Modify NAS-Port-Id value in mysql

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