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FreeRADIUS defines the connected RADIUS clients in the file /etc/raddb/clients.conf. This file needs to hold all your connected Access Points (and/or wired eduroam-enabled switches, if you have these instead of Access Points). You set a shared secret for each client and define these in the config file as follows:
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Use the most recent version available (3.0.10 at the time of writing) because of known issues in older versions (ranging from filters that prevent people to get online with mixed usernames to TLS-related bugs).