Security Guide

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Red Hat Enterprise Linux 6

A Guide to Securing Red Hat Enterprise Linux


Mirek Jahoda

Red Hat Customer Content Services

Robert Krátký

Red Hat Customer Content Services

Martin Prpič

Red Hat Customer Content Services

Tomáš Čapek

Red Hat Customer Content Services

Stephen Wadeley

Red Hat Customer Content Services

Yoana Ruseva

Red Hat Customer Content Services

Miroslav Svoboda

Red Hat Customer Content Services


This book assists users and administrators in learning the processes and practices of securing workstations and servers against local and remote intrusion, exploitation and malicious activity.
Focused on Red Hat Enterprise Linux but detailing concepts and techniques valid for all Linux systems, this guide details the planning and the tools involved in creating a secured computing environment for the data center, workplace, and home.
With proper administrative knowledge, vigilance, and tools, systems running Linux can be both fully functional and secured from most common intrusion and exploit methods.

Chapter 1. Security Overview

Due to the increased reliance on powerful, networked computers to help run businesses and keep track of our personal information, entire industries have been formed around the practice of network and computer security. Enterprises have solicited the knowledge and skills of security experts to properly audit systems and tailor solutions to fit the operating requirements of their organization. Because most organizations are increasingly dynamic in nature, their workers are accessing critical company IT resources locally and remotely, hence the need for secure computing environments has become more pronounced.
Unfortunately, many organizations (as well as individual users) regard security as more of an afterthought, a process that is overlooked in favor of increased power, productivity, convenience, ease of use, and budgetary concerns. Proper security implementation is often enacted postmortem — after an unauthorized intrusion has already occurred. Taking the correct measures prior to connecting a site to an untrusted network, such as the Internet, is an effective means of thwarting many attempts at intrusion.


This document makes several references to files in the /lib directory. When using 64-bit systems, some of the files mentioned may instead be located in /lib64.

1.1. Introduction to Security

1.1.1. What is Computer Security?

Computer security is a general term that covers a wide area of computing and information processing. Industries that depend on computer systems and networks to conduct daily business transactions and access critical information regard their data as an important part of their overall assets. Several terms and metrics have entered our daily business vocabulary, such as total cost of ownership (TCO), return on investment (ROI), and quality of service (QoS). Using these metrics, industries can calculate aspects such as data integrity and high-availability (HA) as part of their planning and process management costs. In some industries, such as electronic commerce, the availability and trustworthiness of data can mean the difference between success and failure. How did Computer Security come about?
Information security has evolved over the years due to the increasing reliance on public networks not to disclose personal, financial, and other restricted information. There are numerous instances such as the Mitnick[1] and the Vladimir Levin[2] cases that prompted organizations across all industries to re-think the way they handle information, including its transmission and disclosure. The popularity of the Internet was one of the most important developments that prompted an intensified effort in data security.
An ever-growing number of people are using their personal computers to gain access to the resources that the Internet has to offer. From research and information retrieval to electronic mail and commerce transactions, the Internet has been regarded as one of the most important developments of the 20th century.
The Internet and its earlier protocols, however, were developed as a trust-based system. That is, the Internet Protocol (IP) was not designed to be secure in itself. There are no approved security standards built into the TCP/IP communications stack, leaving it open to potentially malicious users and processes across the network. Modern developments have made Internet communication more secure, but there are still several incidents that gain national attention and alert us to the fact that nothing is completely safe. Security Today
In February of 2000, a Distributed Denial of Service (DDoS) attack was unleashed on several of the most heavily-trafficked sites on the Internet. The attack rendered,,,, and several other sites completely unreachable to normal users, as it tied up routers for several hours with large-byte ICMP packet transfers, also called a ping flood. The attack was brought on by unknown attackers using specially created, widely available programs that scanned vulnerable network servers, installed client applications called Trojans on the servers. Then they timed an attack with every infected server flooding the victim sites and rendering them unavailable. Many blame the attack on fundamental flaws in the way routers and the protocols used are structured to accept all incoming data, regardless of the purpose of the packets or where they were sent to.will possibly be removed
In 2007, a data breach exploiting the widely-known weaknesses of the Wired Equivalent Privacy (WEP) wireless encryption protocol resulted in the theft from a global financial institution of over 45 million credit card numbers.
Unfortunately, system and network security can be a difficult proposition, requiring an intricate knowledge of how an organization regards, uses, manipulates, and transmits its information. Understanding the way an organization (and the people who make up the organization) conducts business is paramount to implementing a proper security plan. Standardizing Security
Enterprises in every industry rely on regulations and rules that are set by standards-making bodies such as the American Medical Association (AMA) or the Institute of Electrical and Electronics Engineers (IEEE). The same ideals hold true for information security. Many security consultants and vendors agree upon the standard security model known as CIA, or Confidentiality, Integrity, and Availability. This three-tiered model is a generally accepted component to assessing risks of sensitive information and establishing security policy. The following describes the CIA model in further detail:
  • Confidentiality — Sensitive information must be available only to a set of pre-defined individuals. Unauthorized transmission and usage of information should be restricted. For example, confidentiality of information ensures that a customer's personal or financial information is not obtained by an unauthorized individual for malicious purposes such as identity theft or credit fraud.
  • Integrity — Information should not be altered in ways that render it incomplete or incorrect. Unauthorized users should be restricted from the ability to modify or destroy sensitive information.
  • Availability — Information should be accessible to authorized users any time that it is needed. Availability is a warranty that information can be obtained with an agreed-upon frequency and timeliness. This is often measured in terms of percentages and agreed to formally in Service Level Agreements (SLAs) used by network service providers and their enterprise clients.

1.1.2. SELinux

Red Hat Enterprise Linux includes an enhancement to the Linux kernel called SELinux, which implements a Mandatory Access Control (MAC) architecture that provides a fine-grained level of control over files, processes, users and applications in the system. Detailed discussion of SELinux is beyond the scope of this document; however, for more information on SELinux and its use in Red Hat Enterprise Linux, see the Red Hat Enterprise Linux SELinux User Guide. For more information on configuring and running services that are protected by SELinux, see the SELinux Managing Confined Services Guide. Other available resources for SELinux are listed in Chapter 11, References.

1.1.3. Security Controls

Computer security is often divided into three distinct master categories, commonly referred to as controls:
  • Physical
  • Technical
  • Administrative
These three broad categories define the main objectives of proper security implementation. Within these controls are sub-categories that further detail the controls and how to implement them. Physical Controls
Physical control is the implementation of security measures in a defined structure used to deter or prevent unauthorized access to sensitive material. Examples of physical controls are:
  • Closed-circuit surveillance cameras
  • Motion or thermal alarm systems
  • Security guards
  • Picture IDs
  • Locked and dead-bolted steel doors
  • Biometrics (includes fingerprint, voice, face, iris, handwriting, and other automated methods used to recognize individuals) Technical Controls
Technical controls use technology as a basis for controlling the access and usage of sensitive data throughout a physical structure and over a network. Technical controls are far-reaching in scope and encompass such technologies as:
  • Encryption
  • Smart cards
  • Network authentication
  • Access control lists (ACLs)
  • File integrity auditing software Administrative Controls
Administrative controls define the human factors of security. They involve all levels of personnel within an organization and determine which users have access to what resources and information by such means as:
  • Training and awareness
  • Disaster preparedness and recovery plans
  • Personnel recruitment and separation strategies
  • Personnel registration and accounting

1.1.4. Conclusion

Now that you have learned about the origins, reasons, and aspects of security, you will find it easier to determine the appropriate course of action with regard to Red Hat Enterprise Linux. It is important to know what factors and conditions make up security in order to plan and implement a proper strategy. With this information in mind, the process can be formalized and the path becomes clearer as you delve deeper into the specifics of the security process.

1.2. Vulnerability Assessment

Given time, resources, and motivation, an attacker can break into nearly any system. All of the security procedures and technologies currently available cannot guarantee that any systems are completely safe from intrusion. Routers help secure gateways to the Internet. Firewalls help secure the edge of the network. Virtual Private Networks safely pass data in an encrypted stream. Intrusion detection systems warn you of malicious activity. However, the success of each of these technologies is dependent upon a number of variables, including:
  • The expertise of the staff responsible for configuring, monitoring, and maintaining the technologies.
  • The ability to patch and update services and kernels quickly and efficiently.
  • The ability of those responsible to keep constant vigilance over the network.
Given the dynamic state of data systems and technologies, securing corporate resources can be quite complex. Due to this complexity, it is often difficult to find expert resources for all of your systems. While it is possible to have personnel knowledgeable in many areas of information security at a high level, it is difficult to retain staff who are experts in more than a few subject areas. This is mainly because each subject area of information security requires constant attention and focus. Information security does not stand still.

1.2.1. Thinking Like the Enemy

Suppose that you administer an enterprise network. Such networks commonly comprise operating systems, applications, servers, network monitors, firewalls, intrusion detection systems, and more. Now imagine trying to keep current with each of those. Given the complexity of today's software and networking environments, exploits and bugs are a certainty. Keeping current with patches and updates for an entire network can prove to be a daunting task in a large organization with heterogeneous systems.
Combine the expertise requirements with the task of keeping current, and it is inevitable that adverse incidents occur, systems are breached, data is corrupted, and service is interrupted.
To augment security technologies and aid in protecting systems, networks, and data, you must think like an attacker and gauge the security of your systems by checking for weaknesses. Preventative vulnerability assessments against your own systems and network resources can reveal potential issues that can be addressed before an attacker exploits it.
A vulnerability assessment is an internal audit of your network and system security; the results of which indicate the confidentiality, integrity, and availability of your network (as explained in Section, “Standardizing Security”). Typically, vulnerability assessment starts with a reconnaissance phase, during which important data regarding the target systems and resources is gathered. This phase leads to the system readiness phase, whereby the target is essentially checked for all known vulnerabilities. The readiness phase culminates in the reporting phase, where the findings are classified into categories of high, medium, and low risk; and methods for improving the security (or mitigating the risk of vulnerability) of the target are discussed.
If you were to perform a vulnerability assessment of your home, you would likely check each door to your home to see if they are closed and locked. You would also check every window, making sure that they closed completely and latch correctly. This same concept applies to systems, networks, and electronic data. Malicious users are the thieves and vandals of your data. Focus on their tools, mentality, and motivations, and you can then react swiftly to their actions.

1.2.2. Defining Assessment and Testing

Vulnerability assessments may be broken down into one of two types: outside looking in and inside looking around.
When performing an outside-looking-in vulnerability assessment, you are attempting to compromise your systems from the outside. Being external to your company provides you with the attacker's viewpoint. You see what an attacker sees — publicly-routable IP addresses, systems on your DMZ, external interfaces of your firewall, and more. DMZ stands for "demilitarized zone", which corresponds to a computer or small subnetwork that sits between a trusted internal network, such as a corporate private LAN, and an untrusted external network, such as the public Internet. Typically, the DMZ contains devices accessible to Internet traffic, such as Web (HTTP) servers, FTP servers, SMTP (e-mail) servers and DNS servers.
When you perform an inside-looking-around vulnerability assessment, you are at an advantage since you are internal and your status is elevated to trusted. This is the viewpoint you and your co-workers have once logged on to your systems. You see print servers, file servers, databases, and other resources.
There are striking distinctions between the two types of vulnerability assessments. Being internal to your company gives you more privileges than an outsider. In most organizations, security is configured to keep intruders out. Very little is done to secure the internals of the organization (such as departmental firewalls, user-level access controls, and authentication procedures for internal resources). Typically, there are many more resources when looking around inside as most systems are internal to a company. Once you are outside the company, your status is untrusted. The systems and resources available to you externally are usually very limited.
Consider the difference between vulnerability assessments and penetration tests. Think of a vulnerability assessment as the first step to a penetration test. The information gleaned from the assessment is used for testing. Whereas the assessment is undertaken to check for holes and potential vulnerabilities, the penetration testing actually attempts to exploit the findings.
Assessing network infrastructure is a dynamic process. Security, both information and physical, is dynamic. Performing an assessment on the system shows an overview, which can turn up false positives and false negatives. A false positive is a result, where the tool finds vulnerabilities which in reality do not exist. A false negative is when it omits actual vulnerabilities.
Security administrators are only as good as the tools they use and the knowledge they retain. Take any of the assessment tools currently available, run them against your system, and it is almost a guarantee that there are some false positives. Whether by program fault or user error, the result is the same. The tool may find false positives, or, even worse, false negatives.
Now that the difference between a vulnerability assessment and a penetration test is defined, take the findings of the assessment and review them carefully before conducting a penetration test as part of your new best practices approach.


Do not attempt to exploit vulnerabilities on production systems. Doing so can have adverse effects on productivity and efficiency of your systems and network.
The following list examines some of the benefits to performing vulnerability assessments.
  • Creates proactive focus on information security.
  • Finds potential exploits before attackers find them.
  • Results in systems being kept up to date and patched.
  • Promotes growth and aids in developing staff expertise.
  • Abates financial loss and negative publicity. Establishing a Methodology
To aid in the selection of tools for a vulnerability assessment, it is helpful to establish a vulnerability assessment methodology. Unfortunately, there is no predefined or industry approved methodology at this time; however, common sense and best practices can act as a sufficient guide.
What is the target? Are we looking at one server, or are we looking at our entire network and everything within the network? Are we external or internal to the company? The answers to these questions are important as they help determine not only which tools to select but also the manner in which they are used.
To learn more about establishing methodologies, see the following websites:

1.2.3. Evaluating the Tools

An assessment can start by using some form of an information gathering tool. When assessing the entire network, map the layout first to find the hosts that are running. Once located, examine each host individually. Focusing on these hosts requires another set of tools. Knowing which tools to use may be the most crucial step in finding vulnerabilities.
Just as in any aspect of everyday life, there are many different tools that perform the same job. This concept applies to performing vulnerability assessments as well. There are tools specific to operating systems, applications, and even networks (based on the protocols used). Some tools are free; others are not. Some tools are intuitive and easy to use, while others are cryptic and poorly documented but have features that other tools do not.
Finding the right tools may be a daunting task and in the end, experience counts. If possible, set up a test lab and try out as many tools as you can, noting the strengths and weaknesses of each. Read documentation that comes with the tool (for example, in a README file or a manual page). For more information, search articles, step-by-step guides, or even mailing lists specific to a tool on the Internet.
The tools discussed below are just a small sampling of the available tools. Scanning Hosts with Nmap
Nmap is a popular tool that can be used to determine the layout of a network. Nmap has been available for many years and is probably the most often used tool when gathering information. An excellent manual page is included that provides detailed descriptions of its options and usage. Administrators can use Nmap on a network to find host systems and open ports on those systems.
Nmap is a competent first step in vulnerability assessment. You can map out all the hosts within your network and even pass an option that allows Nmap to attempt to identify the operating system running on a particular host. Nmap is a good foundation for establishing a policy of using secure services and restricting unused services.
To install Nmap, run the yum install nmap command as the root user. Using Nmap
Nmap can be run from a shell prompt by typing the nmap command followed by the host name or IP address of the machine to scan:
nmap <host name>
For example, to scan a machine with host name, type the following at a shell prompt:
~]$ nmap
The results of a basic scan (which could take up to a few minutes, depending on where the host is located and other network conditions) look similar to the following:
Interesting ports on
Not shown: 1710 filtered ports
22/tcp  open   ssh
53/tcp  open   domain
80/tcp  open   http
113/tcp closed auth
Nmap tests the most common network communication ports for listening or waiting services. This knowledge can be helpful to an administrator who wants to close down unnecessary or unused services.
For more information about using Nmap, see the official homepage at the following URL: Nessus
Nessus is a full-service security scanner. The plug-in architecture of Nessus allows users to customize it for their systems and networks. As with any scanner, Nessus is only as good as the signature database it relies upon. Fortunately, Nessus is frequently updated and features full reporting, host scanning, and real-time vulnerability searches. Remember that there could be false positives and false negatives, even in a tool as powerful and as frequently updated as Nessus.


The Nessus client and server software requires a subscription to use. It has been included in this document as a reference to users who may be interested in using this popular application.
For more information about Nessus, see the official website at the following URL: Nikto
Nikto is an excellent common gateway interface (CGI) script scanner. Nikto not only checks for CGI vulnerabilities but does so in an evasive manner, so as to elude intrusion detection systems. It comes with thorough documentation which should be carefully reviewed prior to running the program. If you have Web servers serving up CGI scripts, Nikto can be an excellent resource for checking the security of these servers.
More information about Nikto can be found at the following URL: Anticipating Your Future Needs
Depending upon your target and resources, there are many tools available. There are tools for wireless networks, Novell networks, Windows systems, Linux systems, and more. Another essential part of performing assessments may include reviewing physical security, personnel screening, or voice/PBX network assessment. Concepts such as war walking and wardriving, which involves scanning the perimeter of your enterprise's physical structures for wireless network vulnerabilities, are some concepts that you should investigate and, if needed, incorporate into your assessments. Imagination and exposure are the only limits of planning and conducting vulnerability assessments.

1.3. Security Threats

To plan and implement a good security strategy, first be aware of some of the issues which determined, motivated attackers exploit to compromise systems.

1.3.1. Threats to Network Security

Bad practices when configuring the following aspects of a network can increase the risk of attack. Insecure Architectures
A misconfigured network is a primary entry point for unauthorized users. Leaving a trust-based, open local network vulnerable to the highly-insecure Internet is much like leaving a door ajar in a crime-ridden neighborhood — nothing may happen for an arbitrary amount of time, but eventually someone exploits the opportunity. Broadcast Networks
System administrators often fail to realize the importance of networking hardware in their security schemes. Simple hardware such as hubs and routers rely on the broadcast or non-switched principle; that is, whenever a node transmits data across the network to a recipient node, the hub or router sends a broadcast of the data packets until the recipient node receives and processes the data. This method is the most vulnerable to address resolution protocol (ARP) or media access control (MAC) address spoofing by both outside intruders and unauthorized users on local hosts. Centralized Servers
Another potential networking pitfall is the use of centralized computing. A common cost-cutting measure for many businesses is to consolidate all services to a single powerful machine. This can be convenient as it is easier to manage and costs considerably less than multiple-server configurations. However, a centralized server introduces a single point of failure on the network. If the central server is compromised, it may render the network completely useless or worse, prone to data manipulation or theft. In these situations, a central server becomes an open door which allows access to the entire network.

