Different LDAP base DN for different realms

I have an LDAP directory organized as follows: dc=example,dc=org |-ou=first | \-ou=people | \-uid=john | |-ou=second | \-ou=people | \-uid=john | \-ou=third \-ou=people \-uid=john I would like to tell FR to do look in the appropriate OU based on the relam the user authenticates, for example: john@first will try to bind on uid=john,ou=people,ou=first,dc=example,dc=org john@second on uid=john,ou=people,ou=second,dc=example,dc=org ans so on... How can I achieve this with virtual servers or unlang? Thank you very much!