Security and Hardening Guide

Deals with the particulars of installing and setting up a secure SUSE Linux Enterprise Server, and additional post-installation processes required to further secure and harden that installation. Supports the administrator with security-related choices and decisions.

The SUSE Linux Enterprise Server Security and Hardening Guide deals with the particulars of installation and set up of a secure SUSE Linux Enterprise Server and additional post-install processes required to further secure and harden that installation. Security and hardening elements and procedures are best applied to a server both during installation and post-installation and aim to improve the fitness of the system for the purposes demanded by its administrator.

This guide supports administrator in making security related choices and decisions. The individual steps and procedures should be seen as proposals, not as strict rules. You will often need to evaluate the usefulness of measures for your organization yourself.

The objective is to improve the security value of the system. Definitions about the meaning of the term security vary, but we want to settle on one that is both simple and abstract:

A good system does what it is expected to do, and it does it well.

A secure system is a good system that does nothing else.

The focus of this guide lies on doing nothing else. The Linux system is constructed in such way that security policies are enforced. These policies consist of the following concepts (fairly generic and incomplete list):

  • DAC (Discretionary Access Control): File and directory permissions, as set by chmod and chown.

  • Privileged ports: TCP and UDP ports 0-1023 and raw sockets can only be used by root.

  • Other privileged operations: Loading kernel modules, configuring network interfaces, all security relevant settings of the Linux kernel. These are operations that can only be done by the root user, that is the user with the user ID 0, or any other process with the necessary capabilities.

Attacking a system means to attempt to overcome privilege boundaries, for example by circumventing or breaking them. That means the administrator or programmer of the system has not anticipated this scenario.

A hardened system raises the bar by reducing the area that the system exposes to the attacker (often called attack surface). A hardened system can also provide measures to reduce the impact of vulnerabilities in the parts of the systems that must be exposed to a potential attacker.

Security is about decisions, and whenever security is in (apparent) opposition to function, these decisions become trade-offs. While it can be argued that all systems should be set up to be as securely as possible, some levels of security and hardening may very well be overkill in some cases. Each system's operational environment has its own security requirements derived from business drivers or regulatory compliance mandates. SUSE Linux Enterprise Server can, for example, be configured to comply with security standards, such as SOX, HIPAA and PCIDSS. It can also be set up to fulfill the requirements from the German Federal Office of Information Security (Bundesamt für Sicherheit in der Informationstechnik) as described in BSI TR-02102-1. An effective business requirements analysis should be performed to determine the right level of security and hardening to be applied to a server or defined as part of a baseline server build.

As a final note before we begin: You may encounter individual requirements in regulatory compliance frameworks that may not make sense from a technical perspective, or they do not serve the purpose of improving security. It may be a productive attitude to simply implement what is required, but whenever there is a contradiction to security, an informed discussion in the documentation serves the overall purpose of your regulative compliance framework much more than blindly obeying the specifications. Feel encouraged to dispute list items that you think are counterproductive.

Assumptions and Scope

References in this document will usually be made to a single server target or host, however the scope can generally be applied to more than one machine. We generally assume that the security target can cover one or more systems running SUSE Linux Enterprise Server.

We explicitly do not make any assumptions about the hostility of the network that the systems are connected to, or the cooperative nature of the users that leverage the services provided by the systems.

In turn, this means that you partially define your context on your own when reading through this document. You will need to broaden the meaning of individual portions to adapt it to your environment. In some cases, such as the use case of a server that is exposed to the Internet, this document may even be insufficient or incomplete; however, it may still serve as a good starting point on your journey toward an increased level of confidence that your system will behave like you want it to.

About trust: Trust relationships exist among all systems that participate in networked transactions. In this way, the trust relationship between the people that use the systems is transported across these systems. The chain that is formed by your trust relationships is only as strong as the weakest link. It is good practice to graphically visualize the trust relationships with the services in a schematic overview or map of your network. Generally, it is up to the owner of a resource to enforce the policies imposed on that resource; this would usually be the server that provides the resource. The client that opens a connection to request the resource can only be made responsible for the actions that it performs. This refers to the action of opening the connection to start with, but to nothing else as such.

The case of hostile users is special and unique: The Human Resources department may be able to solve some security problems in your computing environment; in addition, some technical measures can be taken. Make sure that the necessary regulations in your environment fit your needs, and that they back your intentions instead of obstructing them if you need to work around a missing support from your HR department (and your management).