1.3.2. Threats to Server Security

Server security is as important as network security because servers often hold a great deal of an organization's vital information. If a server is compromised, all of its contents may become available for the attacker to steal or manipulate at will. The following sections detail some of the main issues. Unused Services and Open Ports
A full installation of Red Hat Enterprise Linux 7 contains 1000+ application and library packages. However, most server administrators do not opt to install every single package in the distribution, preferring instead to install a base installation of packages, including several server applications.
A common occurrence among system administrators is to install the operating system without paying attention to what programs are actually being installed. This can be problematic because unneeded services may be installed, configured with the default settings, and possibly turned on. This can cause unwanted services, such as Telnet, DHCP, or DNS, to run on a server or workstation without the administrator realizing it, which in turn can cause unwanted traffic to the server, or even, a potential pathway into the system for attackers. Refer To Section 2.2, “Server Security” for information on closing ports and disabling unused services. Inattentive Administration
Administrators who fail to patch their systems are one of the greatest threats to server security. According to the SysAdmin, Audit, Network, Security Institute (SANS), the primary cause of computer security vulnerability is to "assign untrained people to maintain security and provide neither the training nor the time to make it possible to do the job. This applies as much to inexperienced administrators as it does to overconfident or amotivated administrators.
Some administrators fail to patch their servers and workstations, while others fail to watch log messages from the system kernel or network traffic. Another common error is when default passwords or keys to services are left unchanged. For example, some databases have default administration passwords because the database developers assume that the system administrator changes these passwords immediately after installation. If a database administrator fails to change this password, even an inexperienced attacker can use a widely-known default password to gain administrative privileges to the database. These are only a few examples of how inattentive administration can lead to compromised servers. Inherently Insecure Services
Even the most vigilant organization can fall victim to vulnerabilities if the network services they choose are inherently insecure. For instance, there are many services developed under the assumption that they are used over trusted networks; however, this assumption fails as soon as the service becomes available over the Internet — which is itself inherently untrusted.
One category of insecure network services are those that require unencrypted user names and passwords for authentication. Telnet and FTP are two such services. If packet sniffing software is monitoring traffic between the remote user and such a service user names and passwords can be easily intercepted.
Inherently, such services can also more easily fall prey to what the security industry terms the man-in-the-middle attack. In this type of attack, an attacker redirects network traffic by tricking a cracked name server on the network to point to his machine instead of the intended server. Once someone opens a remote session to the server, the attacker's machine acts as an invisible conduit, sitting quietly between the remote service and the unsuspecting user capturing information. In this way an attacker can gather administrative passwords and raw data without the server or the user realizing it.
Another category of insecure services include network file systems and information services such as NFS or NIS, which are developed explicitly for LAN usage but are, unfortunately, extended to include WANs (for remote users). NFS does not, by default, have any authentication or security mechanisms configured to prevent an attacker from mounting the NFS share and accessing anything contained therein. NIS, as well, has vital information that must be known by every computer on a network, including passwords and file permissions, within a plain text ASCII or DBM (ASCII-derived) database. An attacker who gains access to this database can then access every user account on a network, including the administrator's account.
By default, Red Hat Enterprise Linux is released with all such services turned off. However, since administrators often find themselves forced to use these services, careful configuration is critical. Refer to Section 2.2, “Server Security” for more information about setting up services in a safe manner.

1.3.3. Threats to Workstation and Home PC Security

Workstations and home PCs may not be as prone to attack as networks or servers, but since they often contain sensitive data, such as credit card information, they are targeted by system attackers. Workstations can also be co-opted without the user's knowledge and used by attackers as "slave" machines in coordinated attacks. For these reasons, knowing the vulnerabilities of a workstation can save users the headache of reinstalling the operating system, or worse, recovering from data theft. Bad Passwords
Bad passwords are one of the easiest ways for an attacker to gain access to a system. For more on how to avoid common pitfalls when creating a password, see Section 2.1.3, “Password Security”. Vulnerable Client Applications
Although an administrator may have a fully secure and patched server, that does not mean remote users are secure when accessing it. For instance, if the server offers Telnet or FTP services over a public network, an attacker can capture the plain text user names and passwords as they pass over the network, and then use the account information to access the remote user's workstation.
Even when using secure protocols, such as SSH, a remote user may be vulnerable to certain attacks if they do not keep their client applications updated. For instance, v.1 SSH clients are vulnerable to an X-forwarding attack from malicious SSH servers. Once connected to the server, the attacker can quietly capture any keystrokes and mouse clicks made by the client over the network. This problem was fixed in the v.2 SSH protocol, but it is up to the user to keep track of what applications have such vulnerabilities and update them as necessary.
Section 2.1, “Workstation Security” discusses in more detail what steps administrators and home users should take to limit the vulnerability of computer workstations.

1.4. Common Exploits and Attacks

Table 1.1, “Common Exploits” details some of the most common exploits and entry points used by intruders to access organizational network resources. Key to these common exploits are the explanations of how they are performed and how administrators can properly safeguard their network against such attacks.
Table 1.1. Common Exploits
Exploit Description Notes
Null or Default Passwords Leaving administrative passwords blank or using a default password set by the product vendor. This is most common in hardware such as routers and firewalls, but some services that run on Linux can contain default administrator passwords as well (though Red Hat Enterprise Linux does not ship with them).
Commonly associated with networking hardware such as routers, firewalls, VPNs, and network attached storage (NAS) appliances.
Common in many legacy operating systems, especially those that bundle services (such as UNIX and Windows.)
Administrators sometimes create privileged user accounts in a rush and leave the password null, creating a perfect entry point for malicious users who discover the account.
Default Shared Keys Secure services sometimes package default security keys for development or evaluation testing purposes. If these keys are left unchanged and are placed in a production environment on the Internet, all users with the same default keys have access to that shared-key resource, and any sensitive information that it contains. Most common in wireless access points and preconfigured secure server appliances.
IP Spoofing A remote machine acts as a node on your local network, finds vulnerabilities with your servers, and installs a backdoor program or Trojan horse to gain control over your network resources.
Spoofing is quite difficult as it involves the attacker predicting TCP/IP sequence numbers to coordinate a connection to target systems, but several tools are available to assist attackers in performing such a vulnerability.
Depends on target system running services (such as rsh, telnet, FTP and others) that use source-based authentication techniques, which are not recommended when compared to PKI or other forms of encrypted authentication used in ssh or SSL/TLS.
Eavesdropping Collecting data that passes between two active nodes on a network by eavesdropping on the connection between the two nodes.
This type of attack works mostly with plain text transmission protocols such as Telnet, FTP, and HTTP transfers.
Remote attacker must have access to a compromised system on a LAN in order to perform such an attack; usually the attacker has used an active attack (such as IP spoofing or man-in-the-middle) to compromise a system on the LAN.
Preventative measures include services with cryptographic key exchange, one-time passwords, or encrypted authentication to prevent password snooping; strong encryption during transmission is also advised.
Service Vulnerabilities An attacker finds a flaw or loophole in a service run over the Internet; through this vulnerability, the attacker compromises the entire system and any data that it may hold, and could possibly compromise other systems on the network.
HTTP-based services such as CGI are vulnerable to remote command execution and even interactive shell access. Even if the HTTP service runs as a non-privileged user such as "nobody", information such as configuration files and network maps can be read, or the attacker can start a denial of service attack which drains system resources or renders it unavailable to other users.
Services sometimes can have vulnerabilities that go unnoticed during development and testing; these vulnerabilities (such as buffer overflows, where attackers crash a service using arbitrary values that fill the memory buffer of an application, giving the attacker an interactive command prompt from which they may execute arbitrary commands) can give complete administrative control to an attacker.
Administrators should make sure that services do not run as the root user, and should stay vigilant of patches and errata updates for applications from vendors or security organizations such as CERT and CVE.
Application Vulnerabilities Attackers find faults in desktop and workstation applications (such as e-mail clients) and execute arbitrary code, implant Trojan horses for future compromise, or crash systems. Further exploitation can occur if the compromised workstation has administrative privileges on the rest of the network.
Workstations and desktops are more prone to exploitation as workers do not have the expertise or experience to prevent or detect a compromise; it is imperative to inform individuals of the risks they are taking when they install unauthorized software or open unsolicited email attachments.
Safeguards can be implemented such that email client software does not automatically open or execute attachments. Additionally, the automatic update of workstation software using Red Hat Network or other system management services can alleviate the burdens of multi-seat security deployments.
Denial of Service (DoS) Attacks Attacker or group of attackers coordinate against an organization's network or server resources by sending unauthorized packets to the target host (either server, router, or workstation). This forces the resource to become unavailable to legitimate users.
Source packets are usually forged (as well as rebroadcast), making investigation as to the true source of the attack difficult.
Advances in ingress filtering (IETF rfc2267) using iptables and Network Intrusion Detection Systems such as snort assist administrators in tracking down and preventing distributed DoS attacks.

1.5. Security Updates

As security vulnerabilities are discovered, the affected software must be updated in order to limit any potential security risks. If the software is part of a package within a Red Hat Enterprise Linux distribution that is currently supported, Red Hat is committed to releasing updated packages that fix the vulnerability as soon as is possible. Often, announcements about a given security exploit are accompanied with a patch (or source code that fixes the problem). This patch is then applied to the Red Hat Enterprise Linux package and tested and released as an errata update. However, if an announcement does not include a patch, a developer first works with the maintainer of the software to fix the problem. Once the problem is fixed, the package is tested and released as an errata update.
If an errata update is released for software used on your system, it is highly recommended that you update the affected packages as soon as possible to minimize the amount of time the system is potentially vulnerable.

1.5.1. Updating Packages

When updating software on a system, it is important to download the update from a trusted source. An attacker can easily rebuild a package with the same version number as the one that is supposed to fix the problem but with a different security exploit and release it on the Internet. If this happens, using security measures such as verifying files against the original RPM does not detect the exploit. Thus, it is very important to only download RPMs from trusted sources, such as from Red Hat and to check the signature of the package to verify its integrity.


Red Hat Enterprise Linux includes a convenient panel icon that displays visible alerts when there is an update available.

1.5.2. Verifying Signed Packages

All Red Hat Enterprise Linux packages are signed with the Red Hat GPG key. GPG stands for GNU Privacy Guard, or GnuPG, a free software package used for ensuring the authenticity of distributed files. For example, a private key (secret key) locks the package while the public key unlocks and verifies the package. If the public key distributed by Red Hat Enterprise Linux does not match the private key during RPM verification, the package may have been altered and therefore cannot be trusted.
The RPM utility within Red Hat Enterprise Linux 6 automatically tries to verify the GPG signature of an RPM package before installing it. If the Red Hat GPG key is not installed, install it from a secure, static location, such as a Red Hat installation CD-ROM or DVD.
Assuming the disc is mounted in /mnt/cdrom, use the following command as the root user to import it into the keyring (a database of trusted keys on the system):
~]# rpm --import /mnt/cdrom/RPM-GPG-KEY
Now, the Red Hat GPG key is located in the /etc/pki/rpm-gpg/ directory.
To display a list of all keys installed for RPM verification, execute the following command:
~]# rpm -qa gpg-pubkey*
To display details about a specific key, use the rpm -qi command followed by the output from the previous command, as in this example:
~]# rpm -qi gpg-pubkey-db42a60e-37ea5438
Name        : gpg-pubkey                   Relocations: (not relocatable)
Version     : 2fa658e0                          Vendor: (none)
Release     : 45700c69                      Build Date: Fri 07 Oct 2011 02:04:51 PM CEST
Install Date: Fri 07 Oct 2011 02:04:51 PM CEST      Build Host: localhost
Group       : Public Keys                   Source RPM: (none)
[output truncated]
It is extremely important to verify the signature of the RPM files before installing them to ensure that they have not been altered from the original source of the packages. To verify all the downloaded packages at once, issue the following command:
~]# rpm -K /root/updates/*.rpm
alsa-lib-1.0.22-3.el6.x86_64.rpm: rsa sha1 (md5) pgp md5 OK
alsa-utils-1.0.21-3.el6.x86_64.rpm: rsa sha1 (md5) pgp md5 OK
aspell-0.60.6-12.el6.x86_64.rpm: rsa sha1 (md5) pgp md5 OK
For each package, if the GPG key verifies successfully, the command returns gpg OK. If it does not, make sure you are using the correct Red Hat public key, as well as verifying the source of the content. Packages that do not pass GPG verification should not be installed, as they may have been altered by a third party.
After verifying the GPG key and downloading all the packages associated with the errata report, install the packages as root at a shell prompt.
Alternatively, you may use the Yum utility to verify signed packages. Yum provides secure package management by enabling GPG signature verification on GPG-signed packages to be turned on for all package repositories (that is, package sources), or for individual repositories. When signature verification is enabled, Yum will refuse to install any packages not GPG-signed with the correct key for that repository. This means that you can trust that the RPM packages you download and install on your system are from a trusted source, such as Red Hat, and were not modified during transfer.
In order to have automatic GPG signature verification enabled when installing or updating packages via Yum, ensure you have the following option defined under the [main] section of your /etc/yum.conf file:

1.5.3. Installing Signed Packages

Installation for most packages can be done safely (except kernel packages) by issuing the following command as root:
rpm -Uvh <package>
For example, to install all packages in a new directory, called updates/, under the /tmp directory, run:
~]# rpm -Uvh /tmp/updates/*.rpm
Preparing...                ########################################### [100%]
   1:alsa-lib               ########################################### [ 33%]
   2:alsa-utils             ########################################### [ 67%]
   3:aspell                 ########################################### [100%]
For kernel packages, as root use the command in the following form:
rpm -ivh <kernel-package>
For example, to install kernel-2.6.32-220.el6.x86_64.rpm, type the following at a shell prompt:
~]# rpm -ivh /tmp/updates/kernel-2.6.32-220.el6.x86_64.rpm
Preparing...                ########################################### [100%]
   1:kernel                 ########################################### [100%]
Once the machine has been safely rebooted using the new kernel, the old kernel may be removed using the following command:
rpm -e <old-kernel-package>
For instance, to remove kernel-2.6.32-206.el6.x86_64, type:
~]# rpm -e kernel-2.6.32-206.el6.x86_64
Alternatively, to install packages with Yum, run, as root, the following command:
~]# yum install kernel-2.6.32-220.el6.x86_64.rpm
To install local packages with Yum, run, as root, the following command:
~]# yum localinstall /root/updates/emacs-23.1-21.el6_2.3.x86_64.rpm


It is not a requirement that the old kernel be removed. The default boot loader, GRUB, allows for multiple kernels to be installed, then chosen from a menu at boot time.


Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 1.5.4, “Applying the Changes” for general instructions about applying the changes made by an errata update.

1.5.4. Applying the Changes

After downloading and installing security errata and updates, it is important to halt usage of the older software and begin using the new software. How this is done depends on the type of software that has been updated. The following list itemizes the general categories of software and provides instructions for using the updated versions after a package upgrade.


In general, rebooting the system is the surest way to ensure that the latest version of a software package is used; however, this option is not always required, or available to the system administrator.
User-space applications are any programs that can be initiated by a system user. Typically, such applications are used only when a user, script, or automated task utility launches them and they do not persist for long periods of time.
Once such a user-space application is updated, halt any instances of the application on the system and launch the program again to use the updated version.
The kernel is the core software component for the Red Hat Enterprise Linux operating system. It manages access to memory, the processor, and peripherals as well as schedules all tasks.
Because of its central role, the kernel cannot be restarted without also stopping the computer. Therefore, an updated version of the kernel cannot be used until the system is rebooted.
Shared Libraries
Shared libraries are units of code, such as glibc, which are used by a number of applications and services. Applications utilizing a shared library typically load the shared code when the application is initialized, so any applications using the updated library must be halted and relaunched.
To determine which running applications link against a particular library, use the lsof command:
lsof <path>
For example, to determine which running applications link against the library, type:
~]# lsof /lib64/*
sshd      13600 root mem    REG  253,0    43256 400501 /lib64/
sshd      13603 juan mem    REG  253,0    43256 400501 /lib64/
gnome-set 14898 juan mem    REG  253,0    43256 400501 /lib64/
metacity  14925 juan mem    REG  253,0    43256 400501 /lib64/
[output truncated]
This command returns a list of all the running programs which use TCP wrappers for host access control. Therefore, any program listed must be halted and relaunched if the tcp_wrappers package is updated.
SysV Services
SysV services are persistent server programs launched during the boot process. Examples of SysV services include sshd, vsftpd, and xinetd.
Because these programs usually persist in memory as long as the machine is booted, each updated SysV service must be halted and relaunched after the package is upgraded. This can be done using the Services Configuration Tool or by logging into a root shell prompt and issuing the /sbin/service command:
/sbin/service <service-name> restart
Replace <service-name> with the name of the service, such as sshd.
xinetd Services
Services controlled by the xinetd super service only run when a there is an active connection. Examples of services controlled by xinetd include Telnet, IMAP, and POP3.
Because new instances of these services are launched by xinetd each time a new request is received, connections that occur after an upgrade are handled by the updated software. However, if there are active connections at the time the xinetd controlled service is upgraded, they are serviced by the older version of the software.
To kill off older instances of a particular xinetd controlled service, upgrade the package for the service then halt all processes currently running. To determine if the process is running, use the ps or pgrep command and then use the kill or killall command to halt current instances of the service.
For example, if security errata imap packages are released, upgrade the packages, then type the following command as root into a shell prompt:
~]# pgrep -l imap
1439 imapd
1788 imapd
1793 imapd
This command returns all active IMAP sessions. Individual sessions can then be terminated by issuing the following command as root:
kill <PID>
If this fails to terminate the session, use the following command instead:
kill -9 <PID>
In the previous examples, replace <PID> with the process identification number (found in the second column of the pgrep -l command) for an IMAP session.
To kill all active IMAP sessions, issue the following command:
~]# killall imapd


Chapter 2. Securing Your Network

2.1. Workstation Security

Securing a Linux environment begins with the workstation. Whether locking down a personal machine or securing an enterprise system, sound security policy begins with the individual computer. A computer network is only as secure as its weakest node.

2.1.1. Evaluating Workstation Security

When evaluating the security of a Red Hat Enterprise Linux workstation, consider the following:
  • BIOS and Boot Loader Security — Can an unauthorized user physically access the machine and boot into single user or rescue mode without a password?
  • Password Security — How secure are the user account passwords on the machine?
  • Administrative Controls — Who has an account on the system and how much administrative control do they have?
  • Available Network Services — What services are listening for requests from the network and should they be running at all?
  • Personal Firewalls — What type of firewall, if any, is necessary?
  • Security Enhanced Communication Tools — Which tools should be used to communicate between workstations and which should be avoided?

2.1.2. BIOS and Boot Loader Security

Password protection for the BIOS (or BIOS equivalent) and the boot loader can prevent unauthorized users who have physical access to systems from booting using removable media or obtaining root privileges through single user mode. The security measures you should take to protect against such attacks depends both on the sensitivity of the information on the workstation and the location of the machine.
For example, if a machine is used in a trade show and contains no sensitive information, then it may not be critical to prevent such attacks. However, if an employee's laptop with private, unencrypted SSH keys for the corporate network is left unattended at that same trade show, it could lead to a major security breach with ramifications for the entire company.
If the workstation is located in a place where only authorized or trusted people have access, however, then securing the BIOS or the boot loader may not be necessary. BIOS Passwords
The two primary reasons for password protecting the BIOS of a computer are[3]:
  1. Preventing Changes to BIOS Settings — If an intruder has access to the BIOS, they can set it to boot from a CD-ROM or a flash drive. This makes it possible for an intruder to enter rescue mode or single user mode, which in turn allows them to start arbitrary processes on the system or copy sensitive data.
  2. Preventing System Booting — Some BIOSes allow password protection of the boot process. When activated, an attacker is forced to enter a password before the BIOS launches the boot loader.
Because the methods for setting a BIOS password vary between computer manufacturers, consult the computer's manual for specific instructions.
If you forget the BIOS password, it can either be reset with jumpers on the motherboard or by disconnecting the CMOS battery. For this reason, it is good practice to lock the computer case if possible. However, consult the manual for the computer or motherboard before attempting to disconnect the CMOS battery. Securing Non-x86 Platforms
Other architectures use different programs to perform low-level tasks roughly equivalent to those of the BIOS on x86 systems. For instance, Intel® Itanium™ computers use the Extensible Firmware Interface (EFI) shell.
For instructions on password protecting BIOS-like programs on other architectures, see the manufacturer's instructions. Boot Loader Passwords
The primary reasons for password protecting a Linux boot loader are as follows:
  1. Preventing Access to Single User Mode — If attackers can boot the system into single user mode, they are logged in automatically as root without being prompted for the root password.


