Large-scale commodity clusters are used in an increasing number of domains: academic, research, and industrial environments. At the same time, these clusters are exposed to an increasing number of attacks coming from public networks. Therefore, mechanisms for efficiently and flexibly managing security have now become an essential requirement for clusters. However, despite the growing importance of cluster security, this field has been only minimally addressed by contemporary cluster administration techniques. This paper presents a high-level view of existing security challenges related to clusters and proposes a structured approach for handling security in clustered servers. The goal of this paper is to identify various necessarily-distributed security services and their related characteristics as a means of enhancing cluster security.