12 '13 at 13:27
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Your definition of untrusted is unreasonable. I've never worked on a machine that you would consider "trusted" (because I colocate servers - they don't sit under my desk). If you have full control over the system's OS you should be able to trust it. If you don't you need to seriously re-evaluate your hosting situation. If you would like to continue the philosophical discussion of when a machine is/isn't trusted stop by the SF chat room –  voretaq7 Jun 12 '13 at 15:15

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