    Protecting access to single user mode with a password by editing the SINGLE parameter in the /etc/sysconfig/init file is not recommended. An attacker can bypass the password by specifying a custom initial command (using the init= parameter) on the kernel command line in GRUB. It is recommended to password-protect the GRUB boot loader as specified in Section, “Password Protecting GRUB”.
  2. Preventing Access to the GRUB Console — If the machine uses GRUB as its boot loader, an attacker can use the GRUB editor interface to change its configuration or to gather information using the cat command.
  3. Preventing Access to Insecure Operating Systems — If it is a dual-boot system, an attacker can select an operating system at boot time (for example, DOS), which ignores access controls and file permissions.
Red Hat Enterprise Linux 6 includes the GRUB boot loader on the x86 platform. For a detailed look at GRUB, see the Red Hat Enterprise Linux Installation Guide. Password Protecting GRUB
You can configure GRUB to address the first two issues listed in Section, “Boot Loader Passwords” by adding a password directive to its configuration file. To do this, first choose a strong password, open a shell, log in as root, and then type the following command:
When prompted, type the GRUB password and press Enter. This returns an MD5 hash of the password.
Next, edit the GRUB configuration file /boot/grub/grub.conf. Open the file and below the timeout line in the main section of the document, add the following line:
password --md5 <password-hash>
Replace <password-hash> with the value returned by /sbin/grub-md5-crypt[4].
The next time the system boots, the GRUB menu prevents access to the editor or command interface without first pressing p followed by the GRUB password.
Unfortunately, this solution does not prevent an attacker from booting into an insecure operating system in a dual-boot environment. For this, a different part of the /boot/grub/grub.conf file must be edited.
Look for the title line of the operating system that you want to secure, and add a line with the lock directive immediately beneath it.
For a DOS system, the stanza should begin similar to the following:
title DOS


A password line must be present in the main section of the /boot/grub/grub.conf file for this method to work properly. Otherwise, an attacker can access the GRUB editor interface and remove the lock line.
To create a different password for a particular kernel or operating system, add a lock line to the stanza, followed by a password line.
Each stanza protected with a unique password should begin with lines similar to the following example:
title DOS
	password --md5 <password-hash> Disabling Interactive Startup
Pressing the I key at the beginning of the boot sequence allows you to start up your system interactively. During an interactive startup, the system prompts you to start up each service one by one. However, this may allow an attacker who gains physical access to your system to disable the security-related services and gain access to the system.
To prevent users from starting up the system interactively, as root, disable the PROMPT parameter in the /etc/sysconfig/init file:

2.1.3. Password Security

Passwords are the primary method that Red Hat Enterprise Linux uses to verify a user's identity. This is why password security is so important for protection of the user, the workstation, and the network.
For security purposes, the installation program configures the system to use Secure Hash Algorithm 512 (SHA512) and shadow passwords. It is highly recommended that you do not alter these settings.
If shadow passwords are deselected during installation, all passwords are stored as a one-way hash in the world-readable /etc/passwd file, which makes the system vulnerable to offline password cracking attacks. If an intruder can gain access to the machine as a regular user, he can copy the /etc/passwd file to his own machine and run any number of password cracking programs against it. If there is an insecure password in the file, it is only a matter of time before the password attacker discovers it.
Shadow passwords eliminate this type of attack by storing the password hashes in the file /etc/shadow, which is readable only by the root user.
This forces a potential attacker to attempt password cracking remotely by logging into a network service on the machine, such as SSH or FTP. This sort of brute-force attack is much slower and leaves an obvious trail as hundreds of failed login attempts are written to system files. Of course, if the attacker starts an attack in the middle of the night on a system with weak passwords, the cracker may have gained access before dawn and edited the log files to cover his tracks.
In addition to format and storage considerations is the issue of content. The single most important thing a user can do to protect his account against a password cracking attack is create a strong password. Creating Strong Passwords
When creating a secure password, the user must remember that long passwords are stronger than short and complex ones. It is not a good idea to create a password of just eight characters, even if it contains digits, special characters and uppercase letters. Password cracking tools, such as John The Ripper, are optimized for breaking such passwords, which are also hard to remember by a person.
In information theory, entropy is the level of uncertainty associated with a random variable and is presented in bits. The higher the entropy value, the more secure the password is. According to NIST SP 800-63-1, passwords that are not present in a dictionary comprised of 50000 commonly selected passwords should have at least 10 bits of entropy. As such, a password that consists of four random words contains around 40 bits of entropy. A long password consisting of multiple words for added security is also called a passphrase, for example:
randomword1 randomword2 randomword3 randomword4
If the system enforces the use of uppercase letters, digits, or special characters, the passphrase that follows the above recommendation can be modified in a simple way, for example by changing the first character to uppercase and appending "1!". Note that such a modification does not increase the security of the passphrase significantly.
While there are different approaches to creating a secure password, always avoid the following bad practices:
  • Using a single dictionary word, a word in a foreign language, an inverted word, or only numbers.
  • Using less than 10 characters for a password or passphrase.
  • Using a sequence of keys from the keyboard layout.
  • Writing down your passwords.
  • Using personal information in a password, such as birth dates, anniversaries, family member names, or pet names.
  • Using the same passphrase or password on multiple machines.
While creating secure passwords is imperative, managing them properly is also important, especially for system administrators within larger organizations. The following section details good practices for creating and managing user passwords within an organization.

2.1.4. Creating User Passwords Within an Organization

If an organization has a large number of users, the system administrators have two basic options available to force the use of good passwords. They can create passwords for the user, or they can let users create their own passwords, while verifying the passwords are of acceptable quality.
Creating the passwords for the users ensures that the passwords are good, but it becomes a daunting task as the organization grows. It also increases the risk of users writing their passwords down.
For these reasons, most system administrators prefer to have the users create their own passwords, but actively verify that the passwords are good and, in some cases, force users to change their passwords periodically through password aging. Forcing Strong Passwords
To protect the network from intrusion it is a good idea for system administrators to verify that the passwords used within an organization are strong ones. When users are asked to create or change passwords, they can use the command line application passwd, which is Pluggable Authentication Modules (PAM) aware and therefore checks to see if the password is too short or otherwise easy to crack. This check is performed using the PAM module. In Red Hat Enterprise Linux, the pam_cracklib PAM module can be used to check a password's strength against a set of rules. It can be stacked alongside other PAM modules in the password component of the/etc/pam.d/passwd file to configure a custom set of rules for user login. The pam_cracklib's routine consists of two parts: it checks whether the password provided is found in a dictionary, and, if that is not the case, it continues with a number of additional checks. For a complete list of these checks, see the pam_cracklib(8) manual page.

Example 2.1. Configuring password strength-checking with pam_cracklib

To require a password with a minimum length of 8 characters, including all four classes of characters, add the following line to the password section of the /etc/pam.d/passwd file:
password   required retry=3 minlen=8 minclass=4
To set a password strength-check for consecutive or repetitive characters, add the following line to the password section of the /etc/pam.d/passwd file:
password   required retry=3 maxsequence=3 maxrepeat=3
In this example, the password entered cannot contain more than 3 consecutive characters, such as "abcd" or "1234". Additionally, the number of identical consecutive characters is limited to 3.


As these checks are not performed for the root user, he can set any password for a regular user, despite the warning messages.
Since PAM is customizable, it is possible to add more password integrity checkers, such as pam_passwdqc (available from or to write a new module. For a list of available PAM modules, see For more information about PAM, see the Managing Single Sign-On and Smart Cards guide.
The password check that is performed at the time of their creation does not discover bad passwords as effectively as running a password cracking program against the passwords.
Many password cracking programs are available that run under Red Hat Enterprise Linux, although none ship with the operating system. Below is a brief list of some of the more popular password cracking programs:
  • John The Ripper — A fast and flexible password cracking program. It allows the use of multiple word lists and is capable of brute-force password cracking. It is available online at
  • Crack — Perhaps the most well known password cracking software, Crack is also very fast, though not as easy to use as John The Ripper.
  • SlurpieSlurpie is similar to John The Ripper and Crack, but it is designed to run on multiple computers simultaneously, creating a distributed password cracking attack. It can be found along with a number of other distributed attack security evaluation tools online at


Always get authorization in writing before attempting to crack passwords within an organization. Passphrases
Passphrases and passwords are the cornerstone to security in most of today's systems. Unfortunately, techniques such as biometrics and two-factor authentication have not yet become mainstream in many systems. If passwords are going to be used to secure a system, then the use of passphrases should be considered. Passphrases are longer than passwords and provide better protection than a password even when implemented with non-standard characters such as numbers and symbols. Password Aging
Password aging is another technique used by system administrators to defend against bad passwords within an organization. Password aging means that after a specified period (usually 90 days), the user is prompted to create a new password. The theory behind this is that if a user is forced to change his password periodically, a cracked password is only useful to an intruder for a limited amount of time. The downside to password aging, however, is that users are more likely to write their passwords down.
There are two primary programs used to specify password aging under Red Hat Enterprise Linux: the chage command or the graphical User Manager (system-config-users) application.


Shadow passwords must be enabled to use the chage command. For more information, see the Red Hat Enterprise Linux 6 Deployment Guide.
The -M option of the chage command specifies the maximum number of days the password is valid. For example, to set a user's password to expire in 90 days, use the following command:
chage -M 90 <username>
In the above command, replace <username> with the name of the user. To disable password expiration, it is traditional to use a value of 99999 after the -M option (this equates to a little over 273 years).
For more information on the options available with the chage command, see the table below.
Table 2.1. chage command line options
Option Description
-d days Specifies the number of days since January 1, 1970 the password was changed.
-E date Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used.
-I days Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires.
-l Lists current account aging settings.
-m days Specify the minimum number of days after which the user must change passwords. If the value is 0, the password does not expire.
-M days Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account.
-W days Specifies the number of days before the password expiration date to warn the user.
You can also use the chage command in interactive mode to modify multiple password aging and account details. Use the following command to enter interactive mode:
chage <username>
The following is a sample interactive session using this command:
~]# chage juan
Changing the aging information for juan
Enter the new value, or press ENTER for the default
Minimum Password Age [0]: 10
Maximum Password Age [99999]: 90
Last Password Change (YYYY-MM-DD) [2006-08-18]:
Password Expiration Warning [7]:
Password Inactive [-1]:
Account Expiration Date (YYYY-MM-DD) [1969-12-31]:
You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.
  1. Set up an initial password. There are two common approaches to this step: you can either assign a default password, or you can use a null password.
    To assign a default password, type the following at a shell prompt as root:
    passwd username
    To assign a null password instead, use the following command:
    passwd -d username


    Using a null password, while convenient, is a highly insecure practice, as any third party can log in first and access the system using the insecure user name. Always make sure that the user is ready to log in before unlocking an account with a null password.
  2. Force immediate password expiration by running the following command as root:
    chage -d 0 username
    This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
Upon the initial log in, the user is now prompted for a new password.
You can also use the graphical User Manager application to create password aging policies, as follows. Note: you need Administrator privileges to perform this procedure.
  1. Click the System menu on the Panel, point to Administration and then click Users and Groups to display the User Manager. Alternatively, type the command system-config-users at a shell prompt.
  2. Click the Users tab, and select the required user in the list of users.
  3. Click Properties on the toolbar to display the User Properties dialog box (or choose Properties on the File menu).
  4. Click the Password Info tab, and select the check box for Enable password expiration.
  5. Enter the required value in the Days before change required field, and click OK.
Specifying password aging options

Figure 2.1. Specifying password aging options

screenshot needs to be updated

2.1.5. Locking Inactive Accounts

The pam_lastlog PAM module is used to lock out users who have not logged in recently enough, or to display information about the last login attempt of a user. The module does not perform a check on the root account, so it is never locked out.
The lastlog command displays the last login of the user, аs opposed to the last command, which displays all current and previous login sessions. The commands read respectively from the /var/log/lastlog and /var/log/wtmp files where the data is stored in binary format.
  • To display the number of failed login attempts prior to the last successful login of a user, add, as root, the following line to the session section in the /etc/pam.d/login file:
    session     optional silent noupdate showfailed
Account locking due to inactivity can be configured to work for the console, GUI, or both:
  • To lock out an account after 10 days of inactivity, add, as root, the following line to the auth section of the /etc/pam.d/login file:
    auth  required inactive=10
  • To lock out an account for the GNOME desktop environment, add, as root, the following line to the auth section of the /etc/pam.d/gdm file:
    auth  required inactive=10


Note that for other desktop environments, the respective files of those environments should be edited.

2.1.6. Customizing Access Control

The pam_access PAM module allows an administrator to customize access control based on login names, host or domain names, or IP addresses. By default, the module reads the access rules from the /etc/security/access.conf file if no other is specified. For a complete description of the format of these rules, see the access.conf(5) manual page. By default, in Red Hat Enterprise Linux, pam_access is included in the /etc/pam.d/crond and /etc/pam.d/atd files.
To deny the user john from accessing system from the console and the graphic desktop environment, follow these steps:
  1. Include the following line in the account section of both /etc/pam.d/login and /etc/pam.d/gdm-* files:
    account     required
  2. Specify the following rule in the /etc/security/access.conf file:
    - : john : ALL
    This rule prohibits all logins from user john from any location.
To grant access to all users attempting to log in using SSH except the user john from the IP address, follow these steps:
  1. Include the following line in the account section of /etc/pam.d/sshd:
    account     required
  2. Specify the following rule in the /etc/security/access.conf file:
    + : ALL EXCEPT john :
In order to limit access from other services, the pam_access module should be required in the respective file in the /etc/pam.d directory.
It is possible to call the pam_access module for all services that call the system wide PAM configuration files (*-auth files in the /etc/pam.d directory) using the following command:
authconfig --enablepamaccess --update
Alternatively, you can enable the pam_access module using the Authentication Configuration utility. To start this utility, select SystemAdministration Authentication from the top menu. From the Advanced Options tab, check the "enable local access control option". This will add the pam_access module to the systemwide PAM configuration.

2.1.7. Time-based Restriction of Access

The pam_time PAM module is used to restrict access during a certain time of the day. It can also be configured to control access based on specific days of a week, user name, usage of a system service, and more. By default, the module reads the access rules from the /etc/security/time.conf file. For a complete description of the format of these rules, see the time.conf(5) manual page.
To restrict all users except the root user from logging in from 05:30 PM to 08:00 AM on Monday till Friday and Saturday and Sunday, follow these steps:
  1. Include the following line in the account section of the /etc/pam.d/login file:
    account     required
  2. Specify the following rule in the /etc/security/time.conf file:
    login ; tty* ; ALL ; !root ; !Wk1730-0800
To allow user john to use the SSH service during working hours and working days only (starting with Monday), follow these steps:
  1. Add the following line to the /etc/pam.d/sshd file:
    account     required
  2. Specify the following rule in the /etc/security/time.conf file:
    sshd ; tty* ; john ; Wk0800-1730


For these configurations to be applied to the desktop environment, the pam_time module should be included in the corresponding files in the /etc/pam.d directory.

2.1.8. Applying Account Limits

The pam_limits PAM module is used to:
  • apply limits to user login sessions, such as maximum simultaneous login sessions per user,
  • specify limits to be set by the ulimit utility,
  • and specify priority to be set by the nice utility.
By default, the rules are read from the/etc/security/limits.conf file. For a complete description of the format of these rules, see the limits.conf(5) manual page. Additionally, you can create individual configuration files in the /etc/security/limits.d directory specifically for certain applications or services. By default, the pam_limits module is included in a number of files in the/etc/pam.d/ directory. A default limit of user processes is defined in the /etc/security/limits.d/90-nproc.conf file to prevent malicious denial of service attacks, such as fork bombs. To change the default limit of user processes to 50, change the value in the /etc/security/limits.d/90-nproc.conf, following the format in the file:
* soft nproc 50

Example 2.2. Specifying a maximum number of logins per user

  1. To set a maximum number of simultaneous logins for each user in a group called office, specify the following rule in the /etc/security/limits.conf file:
    @office - maxlogins 4
  2. The following line should be present by default in /etc/pam.d/system-auth. If not, add it manually.
    session  required

2.1.9. Administrative Controls

When administering a home machine, the user must perform some tasks as the root user or by acquiring effective root privileges through a setuid program, such as sudo or su. A setuid program is one that operates with the user ID (UID) of the program's owner rather than the user operating the program. Such programs are denoted by an s in the owner section of a long format listing, as in the following example:
~]$ ls -l /bin/su
-rwsr-xr-x. 1 root root 34904 Mar 10  2011 /bin/su


The s may be upper case or lower case. If it appears as upper case, it means that the underlying permission bit has not been set.
For the system administrators of an organization, however, choices must be made as to how much administrative access users within the organization should have to their machine. Through a PAM module called, some activities normally reserved only for the root user, such as rebooting and mounting removable media are allowed for the first user that logs in at the physical console (see Managing Single Sign-On and Smart Cards for more information about the module.) However, other important system administration tasks, such as altering network settings, configuring a new mouse, or mounting network devices, are not possible without administrative privileges. As a result, system administrators must decide how much access the users on their network should receive. Allowing Root Access
If the users within an organization are trusted and computer-literate, then allowing them root access may not be an issue. Allowing root access by users means that minor activities, like adding devices or configuring network interfaces, can be handled by the individual users, leaving system administrators free to deal with network security and other important issues.
On the other hand, giving root access to individual users can lead to the following issues:
  • Machine Misconfiguration — Users with root access can misconfigure their machines and require assistance to resolve issues. Even worse, they might open up security holes without knowing it.
  • Running Insecure Services — Users with root access might run insecure servers on their machine, such as FTP or Telnet, potentially putting user names and passwords at risk. These services transmit this information over the network in plain text.
  • Running Email Attachments As Root — Although rare, email viruses that affect Linux do exist. The only time they are a threat, however, is when they are run by the root user.
  • Keeping the audit trail intact — Because the root account is often shared by multiple users, so that multiple system administrators can maintain the system, it is impossible to figure out which of those users was root at a given time. When using separate logins, the account a user logs in with, as well as a unique number for session tracking purposes, is put into the task structure, which is inherited by every process that the user starts. When using concurrent logins, the unique number can be used to trace actions to specific logins. When an action generates an audit event, it is recorded with the login account and the session associated with that unique number. Use the aulast command to view these logins and sessions. The --proof option of the aulast command can be used suggest a specific ausearch query to isolate auditable events generated by a particular session. Disallowing Root Access
If an administrator is uncomfortable allowing users to log in as root for these or other reasons, the root password should be kept secret, and access to runlevel one or single user mode should be disallowed through boot loader password protection (see Section, “Boot Loader Passwords” for more information on this topic.)
The following are four different ways that an administrator can further ensure that root logins are disallowed:
Changing the root shell
To prevent users from logging in directly as root, the system administrator can set the root account's shell to /sbin/nologin in the /etc/passwd file.
Table 2.2. Disabling the Root Shell
Effects Does Not Affect
Prevents access to the root shell and logs any such attempts. The following programs are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • su
  • ssh
  • scp
  • sftp
Programs that do not require a shell, such as FTP clients, mail clients, and many setuid programs. The following programs are not prevented from accessing the root account:
  • sudo
  • FTP clients
  • Email clients
Disabling root access through any console device (tty)
To further limit access to the root account, administrators can disable root logins at the console by editing the /etc/securetty file. This file lists all devices the root user is allowed to log into. If the file does not exist at all, the root user can log in through any communication device on the system, whether through the console or a raw network interface. This is dangerous, because a user can log in to their machine as root through Telnet, which transmits the password in plain text over the network.
By default, Red Hat Enterprise Linux's /etc/securetty file only allows the root user to log in at the console physically attached to the machine. To prevent the root user from logging in, remove the contents of this file by typing the following command at a shell prompt as root:
echo > /etc/securetty
To enable securetty support in the KDM, GDM, and XDM login managers, add the following line:
auth [user_unknown=ignore success=ok ignore=ignore default=bad]
to the files listed below:
  • /etc/pam.d/gdm
  • /etc/pam.d/gdm-autologin
  • /etc/pam.d/gdm-fingerprint
  • /etc/pam.d/gdm-password
  • /etc/pam.d/gdm-smartcard
  • /etc/pam.d/kdm
  • /etc/pam.d/kdm-np
  • /etc/pam.d/xdm


