new query verification in sql.conf

[Jean Carlos Oliveira Guandalini]

I use freeradius for authentication of pppoe wifi.

I need to make new sql query in a table with a list of mac-address, if
the CallingStationId will be equal to the some mac-adress of the table
then will not have to be connected.
A system of mac-adress blacklist.
I tried to make adding one query in sql.conf but it does not function.

I find that it would have to add a new function in rlm_sql.c, but am not
habil C programmer.

Somebody can help me?

Sorry for my english

Thanks

Jean