The res_odbc.conf file:
[my_freetds_dsn]
enabled => yes
dsn => my_freetds_dsn
username => my_freetds_user
password => my_freetds_password
pre-connect => yes
sanitysql => select 1

odbcinst.ini:

[FreeTDS]
Description = ODBC for Microsoft SQL
Driver = /usr/lib64/libtdsodbc.so.0
UsageCount = 1
Threading = 2

odbc.ini:
[my_freetds_instance]
Description = my_freetds_instance
Driver = FreeTDS
Database = my_freetds_instanace
Server = my_mssql_server
Trace = no
TDS_Version = 7.2
Port = my_mssql_port
timeout = 10
connect_timeout = 5