A blank /etc/securetty file does not prevent the root user from logging in remotely using the OpenSSH suite of tools because the console is not opened until after authentication.
Table 2.3. Disabling Root Logins
Effects Does Not Affect
Prevents access to the root account using the console or the network. The following programs are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • Other network services that open a tty
Programs that do not log in as root, but perform administrative tasks through setuid or other mechanisms. The following programs are not prevented from accessing the root account:
  • su
  • sudo
  • ssh
  • scp
  • sftp
Disabling root SSH logins
To prevent root logins using the SSH protocol, edit the SSH daemon's configuration file, /etc/ssh/sshd_config, and change the line that reads:
#PermitRootLogin yes
to read as follows:
PermitRootLogin no
Table 2.4. Disabling Root SSH Logins
Effects Does Not Affect
Prevents root access using the OpenSSH suite of tools. The following programs are prevented from accessing the root account:
  • ssh
  • scp
  • sftp
Programs that are not part of the OpenSSH suite of tools.
Using PAM to limit root access to services
PAM, through the /lib/security/ module, allows great flexibility in denying specific accounts. The administrator can use this module to reference a list of users who are not allowed to log in. To limit root access to a system service, edit the file for the target service in the /etc/pam.d/ directory and make sure the module is required for authentication.
The following is an example of how the module is used for the vsftpd FTP server in the /etc/pam.d/vsftpd PAM configuration file (the \ character at the end of the first line is not necessary if the directive is on a single line):
auth   required   /lib/security/   item=user \
 sense=deny file=/etc/vsftpd.ftpusers onerr=succeed
This instructs PAM to consult the /etc/vsftpd.ftpusers file and deny access to the service for any listed user. The administrator can change the name of this file, and can keep separate lists for each service or use one central list to deny access to multiple services.
If the administrator wants to deny access to multiple services, a similar line can be added to the PAM configuration files, such as /etc/pam.d/pop and /etc/pam.d/imap for mail clients, or /etc/pam.d/ssh for SSH clients.
For more information about PAM, see the chapter titled Using Pluggable Authentication Modules (PAM) in the Red Hat Enterprise Linux Managing Single Sign-On and Smart Cards guide.
Table 2.5. Disabling Root Using PAM
Effects Does Not Affect
Prevents root access to network services that are PAM aware. The following services are prevented from accessing the root account:
  • login
  • gdm
  • kdm
  • xdm
  • ssh
  • scp
  • sftp
  • FTP clients
  • Email clients
  • Any PAM aware services
Programs and services that are not PAM aware. Enabling Automatic Logouts
When the user is logged in as root, an unattended login session may pose a significant security risk. To reduce this risk, you can configure the system to automatically log out idle users after a fixed period of time:
  1. Make sure the screen package is installed. You can do so by running the following command as root:
    ~]# yum install screen
    For more information on how to install packages in Red Hat Enterprise Linux, see the Installing Packages section in the Red Hat Enterprise Linux 6 Deployment Guide.
  2. As root, add the following line at the beginning of the /etc/profile file to make sure the processing of this file cannot be interrupted:
    trap "" 1 2 3 15
  3. Add the following lines at the end of the /etc/profile file to start a screen session each time a user logs in to a virtual console or remotely:
    if [ -w $(tty) ]; then
      trap "exec $SCREENEXEC" 1 2 3 15
      echo -n 'Starting session in 10 seconds'
      sleep 10
      exec $SCREENEXEC
    Note that each time a new session starts, a message will be displayed and the user will have to wait ten seconds. To adjust the time to wait before starting a session, change the value after the sleep command.
  4. Add the following lines to the /etc/screenrc configuration file to close the screen session after a given period of inactivity:
    idle 120 quit 
    autodetach off
    This will set the time limit to 120 seconds. To adjust this limit, change the value after the idle directive.
    Alternatively, you can configure the system to only lock the session by using the following lines instead:
    idle 120 lockscreen
    autodetach off
    This way, a password will be required to unlock the session.
The changes take effect the next time a user logs in to the system. Limiting Root Access
Rather than completely denying access to the root user, the administrator may want to allow access only by setuid programs, such as su or sudo. For more information on su and sudo, see the Red Hat Enterprise Linux 6 Deployment Guide and the su(1) and sudo(8) man pages. Account Locking
In Red Hat Enterprise Linux 6, the pam_faillock PAM module allows system administrators to lock out user accounts after a specified number of failed attempts. Limiting user login attempts serves mainly as a security measure that aims to prevent possible brute force attacks targeted to obtain a user's account password.
With the pam_faillock module, failed login attempts are stored in a separate file for each user in the /var/run/faillock directory.


The order of lines in the failed attempt log files is important. Any change in this order can lock all user accounts, including the root user account when the even_deny_root option is used.
Follow these steps to configure account locking:
  1. To lock out any non-root user after three unsuccessful attempts and unlock that user after 10 minutes, add the following lines to the auth section of the /etc/pam.d/system-auth and /etc/pam.d/password-auth files:
    auth        required preauth silent audit deny=3 unlock_time=600
    auth        sufficient nullok try_first_pass
    auth        [default=die] authfail audit deny=3 unlock_time=600
  2. Add the following line to the account section of both files specified in the previous step:
    account     required
  3. To apply account locking for the root user as well, add the even_deny_root option to the pam_faillock entries in the /etc/pam.d/system-auth and /etc/pam.d/password-auth files:
    auth        required preauth silent audit deny=3 even_deny_root unlock_time=600
    auth        sufficient nullok try_first_pass
    auth        [default=die] authfail audit deny=3 even_deny_root unlock_time=600
    account     required
When user john attempts to log in for the fourth time after failing to log in three times previously, his account is locked upon the fourth attempt:
[user@localhost ~]$ su - john
Account locked due to 3 failed logins
su: incorrect password
To prevent the system from locking users out even after multiple failed logins, add the following line just above the line where pam_faillock is called for the first time in both /etc/pam.d/system-auth and /etc/pam.d/password-auth. Also replace user1, user2, user3 with the actual user names.
auth [success=1 default=ignore] user in user1:user2:user3
To view the number of failed attempts per user, run, as root, the following command:
[root@localhost ~]# faillock 
When                Type  Source                                           Valid
2013-03-05 11:44:14 TTY   pts/0                                                V
To unlock a user's account, run, as root, the following command:
faillock --user <username> --reset
When modifying authentication configuration using the authconfig utility, the system-auth and password-auth files are overwritten with the settings from the authconfig utility. This can be avoided by creating symbolic links in place of the configuration files, which authconfig recognizes and does not overwrite. In order to use custom settings in the configuration files and authconfig simultaneously, configure account locking using the following steps:
  1. Rename the configuration files:
    ~]# mv /etc/pam.d/system-auth /etc/pam.d/system-auth-local
    ~]# mv /etc/pam.d/password-auth /etc/pam.d/password-auth-local
  2. Create the following symbolic links:
    ~]# ln -s /etc/pam.d/system-auth-local /etc/pam.d/system-auth
    ~]# ln -s /etc/pam.d/password-auth-local /etc/pam.d/password-auth
  3. The /etc/pam.d/system-auth-local file should contain the following lines:
    auth        required preauth silent audit deny=3 unlock_time=600
    auth        include        system-auth-ac
    auth        [default=die] authfail silent audit deny=3 unlock_time=600
    account     required
    account     include        system-auth-ac
    password    include        system-auth-ac
    session     include        system-auth-ac
  4. The /etc/pam.d/password-auth-local file should contain the following lines:
    auth        required preauth silent audit deny=3 unlock_time=600
    auth        include        password-auth-ac
    auth        [default=die] authfail silent audit deny=3 unlock_time=600
    account     required
    account     include        password-auth-ac
    password    include        system-auth-ac
    session     include        system-auth-ac
For more information on various pam_faillock configuration options, see the pam_faillock(8) man page.

2.1.10. Session Locking

Users may need to leave their workstation unattended for a number of reasons during everyday operation. This could present an opportunity for an attacker to physically access the machine, especially in environments with insufficient physical security measures (see Section, “Physical Controls”). Laptops are especially exposed since their mobility interferes with physical security. You can alleviate these risks by using session locking features which prevent access to the system until a correct password is entered.


The main advantage of locking the screen instead of logging out is that a lock allows the user's processes (such as file transfers) to continue running. Logging out would stop these processes. Locking GNOME Using gnome-screensaver-command
The default desktop environment for Red Hat Enterprise Linux 6, GNOME, includes a feature which allows users to lock their screen at any time. There are several ways to activate the lock:
  • Press the key combination specified in SystemPreferencesKeyboard ShortcutsDesktopLock screen. The default combination is Ctrl+Alt+L.
  • Select SystemLock screen on the panel.
  • Execute the following command from a command line interface:
    gnome-screensaver-command -l
All of the techniques described have the same result: the screen saver is activated and the screen is locked. Users can then press any key to deactivate the screen saver, enter their password and continue working.
Keep in mind that this function requires the gnome-screensaver process to be running. You can check whether this is the case by using any command which provides information about processes. For example, execute the following command from the terminal:
pidof gnome-screensaver
If the gnome-screensaver process is currently running, a number denoting its identification number (PID) will be displayed on the screen after executing the command. If the process is not currently running, the command will provide no output at all.
Refer to the gnome-screensaver-command(1) man page for additional information.


The means of locking the screen described above rely on manual activation. Administrators should therefore advise their users to lock their computers every time they leave them unattended, even if only for a short period of time. Automatic Lock on Screen Saver Activation
As the name gnome-screensaver-command suggests, the locking functionality is tied to GNOME's screen saver. It is possible to tie the lock to the screen saver's activation, locking the workstation every time it is left unattended for a set period of time. This function is activated by default with a five minute timeout.
To change the automatic locking settings, select SystemPreferencesScreensaver on the main panel. This opens a window which allows setting the timeout period (the Regard the computer as idle after slider) and activating or deactivating the automatic lock (the Lock screen when screensaver is active check box).
Changing the screen saver preferences

Figure 2.2. Changing the screen saver preferences


Disabling the Activate screensaver when computer is idle option in the Screensaver Preferences dialog prevents the screen saver from starting automatically. Automatic locking is therefore disabled as well, but it is still possible to lock the workstation manually using the techniques described in Section, “Locking GNOME Using gnome-screensaver-command”. Remote Session Locking
You can also lock a GNOME session remotely using ssh as long as the target workstation accepts connections over this protocol. To remotely lock the screen on a machine you have access to, execute the following command:
ssh -X <username>@<server> "export DISPLAY=:0; gnome-screensaver-command -l"
Replace <username> with your user name and <server> with the IP address of the workstation you want to lock.
Refer to Section 3.2.2, “Secure Shell” for more information regarding ssh. Locking Virtual Consoles Using vlock
Users may also need to lock a virtual console. This can be done using a utility called vlock. To install this utility, execute the following command as root:
~]# yum install vlock
After installation, any console session can be locked using the vlock command without any additional parameters. This locks the currently active virtual console session while still allowing access to the others. To prevent access to all virtual consoles on the workstation, execute the following:
vlock -a
In this case, vlock locks the currently active console and the -a option prevents switching to other virtual consoles.
Refer to the vlock(1) man page for additional information.


There are several known issues relevant to the version of vlock currently available for Red Hat Enterprise Linux 6:
  • The program does not currently allow unlocking consoles using the root password. Additional information can be found in BZ#895066.
  • Locking a console does not clear the screen and scrollback buffer, allowing anyone with physical access to the workstation to view previously issued commands and any output displayed in the console. Refer to BZ#807369 for more information.

2.1.11. Available Network Services

While user access to administrative controls is an important issue for system administrators within an organization, monitoring which network services are active is of paramount importance to anyone who administers and operates a Linux system.
Many services under Red Hat Enterprise Linux 6 behave as network servers. If a network service is running on a machine, then a server application (called a daemon), is listening for connections on one or more network ports. Each of these servers should be treated as a potential avenue of attack. Risks To Services
Network services can pose many risks for Linux systems. Below is a list of some of the primary issues:
  • Denial of Service Attacks (DoS) — By flooding a service with requests, a denial of service attack can render a system unusable as it tries to log and answer each request.
  • Distributed Denial of Service Attack (DDoS) — A type of DoS attack which uses multiple compromised machines (often numbering in the thousands or more) to direct a coordinated attack on a service, flooding it with requests and making it unusable.
  • Script Vulnerability Attacks — If a server is using scripts to execute server-side actions, as Web servers commonly do, an attacker can attack improperly written scripts. These script vulnerability attacks can lead to a buffer overflow condition or allow the attacker to alter files on the system.
  • Buffer Overflow Attacks — Services that connect to ports numbered 0 through 1023 must run as an administrative user. If the application has an exploitable buffer overflow, an attacker could gain access to the system as the user running the daemon. Because exploitable buffer overflows exist, attackers use automated tools to identify systems with vulnerabilities, and once they have gained access, they use automated rootkits to maintain their access to the system.


The threat of buffer overflow vulnerabilities is mitigated in Red Hat Enterprise Linux by ExecShield, an executable memory segmentation and protection technology supported by x86-compatible uni- and multi-processor kernels. ExecShield reduces the risk of buffer overflow by separating virtual memory into executable and non-executable segments. Any program code that tries to execute outside of the executable segment (such as malicious code injected from a buffer overflow exploit) triggers a segmentation fault and terminates.
Execshield also includes support for No eXecute (NX) technology on AMD64 platforms and eXecute Disable (XD) technology on Itanium and Intel® 64 systems. These technologies work in conjunction with ExecShield to prevent malicious code from running in the executable portion of virtual memory with a granularity of 4KB of executable code, lowering the risk of attack from buffer overflow exploits.


To limit exposure to attacks over the network, disable all services that are unused. Identifying and Configuring Services
To enhance security, most network services installed with Red Hat Enterprise Linux are turned off by default. There are, however, some notable exceptions:
  • cupsd — The default print server for Red Hat Enterprise Linux.
  • lpd — An alternative print server.
  • xinetd — A super server that controls connections to a range of subordinate servers, such as gssftp and telnet.
  • sendmail — The Sendmail Mail Transport Agent (MTA) is enabled by default, but only listens for connections from the localhost.
  • sshd — The OpenSSH server, which is a secure replacement for Telnet.
When determining whether to leave these services running, it is best to use common sense and avoid taking any risks. For example, if a printer is not available, do not leave cupsd running. The same is true for portmap. If you do not mount NFSv3 volumes or use NIS (the ypbind service), then portmap should be disabled.
Services Configuration Tool

Figure 2.3. Services Configuration Tool

If unsure of the purpose for a particular service, the Services Configuration Tool has a description field, illustrated in Figure 2.3, “Services Configuration Tool”, that provides additional information.
Checking which network services are available to start at boot time is not sufficient. It is recommended to also check which ports are open and listening. Refer to Section 2.2.9, “Verifying Which Ports Are Listening” for more information. Insecure Services
Potentially, any network service is insecure. This is why turning off unused services is so important. Exploits for services are routinely revealed and patched, making it very important to regularly update packages associated with any network service. Refer to Section 1.5, “Security Updates” for more information.
Some network protocols are inherently more insecure than others. These include any services that:
  • Transmit Usernames and Passwords Over a Network Unencrypted — Many older protocols, such as Telnet and FTP, do not encrypt the authentication session and should be avoided whenever possible.
  • Transmit Sensitive Data Over a Network Unencrypted — Many protocols transmit data over the network unencrypted. These protocols include Telnet, FTP, HTTP, and SMTP. Many network file systems, such as NFS and SMB, also transmit information over the network unencrypted. It is the user's responsibility when using these protocols to limit what type of data is transmitted.
    Remote memory dump services, like netdump, transmit the contents of memory over the network unencrypted. Memory dumps can contain passwords or, even worse, database entries and other sensitive information.
    Other services like finger and rwhod reveal information about users of the system.
Examples of inherently insecure services include rlogin, rsh, telnet, and vsftpd.
All remote login and shell programs (rlogin, rsh, and telnet) should be avoided in favor of SSH. Refer to Section 2.1.13, “Security Enhanced Communication Tools” for more information about sshd.
FTP is not as inherently dangerous to the security of the system as remote shells, but FTP servers must be carefully configured and monitored to avoid problems. Refer to Section 2.2.6, “Securing FTP” for more information about securing FTP servers.
Services that should be carefully implemented and behind a firewall include:
  • finger
  • authd (this was called identd in previous Red Hat Enterprise Linux releases.)
  • netdump
  • netdump-server
  • nfs
  • rwhod
  • sendmail
  • smb (Samba)
  • yppasswdd
  • ypserv
  • ypxfrd
More information on securing network services is available in Section 2.2, “Server Security”.
The next section discusses tools available to set up a simple firewall.

2.1.12. Personal Firewalls

After the necessary network services are configured, it is important to implement a firewall.


Configure the necessary services and implement a firewall before connecting to the Internet or any other network that you do not trust.
Firewalls prevent network packets from accessing the system's network interface. If a request is made to a port that is blocked by a firewall, the request is ignored. If a service is listening on one of these blocked ports, it does not receive the packets and is effectively disabled. For this reason, ensure that you block access to ports not in use when configuring a firewall, while not blocking access to ports used by configured services.
For most users, the best tool for configuring a simple firewall is the graphical firewall configuration tool which includes Red Hat Enterprise Linux: the Firewall Configuration Tool (system-config-firewall). This tool creates broad iptables rules for a general-purpose firewall using a control panel interface.
Refer to Section 2.8.2, “Basic Firewall Configuration” for more information about using this application and its available options.
For advanced users and server administrators, manually configuring a firewall with iptables is preferable. Refer to Section 2.8, “Firewalls” for more information. Refer to Section 2.8.9, “IPTables” for a comprehensive guide to the iptables command.

2.1.13. Security Enhanced Communication Tools

As the size and popularity of the Internet has grown, so has the threat of communication interception. Over the years, tools have been developed to encrypt communications as they are transferred over the network.
Red Hat Enterprise Linux 6 includes two basic tools that use high-level, public-key-cryptography-based encryption algorithms to protect information as it travels over the network.
  • OpenSSH — A free implementation of the SSH protocol for encrypting network communication.
  • Gnu Privacy Guard (GPG) — A free implementation of the PGP (Pretty Good Privacy) encryption application for encrypting data.
OpenSSH is a safer way to access a remote machine and replaces older, unencrypted services like telnet and rsh. OpenSSH includes a network service called sshd and three command line client applications:
  • ssh — A secure remote console access client.
  • scp — A secure remote copy command.
  • sftp — A secure pseudo-ftp client that allows interactive file transfer sessions.
Refer to Section 3.2.2, “Secure Shell” for more information regarding OpenSSH.


Although the sshd service is inherently secure, the service must be kept up-to-date to prevent security threats. Refer to Section 1.5, “Security Updates” for more information.
GPG is one way to ensure private email communication. It can be used both to email sensitive data over public networks and to protect sensitive data on hard drives.

