Link MAC adress to user after 1st login

[Jurgen van Vliet]

Hi people, 

Im wondering if the following is possible with freeradius:

At the 1st login of a client, the MAC adress is put into the database
At all next logins, if the MAC adress of that client does not match the
allready recorded MAC adress the login gets rejected.

I presume the MAC adress of the client requesting authentication is a
variable in freeradius ? 
If so, I allready use a stored procedure in mysql as post_auth, I can easy
write the mac into the DB.

My question is how/where do I put the check if the MAC allready is registred
and if the current MAC matches the registred one ?

Thank you in advance for thinking with me.

Jurgen