1. In a directory server, a system capable of interacting with entries organized in a tree structure, each entry having attributes, the attributes comprising real attributes each having a value stored in the entry, the system comprising: a mechanism capable of associating a virtual attribute to an entry subject to a virtual attribute condition being verified, the virtual attribute condition being derived from data located elsewhere in the tree structure; and a resolving function capable of receiving a first filter expression based on a virtual attribute, the resolving function for converting the first filter expression into one or more second filter expressions comprising real attributes and computed from the first filter expression and from the virtual attribute condition.