2.1.14. Enforcing Read-Only Mounting of Removable Media

To enforce read-only mounting of removable media (such as USB flash disks), the administrator can use a udev rule to detect removable media and configure them to be mounted read-only using the blockdev utility. Starting with Red Hat Enterprise Linux 6.7, a special parameter can be also passed to the udisks disk manager to force read-only mounting of file systems.
While the udev rule that triggers the blockdev utility is sufficient for enforcing read-only mounting of physical media, the udisks parameter can be used to enforce read-only mounting of filesystems on read-write mounted media.
Using blockdev to Force Read-Only Mounting of Removable Media
To force all removable media to be mounted read-only, create a new udev configuration file named, for example, 80-readonly-removables.rules in the /etc/udev/rules.d/ directory with the following content:
SUBSYSTEM=="block",ATTRS{removable}=="1",RUN{program}="/sbin/blockdev --setro %N"
The above udev rule ensures that any newly connected removable block (storage) device is automatically configured as read-only using the blockdev utility.
Using udisks to Force Read-Only Mounting of Filesystems
To force all file systems to be mounted read-only, a special udisks parameter needs to be set through udev. Create a new udev configuration file named, for example, 80-udisks.rules in the /etc/udev/rules.d/ directory with the following content (or add the following lines to this file if it already exists):
Note that a default 80-udisks.rules file is installed with the udisks package in the /lib/udev/rules.d/ directory. This file contains the above rules, but they are commented out.
The above udev rules instruct the udisks disk manager to only allow read-only mounting of file systems. Also, the noexec parameter forbids direct execution of any binaries on the mounted file systems. This policy is enforced regardless of the way the actual physical device is mounted. That is, file systems are mounted read-only even on read-write mounted devices.
Applying New udev and udisks Settings
For these settings to take effect, the new udev rules need to be applied. The udev service automatically detects changes to its configuration files, but new settings are not applied to already existing devices. Only newly connected devices are affected by the new settings. Therefore, you need to unmount and unplug all connected removable media to ensure that the new settings are applied to them when they are next plugged in.
To force udev to re-apply all rules to already existing devices, enter the following command as root:
~# udevadm trigger
Note that forcing udev to re-apply all rules using the above command does not affect any storage devices that are already mounted.
To force udev to reload all rules (in case the new rules are not automatically detected for some reason), use the following command:
~# udevadm control --reload

2.2. Server Security

When a system is used as a server on a public network, it becomes a target for attacks. Hardening the system and locking down services is therefore of paramount importance for the system administrator.
Before delving into specific issues, review the following general tips for enhancing server security:
  • Keep all services current, to protect against the latest threats.
  • Use secure protocols whenever possible.
  • Serve only one type of network service per machine whenever possible.
  • Monitor all servers carefully for suspicious activity.

2.2.1. Securing Services With TCP Wrappers and xinetd

TCP Wrappers provide access control to a variety of services. Most modern network services, such as SSH, Telnet, and FTP, make use of TCP Wrappers, which stand guard between an incoming request and the requested service.
The benefits offered by TCP Wrappers are enhanced when used in conjunction with xinetd, a super server that provides additional access, logging, binding, redirection, and resource utilization control.


It is a good idea to use iptables firewall rules in conjunction with TCP Wrappers and xinetd to create redundancy within service access controls. Refer to Section 2.8, “Firewalls” for more information about implementing firewalls with iptables commands.
The following subsections assume a basic knowledge of each topic and focus on specific security options. Enhancing Security With TCP Wrappers
TCP Wrappers are capable of much more than denying access to services. This section illustrates how they can be used to send connection banners, warn of attacks from particular hosts, and enhance logging functionality. Refer to the hosts_options man page for information about the TCP Wrapper functionality and control language. Refer to the xinetd.conf man page available online at for available flags, which act as options you can apply to a service. TCP Wrappers and Connection Banners
Displaying a suitable banner when users connect to a service is a good way to let potential attackers know that the system administrator is being vigilant. You can also control what information about the system is presented to users. To implement a TCP Wrappers banner for a service, use the banner option.
This example implements a banner for vsftpd. To begin, create a banner file. It can be anywhere on the system, but it must have same name as the daemon. For this example, the file is called /etc/banners/vsftpd and contains the following lines:
220-Hello, %c 
          220-All activity on is logged.
          220-Inappropriate use will result in your access privileges being removed.
The %c token supplies a variety of client information, such as the user name and hostname, or the user name and IP address to make the connection even more intimidating.
For this banner to be displayed to incoming connections, add the following line to the /etc/hosts.allow file:
vsftpd : ALL : banners /etc/banners/ TCP Wrappers and Attack Warnings
If a particular host or network has been detected attacking the server, TCP Wrappers can be used to warn the administrator of subsequent attacks from that host or network using the spawn directive.
In this example, assume that an attacker from the network has been detected attempting to attack the server. Place the following line in the /etc/hosts.deny file to deny any connection attempts from that network, and to log the attempts to a special file:
ALL : : spawn /bin/echo `date` %c %d >> /var/log/intruder_alert
The %d token supplies the name of the service that the attacker was trying to access.
To allow the connection and log it, place the spawn directive in the /etc/hosts.allow file.


Because the spawn directive executes any shell command, it is a good idea to create a special script to notify the administrator or execute a chain of commands in the event that a particular client attempts to connect to the server. TCP Wrappers and Enhanced Logging
If certain types of connections are of more concern than others, the log level can be elevated for that service using the severity option.
For this example, assume that anyone attempting to connect to port 23 (the Telnet port) on an FTP server is an attacker. To denote this, place an emerg flag in the log files instead of the default flag, info, and deny the connection.
To do this, place the following line in /etc/hosts.deny:
in.telnetd : ALL : severity emerg
This uses the default authpriv logging facility, but elevates the priority from the default value of info to emerg, which posts log messages directly to the console. Enhancing Security With xinetd
This section focuses on using xinetd to set a trap service and using it to control resource levels available to any given xinetd service. Setting resource limits for services can help thwart Denial of Service (DoS) attacks. Refer to the man pages for xinetd and xinetd.conf for a list of available options. Setting a Trap
One important feature of xinetd is its ability to add hosts to a global no_access list. Hosts on this list are denied subsequent connections to services managed by xinetd for a specified period or until xinetd is restarted. You can do this using the SENSOR attribute. This is an easy way to block hosts attempting to scan the ports on the server.
The first step in setting up a SENSOR is to choose a service you do not plan on using. For this example, Telnet is used.
Edit the file /etc/xinetd.d/telnet and change the flags line to read:
flags           = SENSOR
Add the following line:
deny_time       = 30
This denies any further connection attempts to that port by that host for 30 minutes. Other acceptable values for the deny_time attribute are FOREVER, which keeps the ban in effect until xinetd is restarted, and NEVER, which allows the connection and logs it.
Finally, the last line should read:
disable         = no
This enables the trap itself.
While using SENSOR is a good way to detect and stop connections from undesirable hosts, it has two drawbacks:
  • It does not work against stealth scans.
  • An attacker who knows that a SENSOR is running can mount a Denial of Service attack against particular hosts by forging their IP addresses and connecting to the forbidden port. Controlling Server Resources
Another important feature of xinetd is its ability to set resource limits for services under its control.
It does this using the following directives:
  • cps = <number_of_connections> <wait_period> — Limits the rate of incoming connections. This directive takes two arguments:
    • <number_of_connections> — The number of connections per second to handle. If the rate of incoming connections is higher than this, the service is temporarily disabled. The default value is fifty (50).
    • <wait_period> — The number of seconds to wait before re-enabling the service after it has been disabled. The default interval is ten (10) seconds.
  • instances = <number_of_connections> — Specifies the total number of connections allowed to a service. This directive accepts either an integer value or UNLIMITED.
  • per_source = <number_of_connections> — Specifies the number of connections allowed to a service by each host. This directive accepts either an integer value or UNLIMITED.
  • rlimit_as = <number[K|M]> — Specifies the amount of memory address space the service can occupy in kilobytes or megabytes. This directive accepts either an integer value or UNLIMITED.
  • rlimit_cpu = <number_of_seconds> — Specifies the amount of time in seconds that a service may occupy the CPU. This directive accepts either an integer value or UNLIMITED.
Using these directives can help prevent any single xinetd service from overwhelming the system, resulting in a denial of service.

2.2.2. Securing Portmap

The portmap service is a dynamic port assignment daemon for RPC services such as NIS and NFS. It has weak authentication mechanisms and has the ability to assign a wide range of ports for the services it controls. For these reasons, it is difficult to secure.


Securing portmap only affects NFSv2 and NFSv3 implementations, since NFSv4 no longer requires it. If you plan to implement an NFSv2 or NFSv3 server, then portmap is required, and the following section applies.
If running RPC services, follow these basic rules. Protect portmap With TCP Wrappers
It is important to use TCP Wrappers to limit which networks or hosts have access to the portmap service since it has no built-in form of authentication.
Further, use only IP addresses when limiting access to the service. Avoid using hostnames, as they can be forged by DNS poisoning and other methods. Protect portmap With iptables
To further restrict access to the portmap service, it is a good idea to add iptables rules to the server and restrict access to specific networks.
Below are two example iptables commands. The first allows TCP connections to the port 111 (used by the portmap service) from the network. The second allows TCP connections to the same port from the localhost. This is necessary for the sgi_fam service used by Nautilus. All other packets are dropped.
~]# iptables -A INPUT -p tcp -s ! --dport 111 -j DROP
~]# iptables -A INPUT -p tcp -s --dport 111 -j ACCEPT
To similarly limit UDP traffic, use the following command:
~]# iptables -A INPUT -p udp -s ! --dport 111 -j DROP


Refer to Section 2.8, “Firewalls” for more information about implementing firewalls with iptables commands.

2.2.3. Securing NIS

The Network Information Service (NIS) is an RPC service, called ypserv, which is used in conjunction with portmap and other related services to distribute maps of user names, passwords, and other sensitive information to any computer claiming to be within its domain.
A NIS server is comprised of several applications. They include the following:
  • /usr/sbin/rpc.yppasswdd — Also called the yppasswdd service, this daemon allows users to change their NIS passwords.
  • /usr/sbin/rpc.ypxfrd — Also called the ypxfrd service, this daemon is responsible for NIS map transfers over the network.
  • /usr/sbin/yppush — This application propagates changed NIS databases to multiple NIS servers.
  • /usr/sbin/ypserv — This is the NIS server daemon.
NIS is somewhat insecure by today's standards. It has no host authentication mechanisms and transmits all of its information over the network unencrypted, including password hashes. As a result, extreme care must be taken when setting up a network that uses NIS. This is further complicated by the fact that the default configuration of NIS is inherently insecure.
It is recommended that anyone planning to implement a NIS server first secure the portmap service as outlined in Section 2.2.2, “Securing Portmap”, then address the following issues, such as network planning. Carefully Plan the Network
Because NIS transmits sensitive information unencrypted over the network, it is important the service be run behind a firewall and on a segmented and secure network. Whenever NIS information is transmitted over an insecure network, it risks being intercepted. Careful network design can help prevent severe security breaches. Use a Password-like NIS Domain Name and Hostname
Any machine within a NIS domain can use commands to extract information from the server without authentication, as long as the user knows the NIS server's DNS hostname and NIS domain name.
For instance, if someone either connects a laptop computer into the network or breaks into the network from outside (and manages to spoof an internal IP address), the following command reveals the /etc/passwd map:
ypcat -d <NIS_domain> -h <DNS_hostname> passwd
If this attacker is a root user, they can obtain the /etc/shadow file by typing the following command:
ypcat -d <NIS_domain> -h <DNS_hostname> shadow


If Kerberos is used, the /etc/shadow file is not stored within a NIS map.
To make access to NIS maps harder for an attacker, create a random string for the DNS hostname, such as Similarly, create a different randomized NIS domain name. This makes it much more difficult for an attacker to access the NIS server. Edit the /var/yp/securenets File
If the /var/yp/securenets file is blank or does not exist (as is the case after a default installation), NIS listens to all networks. One of the first things to do is to put netmask/network pairs in the file so that ypserv only responds to requests from the appropriate network.
Below is a sample entry from a /var/yp/securenets file:


Never start a NIS server for the first time without creating the /var/yp/securenets file.
This technique does not provide protection from an IP spoofing attack, but it does at least place limits on what networks the NIS server services. Assign Static Ports and Use iptables Rules
All of the servers related to NIS can be assigned specific ports except for rpc.yppasswdd — the daemon that allows users to change their login passwords. Assigning ports to the other two NIS server daemons, rpc.ypxfrd and ypserv, allows for the creation of firewall rules to further protect the NIS server daemons from intruders.
To do this, add the following lines to /etc/sysconfig/network:
YPSERV_ARGS="-p 834"
YPXFRD_ARGS="-p 835"
The following iptables rules can then be used to enforce which network the server listens to for these ports:
~]# iptables -A INPUT -p ALL -s ! --dport 834 -j DROP
~]# iptables -A INPUT -p ALL -s ! --dport 835 -j DROP
This means that the server only allows connections to ports 834 and 835 if the requests come from the network, regardless of the protocol.


Refer to Section 2.8, “Firewalls” for more information about implementing firewalls with iptables commands. Use Kerberos Authentication
One of the issues to consider when NIS is used for authentication is that whenever a user logs into a machine, a password hash from the /etc/shadow map is sent over the network. If an intruder gains access to a NIS domain and sniffs network traffic, they can collect user names and password hashes. With enough time, a password cracking program can guess weak passwords, and an attacker can gain access to a valid account on the network.
Since Kerberos uses secret-key cryptography, no password hashes are ever sent over the network, making the system far more secure. Refer to Managing Single Sign-On and Smart Cards for more information about Kerberos.

2.2.4. Securing NFS


The version of NFS included in Red Hat Enterprise Linux 6, NFSv4, no longer requires the portmap service as outlined in Section 2.2.2, “Securing Portmap”. NFS traffic now utilizes TCP in all versions, rather than UDP, and requires it when using NFSv4. NFSv4 now includes Kerberos user and group authentication, as part of the RPCSEC_GSS kernel module. Information on portmap is still included, since Red Hat Enterprise Linux 6 supports NFSv2 and NFSv3, both of which utilize portmap. Carefully Plan the Network
NFSv2 and NFSv3 traditionally passed data insecurely. All versions of NFS now have the ability to authenticate (and optionally encrypt) ordinary file system operations using Kerberos. Under NFSv4 all operations can use Kerberos; under v2 or v3, file locking and mounting still do not use it. When using NFSv4.0, delegations may be turned off if the clients are behind NAT or a firewall. Refer to the section on pNFS in the Storage Administration Guide for information on the use of NFSv4.1 to allow delegations to operate through NAT and firewalls. Securing NFS Mount Options
The use of the mount command in the /etc/fstab file is explained in the Storage Administration Guide. From a security administration point of view it is worthwhile to note that the NFS mount options can also be specified in /etc/nfsmount.conf, which can be used to set custom default options. Review the NFS Server


Only export entire file systems. Exporting a subdirectory of a file system can be a security issue. It is possible in some cases for a client to "break out" of the exported part of the file system and get to unexported parts (see the section on subtree checking in the exports(5) man page.
Use the ro option to export the file system as read-only whenever possible to reduce the number of users able to write to the mounted file system. Only use the rw option when specifically required. Refer to the man exports(5) page for more information. Allowing write access increases the risk from symlink attacks for example. This includes temporary directories such as /tmp and /usr/tmp.
Where directories must be mounted with the rw option avoid making them world-writable whenever possible to reduce risk. Exporting home directories is also viewed as a risk as some applications store passwords in clear text or weakly encrypted. This risk is being reduced as application code is reviewed and improved. Some users do not set passwords on their SSH keys so this too means home directories present a risk. Enforcing the use of passwords or using Kerberos would mitigate that risk.
Restrict exports only to clients that need access. Use the showmount -e command on an NFS server to review what the server is exporting. Do not export anything that is not specifically required.
Do not use the no_root_squash option and review existing installations to make sure it is not used. Refer to Section, “Do Not Use the no_root_squash Option” for more information.
The secure option is the server-side export option used to restrict exports to reserved ports. By default, the server allows client communication only from reserved ports (ports numbered less than 1024), because traditionally clients have only allowed trusted code (such as in-kernel NFS clients) to use those ports. However, on many networks it is not difficult for anyone to become root on some client, so it is rarely safe for the server to assume that communication from a reserved port is privileged. Therefore the restriction to reserved ports is of limited value; it is better to rely on Kerberos, firewalls, and restriction of exports to particular clients.
Most clients still do use reserved ports when possible. However, reserved ports are a limited resource, so clients (especially those with a large number of NFS mounts) may choose to use higher-numbered ports as well. Linux clients may do this using the noresvport mount option. If you want to allow this on an export, you may do so with the insecure export option.
It is good practice not to allow users to login to a server. While reviewing the above settings on an NFS server conduct a review of who and what can access the server. Review the NFS Client
Use the nosuid option to disallow the use of a setuid program. The nosuid option disables the set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program. Use this option on the client and the server side.
The noexec option disables all executable files on the client. Use this to prevent users from inadvertently executing files placed in the file system being shared. The nosuid and noexec options are standard options for most, if not all, file systems.
Use the nodev option to prevent device-files from being processed as a hardware device by the client.
The resvport option is a client-side mount option and secure is the corresponding server-side export option (see explanation above). It restricts communication to a "reserved port". The reserved or "well known" ports are reserved for privileged users and processes such as the root user. Setting this option causes the client to use a reserved source port to communicate with the server.
All versions of NFS now support mounting with Kerberos authentication. The mount option to enable this is: sec=krb5.
NFSv4 supports mounting with Kerberos using krb5i for integrity and krb5p for privacy protection. These are used when mounting with sec=krb5, but need to be configured on the NFS server. Refer to the man page on exports (man 5 exports) for more information.
The NFS man page (man 5 nfs) has a SECURITY CONSIDERATIONS section which explains the security enhancements in NFSv4 and contains all the NFS specific mount options. Beware of Syntax Errors
The NFS server determines which file systems to export and which hosts to export these directories to by consulting the /etc/exports file. Be careful not to add extraneous spaces when editing this file.
For instance, the following line in the /etc/exports file shares the directory /tmp/nfs/ to the host with read/write permissions.
The following line in the /etc/exports file, on the other hand, shares the same directory to the host with read-only permissions and shares it to the world with read/write permissions due to a single space character after the hostname.
/tmp/nfs/ (rw)
It is good practice to check any configured NFS shares by using the showmount command to verify what is being shared:
showmount -e <hostname> Do Not Use the no_root_squash Option
By default, NFS shares change the root user to the nfsnobody user, an unprivileged user account. This changes the owner of all root-created files to nfsnobody, which prevents uploading of programs with the setuid bit set.
If no_root_squash is used, remote root users are able to change any file on the shared file system and leave applications infected by Trojans for other users to inadvertently execute. NFS Firewall Configuration
The ports used for NFS are assigned dynamically by rpcbind, which can cause problems when creating firewall rules. To simplify this process, use the /etc/sysconfig/nfs file to specify which ports are to be used:
  • MOUNTD_PORT — TCP and UDP port for mountd (rpc.mountd)
  • STATD_PORT — TCP and UDP port for status (rpc.statd)
  • LOCKD_TCPPORT — TCP port for nlockmgr (rpc.lockd)
  • LOCKD_UDPPORT — UDP port nlockmgr (rpc.lockd)
Port numbers specified must not be used by any other service. Configure your firewall to allow the port numbers specified, as well as TCP and UDP port 2049 (NFS).
Run the rpcinfo -p command on the NFS server to see which ports and RPC programs are being used.