Persons that have administrative privileges on a system are automatically considered trusted.

A Linux system—without any additional security frameworks such as SELinux—is a single level security system: From a security policy perspective there is only the superuser (root) and non-privileged users. System users are non-root user IDs that have access to files specific to their purpose. The separation of administrative duties is complicated by this simplicity. Some tools help: Use sudo(8) for administrative tasks, but be aware that after the privilege boundary is crossed, a program running with root privileges does not enforce any file access policies for non-privileged users anymore. vi(1) that runs as root can read and write to any file in the system.

Another tool to mitigate the risk of abuse or accidental misuse of administrative privileges is NetIQ's Privileged User Manager product. More information is available here:

Physical security of the server is another assumption made here, where the server is protected from theft and manipulation by unauthorized persons. A common sobering thought among security professionals is the ten-second Denial of Service: Unplug the wires and reboot the server. Physical security must be ensured and physical access must be controlled. Otherwise, all assumptions about at least the availability of these systems are void.

NOTE: Cryptography

The use of cryptography to protect the confidentiality of transactions with the services that your system provides is generally encouraged. The need to implement cryptographic enhancements is strongly dependent on the operational environments of all participating systems. Keep in mind that you need to verify all of the possible security benefits that cryptography can provide, for all of your services, and that these benefits are not delivered automatically by turning on the encrypt option of your service (if you can enjoy the idyllic situation where encryption is available as a button to check):

Confidentiality

Protection against reading the content of a transaction

Privacy

Protection against knowing that a transaction exists, and some properties that it may have, such as size, identities of involved parties, their presence, etc.

Integrity

Protection against alteration of content. Be aware that cryptography does not automatically provide this kind of protection.

Authenticity

Protection against identity fraud. Cryptography that does not know about identities of participating entities cannot deliver this value.

Keep in mind that encryption of data for confidentiality purposes can merely reduce the size of the data to protect from the actual size to the size of the key that is used to encrypt the data. This results in a key exchange problem for encrypted transactions, and in a key management problem for encrypted data storage. Since data is (typically, there are exceptions!) processed in clear, you need your vault unlocked while data within is being worked with. The encryption of such data on the file system or block device layer helps against the theft of the system, but it does not help the confidentiality of the data while the system is running.

If you want to implement a consistent security policy covering multiple hosts on a network then organizational procedures must ensure that all those hosts can be trusted and are configured with compatible security configurations enforcing an organization wide security policy. Isolation of groups of systems that maintain data of the same trust domain can provide an adequate means of control; ultimately, the access controls to these systems, both for end users and for other systems, need to be carefully designed, configured, inspected and monitored.

IMPORTANT: Trusting Data

Data can only be trusted to the degree that is associated with the domain it comes from. If data leaves the domain in which security policies can be enforced, it should consequently be associated with the trust of the target domain.

For a review of industry best practices on security, the development of sound security processes, controls, development, reviews, audit practices and incident management, you can review a public RFC (request for comments). RFC 2196 is the ongoing work of the world-wide community and individual security and process experts. You can review it online here: http://www.faqs.org/rfcs/rfc2196.html. An RFC is an open and living document that invites comments and review. Enhancements and improvements are welcome; you will find instructions on where to send those suggestions within the document itself.

This guide provides initial guidance on how to set up and secure a SUSE Linux Enterprise Server installation but it is not intended to be the only information required for a system administrator to learn how to operate Linux securely. Assumptions are made within this guide that the reader has knowledge and understanding of operating security principles in general, and of Linux administrative commands and configuration options in particular.

Contents of this Book

Section 1.0, Common Criteria contains a reference to Common Criteria and SUSE Linux Enterprise Server. Section 2.0, Linux Security and Service Protection Methods contains more general system security and service protection schemes.

Available Documentation

NOTE: Online Documentation and Latest Updates

Documentation for our products is available at http://www.suse.com/documentation/, where you can also find the latest updates, and browse or download the documentation in various formats.

In addition, the product documentation is usually available in your installed system under /usr/share/doc/manual.

The following documentation is available for this product:

Installation Quick Start, (↑Quick Start Manuals)

This Quick Start guides you step-by-step through the installation of SUSE® Linux Enterprise Server 15.

Deployment Guide, (↑Deployment Guide)

Shows how to install single or multiple systems and how to exploit the product inherent capabilities for a deployment infrastructure. Choose from various approaches, ranging from a local installation or a network installation server to a mass deployment using a remote-controlled, highly-customized, and automated installation technique.

