Book excerpt: CentOS is the enterprise-grade Linux operating system built using the same source code as Red Hat Enterprise Linux (RHEL) to provide a free-to-use alternative to Red Hat's commercial Linux offering. The purpose of this book is to build on your understanding of CentOS and to explore those mission-critical services you are entrusted to manage and maintain. Starting with a brief introduction to the overall subject of troubleshooting a CentOS server, this book will take you on a journey across the whole spectrum of issue-based problem solving, which includes active processes, the networking environment, package management, users, folders, files, shared resources, security, databases, and web-based services. By the end of the book, you will have expert-level competency in identifying and diagnosing the root causes of CentOS storage, network, and administration issues and resolving them.