2.2.5. Securing the Apache HTTP Server

The Apache HTTP Server is one of the most stable and secure services that ships with Red Hat Enterprise Linux. A large number of options and techniques are available to secure the Apache HTTP Server — too numerous to delve into deeply here. The following section briefly explains good practices when running the Apache HTTP Server.
Always verify that any scripts running on the system work as intended before putting them into production. Also, ensure that only the root user has write permissions to any directory containing scripts or CGIs. To do this, run the following commands as the root user:
chown root <directory_name>
chmod 755 <directory_name>
System administrators should be careful when using the following configuration options (configured in /etc/httpd/conf/httpd.conf):
This directive is enabled by default, so be sure to use caution when creating symbolic links to the document root of the Web server. For instance, it is a bad idea to provide a symbolic link to /.
This directive is enabled by default, but may not be desirable. To prevent visitors from browsing files on the server, remove this directive.
The UserDir directive is disabled by default because it can confirm the presence of a user account on the system. To enable user directory browsing on the server, use the following directives:
UserDir enabled
UserDir disabled root
These directives activate user directory browsing for all user directories other than /root/. To add users to the list of disabled accounts, add a space-delimited list of users on the UserDir disabled line.
The ServerTokens directive controls the server response header field which is sent back to clients. It includes various information which can be customized using the following parameters:
  • ServerTokens Full (default option) — provides all available information (OS type and used modules), for example:
    Apache/2.0.41 (Unix) PHP/4.2.2 MyMod/1.2
  • ServerTokens Prod or ServerTokens ProductOnly — provides the following information:
  • ServerTokens Major — provides the following information:
  • ServerTokens Minor — provides the following information:
  • ServerTokens Min or ServerTokens Minimal — provides the following information:
  • ServerTokens OS — provides the following information:
    Apache/2.0.41 (Unix)
It is recommended to use the ServerTokens Prod option so that a possible attacker does not gain any valuable information about your system.


Do not remove the IncludesNoExec directive. By default, the Server-Side Includes (SSI) module cannot execute commands. It is recommended that you do not change this setting unless absolutely necessary, as it could, potentially, enable an attacker to execute commands on the system.
Removing httpd Modules
In certain scenarios, it is beneficial to remove certain httpd modules to limit the functionality of the HTTP Server. To do so, simply comment out the entire line which loads the module you want to remove in the /etc/httpd/conf/httpd.conf file. For example, to remove the proxy module, comment out the following line by prepending it with a hash sign:
#LoadModule proxy_module modules/
Note that the /etc/httpd/conf.d/ directory contains configuration files which are used to load modules as well.
httpd and SELinux
For information regarding the Apache HTTP Server and SELinux, see the Managing Confined Services Guide.

2.2.6. Securing FTP

The File Transfer Protocol (FTP) is an older TCP protocol designed to transfer files over a network. Because all transactions with the server, including user authentication, are unencrypted, it is considered an insecure protocol and should be carefully configured.
Red Hat Enterprise Linux provides three FTP servers.
  • gssftpd — A Kerberos-aware xinetd-based FTP daemon that does not transmit authentication information over the network.
  • Red Hat Content Accelerator (tux) — A kernel-space Web server with FTP capabilities.
  • vsftpd — A standalone, security oriented implementation of the FTP service.
The following security guidelines are for setting up the vsftpd FTP service. FTP Greeting Banner
Before submitting a user name and password, all users are presented with a greeting banner. By default, this banner includes version information useful to attackers trying to identify weaknesses in a system.
To change the greeting banner for vsftpd, add the following directive to the /etc/vsftpd/vsftpd.conf file:
Replace <insert_greeting_here> in the above directive with the text of the greeting message.
For mutli-line banners, it is best to use a banner file. To simplify management of multiple banners, place all banners in a new directory called /etc/banners/. The banner file for FTP connections in this example is /etc/banners/ftp.msg. Below is an example of what such a file may look like:
######### Hello, all activity on is logged. #########


It is not necessary to begin each line of the file with 220 as specified in Section, “TCP Wrappers and Connection Banners”.
To reference this greeting banner file for vsftpd, add the following directive to the /etc/vsftpd/vsftpd.conf file:
It also is possible to send additional banners to incoming connections using TCP Wrappers as described in Section, “TCP Wrappers and Connection Banners”. Anonymous Access
The presence of the /var/ftp/ directory activates the anonymous account.
The easiest way to create this directory is to install the vsftpd package. This package establishes a directory tree for anonymous users and configures the permissions on directories to read-only for anonymous users.
By default the anonymous user cannot write to any directories.


If enabling anonymous access to an FTP server, be aware of where sensitive data is stored.

Procedure 2.1. Anonymous Upload

  1. To allow anonymous users to upload files, it is recommended to create a write-only directory within the /var/ftp/pub/ directory. Run the following command as root to create such directory named /upload/:
    ~]# mkdir /var/ftp/pub/upload
  2. Next, change the permissions so that anonymous users cannot view the contents of the directory:
    ~]# chmod 730 /var/ftp/pub/upload
    A long format listing of the directory should look like this:
    ~]# ls -ld /var/ftp/pub/upload
    drwx-wx---. 2 root ftp 4096 Nov 14 22:57 /var/ftp/pub/upload


    Administrators who allow anonymous users to read and write in directories often find that their servers become a repository of stolen software.
  3. Under vsftpd, add the following line to the /etc/vsftpd/vsftpd.conf file:
  4. In Red Hat Enterprise Linux, the SELinux is running in Enforcing mode by default. Therefore, the allow_ftpd_anon_write Boolean must be enabled in order to allow vsftpd to upload files:
    ~]# setsebool -P allow_ftpd_anon_write=1
  5. Label the /upload/ directory and its files with the public_content_rw_t SELinux context:
    ~]# semanage fcontext -a -t public_content_rw_t '/var/ftp/pub/upload(/.*)'


    The semanage utility is provided by the policycoreutils-python package, which is not installed by default. To install it, use the following command as root:
    ~]# yum install policycoreutils-python
  6. Use the restorecon utility to change the type of /upload/ and its files:
    ~]# restorecon -R -v /var/ftp/pub/upload
    The directory is now properly labeled with public_content_rw_t so that SELinux in Enforcing mode allows anonymous users to upload files to it:
    ~]$ ls -dZ /var/ftp/pub/upload
    drwx-wx---. root root unconfined_u:object_r:public_content_t:s0 /var/ftp/pub/upload/
    For further information about using SELinux, see the Security-Enhanced Linux User Guide and Managing Confined Services guides. User Accounts
Because FTP transmits unencrypted user names and passwords over insecure networks for authentication, it is a good idea to deny system users access to the server from their user accounts.
To disable all user accounts in vsftpd, add the following directive to /etc/vsftpd/vsftpd.conf:
local_enable=NO Restricting User Accounts
To disable FTP access for specific accounts or specific groups of accounts, such as the root user and those with sudo privileges, the easiest way is to use a PAM list file as described in Section, “Disallowing Root Access”. The PAM configuration file for vsftpd is /etc/pam.d/vsftpd.
It is also possible to disable user accounts within each service directly.
To disable specific user accounts in vsftpd, add the user name to /etc/vsftpd/ftpusers Use TCP Wrappers To Control Access
Use TCP Wrappers to control access to either FTP daemon as outlined in Section, “Enhancing Security With TCP Wrappers”.

2.2.7. Securing Postfix

Postfix is a Mail Transfer Agent (MTA) that uses the Simple Mail Transfer Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although many MTAs are capable of encrypting traffic between one another, most do not, so sending email over any public networks is considered an inherently insecure form of communication.
It is recommended that anyone planning to implement a Postfix server address the following issues. Limiting a Denial of Service Attack
Because of the nature of email, a determined attacker can flood the server with mail fairly easily and cause a denial of service. The effectiveness of such attacks can be limited by setting limits of the directives in the /etc/postfix/ file. You can change the value of the directives which are already there or you can add the directives you need with the value you want in the following format:
<directive> = <value>
The following is a list of directives that can be used for limiting a denial of service attack:
  • smtpd_client_connection_rate_limit — The maximum number of connection attempts any client is allowed to make to this service per time unit (described below). The default value is 0, which means a client can make as many connections per time unit as Postfix can accept. By default, clients in trusted networks are excluded.
  • anvil_rate_time_unit — This time unit is used for rate limit calculations. The default value is 60 seconds.
  • smtpd_client_event_limit_exceptions — Clients that are excluded from the connection and rate limit commands. By default, clients in trusted networks are excluded.
  • smtpd_client_message_rate_limit — The maximum number of message deliveries a client is allowed to request per time unit (regardless of whether or not Postfix actually accepts those messages).
  • default_process_limit — The default maximum number of Postfix child processes that provide a given service. This limit can be overruled for specific services in the file. By default the value is 100.
  • queue_minfree — The minimum amount of free space in bytes in the queue file system that is needed to receive mail. This is currently used by the Postfix SMTP server to decide if it will accept any mail at all. By default, the Postfix SMTP server rejects MAIL FROM commands when the amount of free space is less than 1.5 times the message_size_limit. To specify a higher minimum free space limit, specify a queue_minfree value that is at least 1.5 times the message_size_limit. By default the queue_minfree value is 0.
  • header_size_limit — The maximum amount of memory in bytes for storing a message header. If a header is larger, the excess is discarded. By default the value is 102400.
  • message_size_limit — The maximum size in bytes of a message, including envelope information. By default the value is 10240000. NFS and Postfix
Never put the mail spool directory, /var/spool/postfix/, on an NFS shared volume.
Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail.


With NFSv4 using Kerberos, this is not the case, since the SECRPC_GSS kernel module does not utilize UID-based authentication. However, it is still considered good practice not to put the mail spool directory on NFS shared volumes. Mail-only Users
To help prevent local user exploits on the Postfix server, it is best for mail users to only access the Postfix server using an email program. Shell accounts on the mail server should not be allowed and all user shells in the /etc/passwd file should be set to /sbin/nologin (with the possible exception of the root user). Disable Postfix Network Listening
By default, Postfix is set up to only listen to the local loopback address. You can verify this by viewing the file /etc/postfix/
View the file /etc/postfix/ to ensure that only the following inet_interfaces line appears:
inet_interfaces = localhost
This ensures that Postfix only accepts mail messages (such as cron job reports) from the local system and not from the network. This is the default setting and protects Postfix from a network attack.
For removal of the localhost restriction and allowing Postfix to listen on all interfaces the inet_interfaces = all setting can be used. Configuring Postfix to Use SASL
The Red Hat Enterprise Linux version of Postfix can use the Dovecot or Cyrus SASL implementations for SMTP Authentication (or SMTP AUTH). SMTP Authentication is an extension of the Simple Mail Transfer Protocol. When enabled, SMTP clients are required to authenticate to the SMTP server using an authentication method supported and accepted by both the server and the client. This section describes how to configure Postfix to make use of the Dovecot SASL implementation.
To install the Dovecot POP/IMAP server, and thus make the Dovecot SASL implementation available on your system, issue the following command as the root user:
~]# yum install dovecot
The Postfix SMTP server can communicate with the Dovecot SASL implementation using either a UNIX-domain socket or a TCP socket. The latter method is only needed in case the Postfix and Dovecot applications are running on separate machines. This guide gives preference to the UNIX-domain socket method, which affords better privacy.
In order to instruct Postfix to use the Dovecot SASL implementation, a number of configuration changes need to be performed for both applications. Follow the procedures below to effect these changes.
Setting Up Dovecot
  1. Modify the main Dovecot configuration file, /etc/dovecot/conf.d/10-master.conf, to include the following lines (the default configuration file already includes most of the relevant section, and the lines just need to be uncommented):
    service auth {
      unix_listener /var/spool/postfix/private/auth {
        mode = 0660
        user = postfix
        group = postfix
    The above example assumes the use of UNIX-domain sockets for communication between Postfix and Dovecot. It also assumes default settings of the Postfix SMTP server, which include the mail queue located in the /var/spool/postfix/ directory, and the application running under the postfix user and group. In this way, read and write permissions are limited to the postfix user and group.
    Alternatively, you can use the following configuration to set up Dovecot to listen for Postfix authentication requests via TCP:
    service auth {
      inet_listener {
        port = 12345
    In the above example, replace 12345 with the number of the port you want to use.
  2. Edit the /etc/dovecot/conf.d/10-auth.conf configuration file to instruct Dovecot to provide the Postfix SMTP server with the plain and login authentication mechanisms:
    auth_mechanisms = plain login
Setting Up Postfix
In the case of Postfix, only the main configuration file, /etc/postfix/, needs to be modified. Add or edit the following configuration directives:
  1. Enable SMTP Authentication in the Postfix SMTP server:
    smtpd_sasl_auth_enable = yes
  2. Instruct Postfix to use the Dovecot SASL implementation for SMTP Authentication:
    smtpd_sasl_type = dovecot
  3. Provide the authentication path relative to the Postfix queue directory (note that the use of a relative path ensures that the configuration works regardless of whether the Postfix server runs in a chroot or not):
    smtpd_sasl_path = private/auth
    This step assumes that you want to use UNIX-domain sockets for communication between Postfix and Dovecot. To configure Postfix to look for Dovecot on a different machine in case you use TCP sockets for communication, use configuration values similar to the following:
    smtpd_sasl_path = inet:
    In the above example, needs to be substituted by the IP address of the Dovecot machine and 12345 by the port specified in Dovecot's /etc/dovecot/conf.d/10-master.conf configuration file.
  4. Specify SASL mechanisms that the Postfix SMTP server makes available to clients. Note that different mechanisms can be specified for encrypted and unencrypted sessions.
    smtpd_sasl_security_options = noanonymous, noplaintext
    smtpd_sasl_tls_security_options = noanonymous
    The above example specifies that during unencrypted sessions, no anonymous authentication is allowed and no mechanisms that transmit unencrypted usernames or passwords are allowed. For encrypted sessions (using TLS), only non-anonymous authentication mechanisms are allowed.
    See for a list of all supported policies for limiting allowed SASL mechanisms.
Additional Resources
The following online resources provide additional information useful for configuring Postfix SMTP Authentication through SASL.

2.2.8. Securing Sendmail

Sendmail is a Mail Transfer Agent (MTA) that uses the Simple Mail Transfer Protocol (SMTP) to deliver electronic messages between other MTAs and to email clients or delivery agents. Although many MTAs are capable of encrypting traffic between one another, most do not, so sending email over any public networks is considered an inherently insecure form of communication.
It is recommended that anyone planning to implement a Sendmail server address the following issues. Limiting a Denial of Service Attack
Because of the nature of email, a determined attacker can flood the server with mail fairly easily and cause a denial of service. By setting limits to the following directives in /etc/mail/, the effectiveness of such attacks is limited.
  • confCONNECTION_RATE_THROTTLE — The number of connections the server can receive per second. By default, Sendmail does not limit the number of connections. If a limit is set and reached, further connections are delayed.
  • confMAX_DAEMON_CHILDREN — The maximum number of child processes that can be spawned by the server. By default, Sendmail does not assign a limit to the number of child processes. If a limit is set and reached, further connections are delayed.
  • confMIN_FREE_BLOCKS — The minimum number of free blocks which must be available for the server to accept mail. The default is 100 blocks.
  • confMAX_HEADERS_LENGTH — The maximum acceptable size (in bytes) for a message header.
  • confMAX_MESSAGE_SIZE — The maximum acceptable size (in bytes) for a single message. NFS and Sendmail
Never put the mail spool directory, /var/spool/mail/, on an NFS shared volume. Because NFSv2 and NFSv3 do not maintain control over user and group IDs, two or more users can have the same UID, and receive and read each other's mail.


With NFSv4 using Kerberos, this is not the case, since the SECRPC_GSS kernel module does not utilize UID-based authentication. However, it is still considered good practice not to put the mail spool directory on NFS shared volumes. Mail-only Users
To help prevent local user exploits on the Sendmail server, it is best for mail users to only access the Sendmail server using an email program. Shell accounts on the mail server should not be allowed and all user shells in the /etc/passwd file should be set to /sbin/nologin (with the possible exception of the root user). Disable Sendmail Network Listening
By default, Sendmail is set up to only listen to the local loopback address. You can verify this by viewing the file /etc/mail/ to ensure that the following line appears:
DAEMON_OPTIONS(`Port=smtp,Addr=, Name=MTA')dnl
This ensures that Sendmail only accepts mail messages (such as cron job reports) from the local system and not from the network. This is the default setting and protects Sendmail from a network attack.
For removal of the localhost restriction, the Addr= string needs to be removed. Changing Sendmail's configuration requires installing the sendmail-cf package, then editing the .mc file, running /etc/mail/make and finally restarting sendmail. The .cf configuration file will be regenerated. Note that the system clock must be correct and working and that there must not be any system clock time shifts between these actions in order for the configuration file to be automatically regenerated.

2.2.9. Verifying Which Ports Are Listening

Unnecessary open ports should be avoided because it increases the attack surface of your system. If after the system has been in service you find unexpected open ports in listening state, that might be signs of intrusion and it should be investigated.
Issue the following command, as root, from the console to determine which ports are listening for connections from the network:
~]# netstat -tanp | grep LISTEN
tcp        0      0     *                   LISTEN      1193/rpc.statd      
tcp        0      0  *                   LISTEN      1241/dnsmasq        
tcp        0      0     *                   LISTEN      1783/cupsd          
tcp        0      0      *                   LISTEN      7696/sendmail       
tcp        0      0       *                   LISTEN      1167/rpcbind        
tcp        0      0   *                   LISTEN      1118/tcsd           
tcp        0      0 :::631                      :::*                        LISTEN      1/init              
tcp        0      0 :::35018                    :::*                        LISTEN      1193/rpc.statd      
tcp        0      0 :::111                      :::*                        LISTEN      1167/rpcbind
Review the output of the command with the services needed on the system, turn off what is not specifically required or authorized, repeat the check. Proceed then to make external checks using nmap from another system connected via the network to the first system. This can be used verify the rules in iptables. Make a scan for every IP address shown in the netstat output (except for localhost or ::1 range) from an external system. Use the -6 option for scanning an IPv6 address. See man nmap(1) for more information.
The following is an example of the command to be issued from the console of another system to determine which ports are listening for TCP connections from the network:
~]# nmap -sT -O
See the netstat(8), nmap(1), and services(5) manual pages for more information.

2.2.10. Disable Source Routing

Source routing is an Internet Protocol mechanism that allows an IP packet to carry information, a list of addresses, that tells a router the path the packet must take. There is also an option to record the hops as the route is traversed. The list of hops taken, the "route record", provides the destination with a return path to the source. This allows the source (the sending host) to specify the route, loosely or strictly, ignoring the routing tables of some or all of the routers. It can allow a user to redirect network traffic for malicious purposes. Therefore, source-based routing should be disabled.
The accept_source_route option causes network interfaces to accept packets with the Strict Source Route (SSR) or Loose Source Routing (LSR) option set. The acceptance of source routed packets is controlled by sysctl settings. Issue the following command as root to drop packets with the SSR or LSR option set:
~]# /sbin/sysctl -w net.ipv4.conf.all.accept_source_route=0
Disabling the forwarding of packets should also be done in conjunction with the above when possible (disabling forwarding may interfere with virtualization). Issue the commands listed below as root:
These commands disable forwarding of IPv4 and IPv6 packets on all interfaces.
~]# /sbin/sysctl -w net.ipv4.conf.all.forwarding=0
~]# /sbin/sysctl -w net.ipv6.conf.all.forwarding=0
These commands disable forwarding of all multicast packets on all interfaces.
~]# /sbin/sysctl -w net.ipv4.conf.all.mc_forwarding=0
~]# /sbin/sysctl -w net.ipv6.conf.all.mc_forwarding=0
Accepting ICMP redirects has few legitimate uses. Disable the acceptance and sending of ICMP redirected packets unless specifically required.
These commands disable acceptance of all ICMP redirected packets on all interfaces:
~]# /sbin/sysctl -w net.ipv4.conf.all.accept_redirects=0
~]# /sbin/sysctl -w net.ipv6.conf.all.accept_redirects=0
This command disables acceptance of secure ICMP redirected packets on all interfaces:
~]# /sbin/sysctl -w net.ipv4.conf.all.secure_redirects=0
This command disables sending of all IPv4 ICMP redirected packets on all interfaces:
~]# /sbin/sysctl -w net.ipv4.conf.all.send_redirects=0


Sending of ICMP redirects remains active if at least one of the net.ipv4.conf.all.send_redirects or net.ipv4.conf.interface.send_redirects options is set to enabled. Ensure that you set the net.ipv4.conf.interface.send_redirects option to the 0 value for every interface. To automatically disable sending of ICMP requests whenever you add a new interface, enter the following command:
~]# /sbin/sysctl -w net.ipv4.conf.default.send_redirects=0
There is only a directive to disable sending of IPv4 redirected packets. Refer to RFC4294 for an explanation of IPv6 Node Requirements, which resulted in this difference between IPv4 and IPv6.
In order to make the settings permanent they must be added to /etc/sysctl.conf.
See the sysctl(8) manual page for more information. Refer to RFC791 for an explanation of the Internet options related to source based routing and its variants.


