Modify NAS-Port-Id value in mysql

[Yannick Champs]

 Hi all,

how can I modify NAS-Port-Id attribute value (truncate string operation)
before it gets recorded in MySQL database.
I've read about ulang and understand that /etc/raddb/sites-enabled/default
file should be modified.
which section "post-auth", "preacct", "accounting" should I modify ?
Is there another way to do it ?

Thank you in advance.

-- 
Yannick