Administration Guide, (↑Administration Guide)

Covers system administration tasks like maintaining, monitoring and customizing an initially installed system.

Virtualization Guide, (↑Virtualization Guide)

Describes virtualization technology in general, and introduces libvirt—the unified interface to virtualization—and detailed information on specific hypervisors.

Storage Administration Guide, (↑Storage Administration Guide)

Provides information about how to manage storage devices on a SUSE Linux Enterprise Server.

AutoYaST Guide, (↑AutoYaST Guide)

AutoYaST is a system for unattended mass deployment of SUSE Linux Enterprise Server systems using an AutoYaST profile containing installation and configuration data. The manual guides you through the basic steps of auto-installation: preparation, installation, and configuration.

Security Guide, (↑Security Guide)

Introduces basic concepts of system security, covering both local and network security aspects. Shows how to use the product inherent security software like AppArmor or the auditing system that reliably collects information about any security-relevant events.

Security and Hardening Guide

Deals with the particulars of installing and setting up a secure SUSE Linux Enterprise Server, and additional post-installation processes required to further secure and harden that installation. Supports the administrator with security-related choices and decisions.

System Analysis and Tuning Guide, (↑System Analysis and Tuning Guide)

An administrator's guide for problem detection, resolution and optimization. Find how to inspect and optimize your system by means of monitoring tools and how to efficiently manage resources. Also contains an overview of common problems and solutions and of additional help and documentation resources.

Repository Mirroring Tool for SLES 15, (↑Repository Mirroring Tool for SLES 15)

An administrator's guide to Subscription Management Tool—a proxy system for SUSE Customer Center with repository and registration targets. Learn how to install and configure a local SMT server, mirror and manage repositories, manage client machines, and configure clients to use SMT.

GNOME User Guide, (↑GNOME User Guide)

Introduces the GNOME desktop of SUSE Linux Enterprise Server. It guides you through using and configuring the desktop and helps you perform key tasks. It is intended mainly for end users who want to make efficient use of GNOME as their default desktop.

Feedback

Several feedback channels are available:

Bugs and Enhancement Requests

For services and support options available for your product, refer to http://www.suse.com/support/.

Help for openSUSE is provided by the community. Refer to https://en.opensuse.org/Portal:Support for more information.

To report bugs for a product component, go to https://scc.suse.com/support/requests, log in, and click Create New.

User Comments

We want to hear your comments about and suggestions for this manual and the other documentation included with this product. Use the User Comments feature at the bottom of each page in the online documentation or go to http://www.suse.com/documentation/feedback.html and enter your comments there.

Mail

For feedback on the documentation of this product, you can also send a mail to doc-team@suse.com. Make sure to include the document title, the product version and the publication date of the documentation. To report errors or suggest enhancements, provide a concise description of the problem and refer to the respective section number and page (or URL).

Documentation Conventions

The following notices and typographical conventions are used in this documentation:

  • /etc/passwd: directory names and file names

  • PLACEHOLDER: replace PLACEHOLDER with the actual value

  • PATH: the environment variable PATH

  • ls, --help: commands, options, and parameters

  • user: users or groups

  • package name : name of a package

  • Alt, Alt+F1: a key to press or a key combination; keys are shown in uppercase as on a keyboard

  • File, File > Save As: menu items, buttons

  • This paragraph is only relevant for the AMD64/Intel 64 architecture. The arrows mark the beginning and the end of the text block.

    This paragraph is only relevant for the architectures IBM Z and POWER. The arrows mark the beginning and the end of the text block.

  • Dancing Penguins (Chapter Penguins, ↑Another Manual): This is a reference to a chapter in another manual.

  • Commands that must be run with root privileges. Often you can also prefix these commands with the sudo command to run them as non-privileged user.

    root # command
    tux > sudo command
  • Commands that can be run by non-privileged users.

    tux > command
  • Notices

    WARNING: Warning Notice

    Vital information you must be aware of before proceeding. Warns you about security issues, potential loss of data, damage to hardware, or physical hazards.

    IMPORTANT: Important Notice

    Important information you should be aware of before proceeding.

    NOTE: Note Notice

    Additional information, for example about differences in software versions.

    HINT: Tip Notice

    Helpful information, like a guideline or a piece of practical advice.