Ethernet networks provide additional ways to redirect traffic, such as ARP or MAC address spoofing, unauthorized DHCP servers, and IPv6 router or neighbor advertisements. In addition, unicast traffic is occasionally broadcast, causing information leaks. These weaknesses can only be addressed by specific countermeasures implemented by the network operator. Host-based countermeasures are not fully effective.

2.2.11. Reverse Path Forwarding

Reverse Path Forwarding is used to prevent packets that arrived via one interface from leaving via a different interface. When outgoing routes and incoming routes are different, it is sometimes referred to as asymmetric routing. Routers often route packets this way, but most hosts should not need to do this. Exceptions are such applications that involve sending traffic out over one link and receiving traffic over another link from a different service provider. For example, using leased lines in combination with xDSL or satellite links with 3G modems. If such a scenario is applicable to you, then turning off reverse path forwarding on the incoming interface is necessary. In short, unless you know that it is required, it is best enabled as it prevents users spoofing IP addresses from local subnets and reduces the opportunity for DDoS attacks.


Red Hat Enterprise Linux 6 (unlike Red Hat Enterprise Linux 5) defaults to using Strict Reverse Path Forwarding. Red Hat Enterprise Linux 6 follows the Strict Reverse Path recommendation from RFC 3704, Ingress Filtering for Multihomed Networks. This currently only applies to IPv4 in Red Hat Enterprise Linux 6.


If forwarding is enabled, then Reverse Path Forwarding should only be disabled if there are other means for source-address validation (such as iptables rules for example).
Reverse Path Forwarding is enabled by means of the rp_filter directive. The rp_filter option is used to direct the kernel to select from one of three modes.
It takes the following form when setting the default behavior:
~]# /sbin/sysctl -w net.ipv4.conf.default.rp_filter=INTEGER
where INTEGER is one of the following:
  • 0 — No source validation.
  • 1 — Strict mode as defined in RFC 3704.
  • 2 — Loose mode as defined in RFC 3704.
The setting can be overridden per network interface using net.ipv4.interface.rp_filter. To make these settings persistent across reboot, modify the /etc/sysctl.conf file. Additional Resources
The following are resources that explain more about Reverse Path Forwarding.
  • Installed Documentation
    usr/share/doc/kernel-doc-version/Documentation/networking/ip-sysctl.txt — This file contains a complete list of files and options available in the /proc/sys/net/ipv4/ directory.
  • Useful Websites — The Red Hat Knowledgebase article about rp_filter.
    See RFC 3704 for an explanation of Ingress Filtering for Multihomed Networks.

2.3. Single Sign-on (SSO)

The Red Hat Enterprise Linux SSO functionality reduces the number of times Red Hat Enterprise Linux desktop users have to enter their passwords. Several major applications leverage the same underlying authentication and authorization mechanisms so that users can log in to Red Hat Enterprise Linux from the log-in screen, and then not need to re-enter their passwords. These applications are detailed below.
For more information on Pluggable Authentication Modules, see the Red  Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards guide.

2.4. Pluggable Authentication Modules (PAM)

Pluggable authentication modules are a common framework for authentication and security. Both of Red Hat Enterprise Linux's single sign-on methods — Kerberos and smart cards — depend on underlying PAM configuration.
For more information on Pluggable Authentication Modules, see the corresponding chapter in the Red  Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards guide.

2.5. Kerberos

Maintaining system security and integrity within a network is critical, and it encompasses every user, application, service, and server within the network infrastructure. It requires an understanding of everything that is running on the network and the manner in which these services are used. At the core of maintaining this security is maintaining access to these applications and services and enforcing that access.
Kerberos provides a mechanism that allows both users and machines to identify themselves to network and receive defined, limited access to the areas and services that the administrator configured. Kerberos authenticates entities by verifying their identity, and Kerberos also secures this authenticating data so that it cannot be accessed and used or tampered with by an outsider.
For more information on Pluggable Authentication Modules, see the corresponding chapter in the Red  Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards guide.

2.6. TCP Wrappers and xinetd

Controlling access to network services is one of the most important security tasks facing a server administrator. Red Hat Enterprise Linux provides several tools for this purpose. For example, an iptables-based firewall filters out unwelcome network packets within the kernel's network stack. For network services that utilize it, TCP Wrappers add an additional layer of protection by defining which hosts are or are not allowed to connect to "wrapped" network services. One such wrapped network service is the xinetd super server. This service is called a super server because it controls connections to a subset of network services and further refines access control.
Figure 2.4, “Access Control to Network Services” is a basic illustration of how these tools work together to protect network services.
Access Control to Network Services

Figure 2.4. Access Control to Network Services

For more information about using firewalls with iptables, see Section 2.8.9, “IPTables”.

2.6.1. TCP Wrappers

The TCP Wrappers packages (tcp_wrappers and tcp_wrappers-libs) are installed by default and provide host-based access control to network services. The most important component within the package is the /lib/ or /lib64/ library. In general terms, a TCP-wrapped service is one that has been compiled against the library.
When a connection attempt is made to a TCP-wrapped service, the service first references the host's access files (/etc/hosts.allow and /etc/hosts.deny) to determine whether or not the client is allowed to connect. In most cases, it then uses the syslog daemon (syslogd) to write the name of the requesting client and the requested service to /var/log/secure or /var/log/messages.
If a client is allowed to connect, TCP Wrappers release control of the connection to the requested service and take no further part in the communication between the client and the server.
In addition to access control and logging, TCP Wrappers can execute commands to interact with the client before denying or releasing control of the connection to the requested network service.
Because TCP Wrappers are a valuable addition to any server administrator's arsenal of security tools, most network services within Red Hat Enterprise Linux are linked to the library. Such applications include /usr/sbin/sshd, /usr/sbin/sendmail, and /usr/sbin/xinetd.


To determine if a network service binary is linked to, type the following command as the root user:
ldd <binary-name> | grep libwrap
Replace <binary-name> with the name of the network service binary. If the command returns straight to the prompt with no output, then the network service is not linked to
The following example indicates that /usr/sbin/sshd is linked to
~]# ldd /usr/sbin/sshd | grep libwrap => /lib/ (0x00655000) Advantages of TCP Wrappers
TCP Wrappers provide the following advantages over other network service control techniques:
  • Transparency to both the client and the wrapped network service — Both the connecting client and the wrapped network service are unaware that TCP Wrappers are in use. Legitimate users are logged and connected to the requested service while connections from banned clients fail.
  • Centralized management of multiple protocols — TCP Wrappers operate separately from the network services they protect, allowing many server applications to share a common set of access control configuration files, making for simpler management.

2.6.2. TCP Wrappers Configuration Files

To determine if a client is allowed to connect to a service, TCP Wrappers reference the following two files, which are commonly referred to as hosts access files:
  • /etc/hosts.allow
  • /etc/hosts.deny
When a TCP-wrapped service receives a client request, it performs the following steps:
  1. It references /etc/hosts.allow — The TCP-wrapped service sequentially parses the /etc/hosts.allow file and applies the first rule specified for that service. If it finds a matching rule, it allows the connection. If not, it moves on to the next step.
  2. It references /etc/hosts.deny — The TCP-wrapped service sequentially parses the /etc/hosts.deny file. If it finds a matching rule, it denies the connection. If not, it grants access to the service.
The following are important points to consider when using TCP Wrappers to protect network services:
  • Because access rules in hosts.allow are applied first, they take precedence over rules specified in hosts.deny. Therefore, if access to a service is allowed in hosts.allow, a rule denying access to that same service in hosts.deny is ignored.
  • The rules in each file are read from the top down and the first matching rule for a given service is the only one applied. The order of the rules is extremely important.
  • If no rules for the service are found in either file, or if neither file exists, access to the service is granted.
  • TCP-wrapped services do not cache the rules from the hosts access files, so any changes to hosts.allow or hosts.deny take effect immediately, without restarting network services.


If the last line of a hosts access file is not a newline character (created by pressing the Enter key), the last rule in the file fails and an error is logged to either /var/log/messages or /var/log/secure. This is also the case for a rule that spans multiple lines without using the backslash character. The following example illustrates the relevant portion of a log message for a rule failure due to either of these circumstances:
warning: /etc/hosts.allow, line 20: missing newline or line too long Formatting Access Rules
The format for both /etc/hosts.allow and /etc/hosts.deny is identical. Each rule must be on its own line. Blank lines or lines that start with a hash (#) are ignored.
Each rule uses the following basic format to control access to network services:
<daemon list> : <client list> [: <option> : <option> : …]
  • <daemon list> — A comma-separated list of process names (not service names) or the ALL wildcard. The daemon list also accepts operators (refer to Section, “Operators”) to allow greater flexibility.
  • <client list> — A comma-separated list of hostnames, host IP addresses, special patterns, or wildcards which identify the hosts affected by the rule. The client list also accepts operators listed in Section, “Operators” to allow greater flexibility.
  • <option> — An optional action or colon-separated list of actions performed when the rule is triggered. Option fields support expansions, launch shell commands, allow or deny access, and alter logging behavior.
The following is a basic sample hosts access rule:
vsftpd :
This rule instructs TCP Wrappers to watch for connections to the FTP daemon (vsftpd) from any host in the domain. If this rule appears in hosts.allow, the connection is accepted. If this rule appears in hosts.deny, the connection is rejected.
The next sample hosts access rule is more complex and uses two option fields:
sshd :  \
	: spawn /bin/echo `/bin/date` access denied>>/var/log/sshd.log \
	: deny
Note that each option field is preceded by the backslash (\). Use of the backslash prevents failure of the rule due to length.
This sample rule states that if a connection to the SSH daemon (sshd) is attempted from a host in the domain, execute the echo command to append the attempt to a special log file, and deny the connection. Because the optional deny directive is used, this line denies access even if it appears in the hosts.allow file. Refer to Section, “Option Fields” for a more detailed look at available options. Wildcards
Wildcards allow TCP Wrappers to more easily match groups of daemons or hosts. They are used most frequently in the client list field of access rules.
The following wildcards are available:
  • ALL — Matches everything. It can be used for both the daemon list and the client list.
  • LOCAL — Matches any host that does not contain a period (.), such as localhost.
  • KNOWN — Matches any host where the hostname and host address are known or where the user is known.
  • UNKNOWN — Matches any host where the hostname or host address are unknown or where the user is unknown.
  • PARANOID — A reverse DNS lookup is done on the source IP address to obtain the host name. Then a DNS lookup is performed to resolve the IP address. If the two IP addresses do not match the connection is dropped and the logs are updated


The KNOWN, UNKNOWN, and PARANOID wildcards should be used with care, because they rely on a functioning DNS server for correct operation. Any disruption to name resolution may prevent legitimate users from gaining access to a service. Patterns
Patterns can be used in the client field of access rules to more precisely specify groups of client hosts.
The following is a list of common patterns for entries in the client field:
  • Hostname beginning with a period (.) — Placing a period at the beginning of a hostname matches all hosts sharing the listed components of the name. The following example applies to any host within the domain:
    ALL :
  • IP address ending with a period (.) — Placing a period at the end of an IP address matches all hosts sharing the initial numeric groups of an IP address. The following example applies to any host within the 192.168.x.x network:
    ALL : 192.168.
  • IP address/netmask pair — Netmask expressions can also be used as a pattern to control access to a particular group of IP addresses. The following example applies to any host with an address range of through
    ALL :


    When working in the IPv4 address space, the address/prefix length (prefixlen) pair declarations (CIDR notation) are not supported. Only IPv6 rules can use this format.
  • [IPv6 address]/prefixlen pair — [net]/prefixlen pairs can also be used as a pattern to control access to a particular group of IPv6 addresses. The following example would apply to any host with an address range of 3ffe:505:2:1:: through 3ffe:505:2:1:ffff:ffff:ffff:ffff:
    ALL : [3ffe:505:2:1::]/64
  • The asterisk (*) — Asterisks can be used to match entire groups of hostnames or IP addresses, as long as they are not mixed in a client list containing other types of patterns. The following example would apply to any host within the domain:
    ALL : *
  • The slash (/) — If a client list begins with a slash, it is treated as a file name. This is useful if rules specifying large numbers of hosts are necessary. The following example refers TCP Wrappers to the /etc/telnet.hosts file for all Telnet connections:
    in.telnetd : /etc/telnet.hosts
Other, less used patterns are also accepted by TCP Wrappers. Refer to the hosts_access man 5 page for more information.


Be very careful when using hostnames and domain names. Attackers can use a variety of tricks to circumvent accurate name resolution. In addition, disruption to DNS service prevents even authorized users from using network services. It is, therefore, best to use IP addresses whenever possible. Portmap and TCP Wrappers
Portmap's implementation of TCP Wrappers does not support host look-ups, which means portmap can not use hostnames to identify hosts. Consequently, access control rules for portmap in hosts.allow or hosts.deny must use IP addresses, or the keyword ALL, for specifying hosts.
Changes to portmap access control rules may not take effect immediately. You may need to restart the portmap service.
Widely used services, such as NIS and NFS, depend on portmap to operate, so be aware of these limitations. Operators
At present, access control rules accept one operator, EXCEPT. It can be used in both the daemon list and the client list of a rule.
The EXCEPT operator allows specific exceptions to broader matches within the same rule.
In the following example from a hosts.allow file, all hosts are allowed to connect to all services except
In another example from a hosts.allow file, clients from the 192.168.0.x network can use all services except for FTP:
ALL EXCEPT vsftpd : 192.168.0.


Organizationally, it is often easier to avoid using EXCEPT operators. This allows other administrators to quickly scan the appropriate files to see what hosts are allowed or denied access to services, without having to sort through EXCEPT operators. Option Fields
In addition to basic rules that allow and deny access, the Red Hat Enterprise Linux implementation of TCP Wrappers supports extensions to the access control language through option fields. By using option fields in hosts access rules, administrators can accomplish a variety of tasks such as altering log behavior, consolidating access control, and launching shell commands. Logging
Option fields let administrators easily change the log facility and priority level for a rule by using the severity directive.
In the following example, connections to the SSH daemon from any host in the domain are logged to the default authpriv syslog facility (because no facility value is specified) with a priority of emerg:
sshd : : severity emerg
It is also possible to specify a facility using the severity option. The following example logs any SSH connection attempts by hosts from the domain to the local0 facility with a priority of alert:
sshd : : severity local0.alert


In practice, this example does not work until the syslog daemon (syslogd) is configured to log to the local0 facility. Refer to the syslog.conf man page for information about configuring custom log facilities. Access Control
Option fields also allow administrators to explicitly allow or deny hosts in a single rule by adding the allow or deny directive as the final option.
For example, the following two rules allow SSH connections from, but deny connections from
sshd : : allow
sshd : : deny
By allowing access control on a per-rule basis, the option field allows administrators to consolidate all access rules into a single file: either hosts.allow or hosts.deny. Some administrators consider this an easier way of organizing access rules. Shell Commands
Option fields allow access rules to launch shell commands through the following two directives:
  • spawn — Launches a shell command as a child process. This directive can perform tasks like using /usr/sbin/safe_finger to get more information about the requesting client or create special log files using the echo command.
    In the following example, clients attempting to access Telnet services from the domain are quietly logged to a special file:
    in.telnetd : \
    	: spawn /bin/echo `/bin/date` from %h>>/var/log/telnet.log \
    	: allow
  • twist — Replaces the requested service with the specified command. This directive is often used to set up traps for intruders (also called "honey pots"). It can also be used to send messages to connecting clients. The twist directive must occur at the end of the rule line.
    In the following example, clients attempting to access FTP services from the domain are sent a message using the echo command:
    vsftpd : \
    	: twist /bin/echo "421 This domain has been black-listed. Access denied!"
For more information about shell command options, see the hosts_options man page. Expansions
Expansions, when used in conjunction with the spawn and twist directives, provide information about the client, server, and processes involved.
The following is a list of supported expansions:
  • %a — Returns the client's IP address.
  • %A — Returns the server's IP address.
  • %c — Returns a variety of client information, such as the user name and hostname, or the user name and IP address.
  • %d — Returns the daemon process name.
  • %h — Returns the client's hostname (or IP address, if the hostname is unavailable).
  • %H — Returns the server's hostname (or IP address, if the hostname is unavailable).
  • %n — Returns the client's hostname. If unavailable, unknown is printed. If the client's hostname and host address do not match, paranoid is printed.
  • %N — Returns the server's hostname. If unavailable, unknown is printed. If the server's hostname and host address do not match, paranoid is printed.
  • %p — Returns the daemon's process ID.
  • %s —Returns various types of server information, such as the daemon process and the host or IP address of the server.
  • %u — Returns the client's user name. If unavailable, unknown is printed.
The following sample rule uses an expansion in conjunction with the spawn command to identify the client host in a customized log file.
When connections to the SSH daemon (sshd) are attempted from a host in the domain, execute the echo command to log the attempt, including the client hostname (by using the %h expansion), to a special file:
sshd :  \
	: spawn /bin/echo `/bin/date` access denied to %h>>/var/log/sshd.log \
	: deny
Similarly, expansions can be used to personalize messages back to the client. In the following example, clients attempting to access FTP services from the domain are informed that they have been banned from the server:
vsftpd : \
: twist /bin/echo "421 %h has been banned from this server!"
For a full explanation of available expansions, as well as additional access control options, see section 5 of the man pages for hosts_access (man 5 hosts_access) and the man page for hosts_options.
Refer to Section 2.6.5, “Additional Resources” for more information about TCP Wrappers.

2.6.3. xinetd

The xinetd daemon is a TCP-wrapped super service which controls access to a subset of popular network services, including FTP, IMAP, and Telnet. It also provides service-specific configuration options for access control, enhanced logging, binding, redirection, and resource utilization control.
When a client attempts to connect to a network service controlled by xinetd, the super service receives the request and checks for any TCP Wrappers access control rules.
If access is allowed, xinetd verifies that the connection is allowed under its own access rules for that service. It also checks that the service is able to have more resources assigned to it and that it is not in breach of any defined rules.
If all these conditions are met (that is, access is allowed to the service; the service has not reached its resource limit; and the service is not in breach of any defined rule), xinetd then starts an instance of the requested service and passes control of the connection to it. After the connection has been established, xinetd takes no further part in the communication between the client and the server.

