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is it possible to exclude particular user to not being logged in the radius.log file ?

Not really. If you're logging user authentications, they *all* get logged.

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I have some users that periodically connect and download config files from the routers and they filled the log quite a lot

Then don't log user authentications in radius.log. Log them in SQL or something else.

Hi alan , thanks for you answer. Actually i'm not logging into radius.log, bu to be precise i'm sending the logs into syslog at the facility local1 Is is the same in this case ? thanks pet