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According to the description of work [19, Annex 1], the XtreemOS operating system is composed of two parts: 1) the foundation layer (XtreemOS-F), which is a modified Linux system, embedding Virtual Organization (VO) support mechanisms and providing an appropriate interface to implement 2) the high level operating system services, (XtreemOS-G). This document presents the specification of Linux-XtreemOS (Linux-XOS), the instance of XtreemOS-F for grid nodes consisting of a single computer (PC). It results mainly from the joint work that has been carried out by INRIA, CCLRC, CNR, NEC, SAP, ICT and TID since the beginning of the project (June 2006) in WorkpackageWP2.1, and from discussions with other XtreemOS partners, especially those involved in SP3 sub-project (XtreemOS-G specification) and in WP4.2 workpackage (application requirements). The presentation follows two main directions: the specification of features to support Virtual Organizations, and that of features to support application checkpointing. In both cases, we put an emphasis on maximizing the acceptance likelihood of the XtreemOS design from the Linux community, by leveraging existing well-established standards and accepted tools in the XtreemOS-F software architecture. The goal of VO support in XtreemOS is to provide mechanisms to set up and manage VOs in a scalable and flexible manner, and mechanisms which ensure access to various resources with fine-grained, mandatory access control without sacrificing site autonomy. We have identified several issues to be considered (§2.1), by analyzing requirements gathered from XtreemOS use cases, and also from other EU funded research projects (EGEE and Akogrimo) as well as the OGSA standard from GGF. The analysis of the state of the art (§2.2) on management of VOs makes evident that despite significant advances, there are still open issues with respect to a) scalability of in-the-large VO management, especially for short-lived and dynamically changing VOs, b) ease of management of VOs and VO identities, and c) security and VO policy enforcement at the node and site level, where current VO middleware cannot fully leverage the native OS. We distinguish two levels in VO management: VO level (administration) and node level. VO-level management is performed by XtreemOS-G services (WPs 3.2, 3.3, 3.4 and 3.5), and it includes distributed information management for membership tracking and accounting of users and resources. The first of the two tasks of WP 2.1 is to add local mechanisms for recognizing, controlling and enforcing usage of global Grid entities to the standard Linux kernel, which is unaware of Grid entities. We have identified the main responsibilities of node-level management, which cover the areas of (Grid) identity management, resource access granting, VO policy checking, auditing, and enforcing. In this document (see §2.3) we give an overview of the basic implementation of VO support in XtreemOS, and how the