2.6.4. xinetd Configuration Files

The configuration files for xinetd are as follows:
  • /etc/xinetd.conf — The global xinetd configuration file.
  • /etc/xinetd.d/ — The directory containing all service-specific files. The /etc/xinetd.conf File
The /etc/xinetd.conf file contains general configuration settings which affect every service under xinetd's control. It is read when the xinetd service is first started, so for configuration changes to take effect, you need to restart the xinetd service. The following is a sample /etc/xinetd.conf file:
	 instances               = 60        
	 log_type                = SYSLOG	authpriv
	 log_on_success          = HOST PID
	 log_on_failure          = HOST
	 cps                     = 25 30
includedir /etc/xinetd.d
These lines control the following aspects of xinetd:
  • instances — Specifies the maximum number of simultaneous requests that xinetd can process.
  • log_type — Configures xinetd to use the authpriv log facility, which writes log entries to the /var/log/secure file. Adding a directive such as FILE /var/log/xinetdlog would create a custom log file called xinetdlog in the /var/log/ directory.
  • log_on_success — Configures xinetd to log successful connection attempts. By default, the remote host's IP address and the process ID of the server processing the request are recorded.
  • log_on_failure — Configures xinetd to log failed connection attempts or if the connection was denied.
  • cps — Configures xinetd to allow no more than 25 connections per second to any given service. If this limit is exceeded, the service is retired for 30 seconds.
  • includedir /etc/xinetd.d/ — Includes options declared in the service-specific configuration files located in the /etc/xinetd.d/ directory. Refer to Section, “The /etc/xinetd.d/ Directory” for more information.


Often, both the log_on_success and log_on_failure settings in /etc/xinetd.conf are further modified in the service-specific configuration files. More information may therefore appear in a given service's log file than the /etc/xinetd.conf file may indicate. Refer to Section, “Logging Options” for further information. The /etc/xinetd.d/ Directory
The /etc/xinetd.d/ directory contains the configuration files for each service managed by xinetd and the names of the files are correlated to the service. As with xinetd.conf, this directory is read only when the xinetd service is started. For any changes to take effect, the administrator must restart the xinetd service.
The format of files in the /etc/xinetd.d/ directory use the same conventions as /etc/xinetd.conf. The primary reason the configuration for each service is stored in a separate file is to make customization easier and less likely to affect other services.
To gain an understanding of how these files are structured, consider the /etc/xinetd.d/krb5-telnet file:
service telnet
	 flags           = REUSE
	 socket_type     = stream
	 wait            = no
	 user            = root
	 server          = /usr/kerberos/sbin/telnetd
	 log_on_failure  += USERID
	 disable         = yes
These lines control various aspects of the telnet service:
  • service — Specifies the service name, usually one of those listed in the /etc/services file.
  • flags — Sets any of a number of attributes for the connection. REUSE instructs xinetd to reuse the socket for a Telnet connection.


    The REUSE flag is deprecated. All services now implicitly use the REUSE flag.
  • socket_type — Sets the network socket type to stream.
  • wait — Specifies whether the service is single-threaded (yes) or multi-threaded (no).
  • user — Specifies which user ID the process runs under.
  • server — Specifies which binary executable to launch.
  • log_on_failure — Specifies logging parameters for log_on_failure in addition to those already defined in xinetd.conf.
  • disable — Specifies whether the service is disabled (yes) or enabled (no).
Refer to the xinetd.conf man page for more information about these options and their usage. Altering xinetd Configuration Files
A range of directives are available for services protected by xinetd. This section highlights some of the more commonly used options. Logging Options
The following logging options are available for both /etc/xinetd.conf and the service-specific configuration files within the /etc/xinetd.d/ directory.
The following is a list of some of the more commonly used logging options:
  • ATTEMPT — Logs the fact that a failed attempt was made (log_on_failure).
  • DURATION — Logs the length of time the service is used by a remote system (log_on_success).
  • EXIT — Logs the exit status or termination signal of the service (log_on_success).
  • HOST — Logs the remote host's IP address (log_on_failure and log_on_success).
  • PID — Logs the process ID of the server receiving the request (log_on_success).
  • USERID — Logs the remote user using the method defined in RFC 1413 for all multi-threaded stream services (log_on_failure and log_on_success).
For a complete list of logging options, see the xinetd.conf man page. Access Control Options
Users of xinetd services can choose to use the TCP Wrappers hosts access rules, provide access control via the xinetd configuration files, or a mixture of both. Refer to Section 2.6.2, “TCP Wrappers Configuration Files” for more information about TCP Wrappers hosts access control files.
This section discusses using xinetd to control access to services.


Unlike TCP Wrappers, changes to access control only take effect if the xinetd administrator restarts the xinetd service.
Also, unlike TCP Wrappers, access control through xinetd only affects services controlled by xinetd.
The xinetd hosts access control differs from the method used by TCP Wrappers. While TCP Wrappers places all of the access configuration within two files, /etc/hosts.allow and /etc/hosts.deny, xinetd's access control is found in each service's configuration file in the /etc/xinetd.d/ directory.
The following hosts access options are supported by xinetd:
  • only_from — Allows only the specified hosts to use the service.
  • no_access — Blocks listed hosts from using the service.
  • access_times — Specifies the time range when a particular service may be used. The time range must be stated in 24-hour format notation, HH:MM-HH:MM.
The only_from and no_access options can use a list of IP addresses or host names, or can specify an entire network. Like TCP Wrappers, combining xinetd access control with the enhanced logging configuration can increase security by blocking requests from banned hosts while verbosely recording each connection attempt.
For example, the following /etc/xinetd.d/telnet file can be used to block Telnet access from a particular network group and restrict the overall time range that even allowed users can log in:
service telnet
	 disable         = no
	 flags           = REUSE
	 socket_type     = stream
	 wait            = no
	 user            = root
	 server          = /usr/kerberos/sbin/telnetd
	 log_on_failure  += USERID
	 no_access       =
	 log_on_success  += PID HOST EXIT
	 access_times    = 09:45-16:15
In this example, when a client system from the network, such as, tries to access the Telnet service, it receives the following message:
Connection closed by foreign host.
In addition, their login attempts are logged in /var/log/messages as follows:
Sep  7 14:58:33 localhost xinetd[5285]: FAIL: telnet address from=
Sep  7 14:58:33 localhost xinetd[5283]: START: telnet pid=5285 from=
Sep  7 14:58:33 localhost xinetd[5283]: EXIT: telnet status=0 pid=5285 duration=0(sec)
When using TCP Wrappers in conjunction with xinetd access controls, it is important to understand the relationship between the two access control mechanisms.
The following is the sequence of events followed by xinetd when a client requests a connection:
  1. The xinetd daemon accesses the TCP Wrappers hosts access rules using a library call. If a deny rule matches the client, the connection is dropped. If an allow rule matches the client, the connection is passed to xinetd.
  2. The xinetd daemon checks its own access control rules both for the xinetd service and the requested service. If a deny rule matches the client, the connection is dropped. Otherwise, xinetd starts an instance of the requested service and passes control of the connection to that service.


Care should be taken when using TCP Wrappers access controls in conjunction with xinetd access controls. Misconfiguration can cause undesirable effects. Binding and Redirection Options
The service configuration files for xinetd support binding the service to an IP address and redirecting incoming requests for that service to another IP address, hostname, or port.
Binding is controlled with the bind option in the service-specific configuration files and links the service to one IP address on the system. When this is configured, the bind option only allows requests to the correct IP address to access the service. You can use this method to bind different services to different network interfaces based on requirements.
This is particularly useful for systems with multiple network adapters or with multiple IP addresses. On such a system, insecure services (for example, Telnet), can be configured to listen only on the interface connected to a private network and not to the interface connected to the Internet.
The redirect option accepts an IP address or hostname followed by a port number. It configures the service to redirect any requests for this service to the specified host and port number. This feature can be used to point to another port number on the same system, redirect the request to a different IP address on the same machine, shift the request to a totally different system and port number, or any combination of these options. A user connecting to a certain service on a system may therefore be rerouted to another system without disruption.
The xinetd daemon is able to accomplish this redirection by spawning a process that stays alive for the duration of the connection between the requesting client machine and the host actually providing the service, transferring data between the two systems.
The advantages of the bind and redirect options are most clearly evident when they are used together. By binding a service to a particular IP address on a system and then redirecting requests for this service to a second machine that only the first machine can see, an internal system can be used to provide services for a totally different network. Alternatively, these options can be used to limit the exposure of a particular service on a multi-homed machine to a known IP address, as well as redirect any requests for that service to another machine especially configured for that purpose.
For example, consider a system that is used as a firewall with this setting for its Telnet service:
service telnet
	 socket_type		= stream
	 wait			= no
	 server			= /usr/kerberos/sbin/telnetd
	 log_on_success		+= DURATION USERID
	 log_on_failure		+= USERID
	 bind                    =
	 redirect                = 23
The bind and redirect options in this file ensure that the Telnet service on the machine is bound to the external IP address (, the one facing the Internet. In addition, any requests for Telnet service sent to are redirected via a second network adapter to an internal IP address ( that only the firewall and internal systems can access. The firewall then sends the communication between the two systems, and the connecting system thinks it is connected to when it is actually connected to a different machine.
This feature is particularly useful for users with broadband connections and only one fixed IP address. When using Network Address Translation (NAT), the systems behind the gateway machine, which are using internal-only IP addresses, are not available from outside the gateway system. However, when certain services controlled by xinetd are configured with the bind and redirect options, the gateway machine can act as a proxy between outside systems and a particular internal machine configured to provide the service. In addition, the various xinetd access control and logging options are also available for additional protection. Resource Management Options
The xinetd daemon can add a basic level of protection from Denial of Service (DoS) attacks. The following is a list of directives which can aid in limiting the effectiveness of such attacks:
  • per_source — Defines the maximum number of instances for a service per source IP address. It accepts only integers as an argument and can be used in both xinetd.conf and in the service-specific configuration files in the xinetd.d/ directory.
  • cps — Defines the maximum number of connections per second. This directive takes two integer arguments separated by white space. The first argument is the maximum number of connections allowed to the service per second. The second argument is the number of seconds that xinetd must wait before re-enabling the service. It accepts only integers as arguments and can be used in either the xinetd.conf file or the service-specific configuration files in the xinetd.d/ directory.
  • max_load — Defines the CPU usage or load average threshold for a service. It accepts a floating point number argument.
    The load average is a rough measure of how many processes are active at a given time. See the uptime, who, and procinfo commands for more information about load average.
There are more resource management options available for xinetd. Refer to the xinetd.conf man page for more information.

2.6.5. Additional Resources

More information about TCP Wrappers and xinetd is available from system documentation and on the Internet. Installed TCP Wrappers Documentation
The documentation on your system is a good place to start looking for additional configuration options for TCP Wrappers, xinetd, and access control.
  • /usr/share/doc/tcp_wrappers-<version>/ — This directory contains a README file that discusses how TCP Wrappers work and the various hostname and host address spoofing risks that exist.
  • /usr/share/doc/xinetd-<version>/ — This directory contains a README file that discusses aspects of access control and a sample.conf file with various ideas for modifying service-specific configuration files in the /etc/xinetd.d/ directory.
  • TCP Wrappers and xinetd-related man pages — A number of man pages exist for the various applications and configuration files involved with TCP Wrappers and xinetd. The following are some of the more important man pages:
    Server Applications
    • man xinetd — The man page for xinetd.
    Configuration Files
    • man 5 hosts_access — The man page for the TCP Wrappers hosts access control files.
    • man hosts_options — The man page for the TCP Wrappers options fields.
    • man xinetd.conf — The man page listing xinetd configuration options.

2.7. Securing Virtual Private Networks (VPNs)

In Red Hat Enterprise Linux 6, a Virtual Private Network (VPN) can be configured using the IPsec tunneling protocol which is supported by the Libreswan application. Libreswan is a fork of the Openswan application and examples in documentation should be interchangeable. The NetworkManager IPsec plug-in is called NetworkManager-openswan.


Libreswan replaced Openswan as the preferred implementation of IPsec in Red Hat Enterprise Linux 6.8. Performing an upgrade from a version earlier than 6.8 replaces the openswan package with libreswan.
Libreswan is an open-source, user-space IPsec implementation available in Red Hat Enterprise Linux 6. It uses the Internet key exchange (IKE) protocol. IKE version 1 and 2 are implemented as a user-level daemon. Manual key establishment is also possible via ip xfrm commands, however this is not recommended. Libreswan interfaces with the Linux kernel using netlink to transfer the encryption keys. Packet encryption and decryption happen in the Linux kernel.
Libreswan uses the network security services (NSS) cryptographic library, which is required for Federal Information Processing Standard (FIPS) security compliance.

2.7.1. IPsec VPN Using Libreswan

To install Libreswan, issue the following command as root. Note that the libreswan package is available from the Extras repository, which needs to be enabled for the installation to succeed. See How to enable/disable a repository using Red Hat Subscription Manager? (The ID of the Extras repository is rhel-6-server-extras-rpms.)
~]# yum install libreswan
To check that Libreswan is installed, issue the following command:
~]$ yum info libreswan
After a new installation of Libreswan the NSS database should be initialized as part of the install process. However, should you need to start a new database, first remove the old database as follows:
~]# rm /etc/ipsec.d/*db
Then, to initialize a new NSS database, issue the following command as root:
~]# ipsec initnss
Initializing NSS database
See 'man pluto' if you want to protect the NSS database with a password
To start the ipsec daemon provided by Libreswan, issue the following command as root:
~]# service ipsec start
To confirm that the daemon is now running:
~]$ service ipsec status
pluto (pid  3496) is running...
To ensure that Libreswan will start when the system starts, issue the following command as root:
~]# chkconfig ipsec on
Configure any intermediate as well as host-based firewalls to permit the ipsec service. See Section 2.8, “Firewalls” for information on firewalls and allowing specific services to pass through. Libreswan requires the firewall to allow the following packets:
  • UDP port 500 for the Internet Key Exchange (IKE) protocol
  • UDP port 4500 for IKE NAT-Traversal
  • Protocol 50 for Encapsulated Security Payload (ESP) IPsec packets
  • Protocol 51 for Authenticated Header (AH) IPsec packets (uncommon)
We present three examples of using Libreswan to set up an IPsec VPN. The first example is for connecting two hosts together so that they may communicate securely. The second example is connecting two sites together to form one network. The third example is supporting roaming users, known as road warriors in this context.

2.7.2. VPN Configurations Using Libreswan

Libreswan does not use the terms source or destination. Instead, it uses the terms left and right to refer to end points (the hosts). This allows the same configuration to be used on both end points in most cases, although most administrators use left for the local host and right for the remote host.
There are three commonly used methods for authentication of endpoints:
  • Raw RSA keys are commonly used for static host-to-host or subnet-to-subnet IPsec configurations. The hosts are manually configured with each other's public RSA key. This method does not scale well when dozens or more hosts all need to setup IPsec tunnels to each other.
  • X.509 certificates are commonly used for large scale deployments where there are many hosts that need to connect to a common IPsec gateway. A central certificate authority (CA) is used to sign RSA certificates for hosts or users. This central CA is responsible for relaying trust, including the revocations of individual hosts or users.
  • Pre-Shared Keys (PSK) is the simplest authentication method. PSK's should consist of random characters and have a length of at least 20 characters. Due to the dangers of non-random and short PSKs, this is the least secure form of authentication and it is recommended to use either raw RSA keys or certificate based authentication instead.

2.7.3. Host-To-Host VPN Using Libreswan

To configure Libreswan to create a host-to-host IPsec VPN, between two hosts referred to as left and right, and enter the following commands as root on both of the hosts (left and right) to create new raw RSA key pairs:
~]# ipsec newhostkey --configdir /etc/ipsec.d \
--output /etc/ipsec.d/myvpn.secrets
Generated RSA key pair using the NSS database
This generates an RSA key pair for the host. The process of generating RSA keys can take many minutes, especially on virtual machines with low entropy.
To view the public key, issue the following command as root on either of the hosts. For example, to view the public key on the left host, run:
~]# ipsec showhostkey --left
ipsec showhostkey loading secrets from "/etc/ipsec.secrets"
ipsec showhostkey loading secrets from "/etc/ipsec.d/myvpn.secrets"
ipsec showhostkey loaded private key for keyid: PPK_RSA:AQOjAKLlL
	# rsakey AQOjAKLlL
	leftrsasigkey=0sAQOjAKLlL4a7YBv [...]
You have to add this key to the configuration file as explained in the following paragraphs.
The secret part is stored in /etc/ipsec.d/*.db files, also called the NSS database.
To make a configuration file for this host-to-host tunnel, the lines leftrsasigkey= and rightrsasigkey= from above, are added to a custom configuration file placed in the /etc/ipsec.d/ directory.
Using an editor running as root, create a file with a corresponding name in the following format:
Edit the file as follows:
conn myvpn
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
    # load and initiate automatically
You can use the identical configuration file on both left and right hosts. They auto-detect if they are left or right. If one of the hosts is a mobile host, which implies the IP address is not known in advance, then on the mobile host use %defaultroute as its IP address. This picks up the dynamic IP address automatically. On the static host that accepts connections from incoming mobile hosts, specify the mobile host using %any for its IP address.
Ensure the leftrsasigkey value is obtained from the left host and the rightrsasigkey value is obtained from the right host.
Restart ipsec to ensure it reads the new configuration:
~]# service ipsec --full-restart
To check the tunnel is succesfully established, and additionally see how much traffic has gone through the tunnel, enter the following command as root:
~]# ipsec whack --trafficstatus
006 #2: "myvpn", type=ESP, add_time=1234567890, inBytes=336, outBytes=336, id='@east'
Alternatively, if not using the auto=start option in the /etc/ipsec.d/*.conf file or if a tunnel is not succesfully established, use the following command as root to load the IPsec tunnel:
~]# ipsec auto --add myvpn
To bring up the tunnel, issue the following command as root, on the left or the right side:
~]# ipsec auto --up myvpn Verify Host-To-Host VPN Using Libreswan
The IKE negotiation takes place on UDP port 500. IPsec packets show up as Encapsulated Security Payload (ESP) packets. When the VPN connection needs to pass through a NAT router, the ESP packets are encapsulated in UDP packets on port 4500.
To verify that packets are being sent via the VPN tunnel, issue a command as root in the following format:
~]# tcpdump -n -i interface esp or udp port 500 or udp port 4500
00:32:32.632165 IP > ESP(spi=0x63ad7e17,seq=0x1a), length 132
00:32:32.632592 IP > ESP(spi=0x4841b647,seq=0x1a), length 132
00:32:32.632592 IP > ICMP echo reply, id 2489, seq 7, length 64
00:32:33.632221 IP > ESP(spi=0x63ad7e17,seq=0x1b), length 132
00:32:33.632731 IP > ESP(spi=0x4841b647,seq=0x1b), length 132
00:32:33.632731 IP > ICMP echo reply, id 2489, seq 8, length 64
00:32:34.632183 IP > ESP(spi=0x63ad7e17,seq=0x1c), length 132
00:32:34.632607 IP > ESP(spi=0x4841b647,seq=0x1c), length 132
00:32:34.632607 IP > ICMP echo reply, id 2489, seq 9, length 64
00:32:35.632233 IP > ESP(spi=0x63ad7e17,seq=0x1d), length 132
00:32:35.632685 IP > ESP(spi=0x4841b647,seq=0x1d), length 132
00:32:35.632685 IP > ICMP echo reply, id 2489, seq 10, length 64
Where interface is the interface known to carry the traffic. To end the capture with tcpdump, press Ctrl+C.


The tcpdump commands interacts a little unexpectedly with IPsec. It only sees the outgoing encrypted packet, not the outgoing plaintext packet. It does see the encrypted incoming packet, as well as the decrypted incoming packet. If possible, run tcpdump on a router between the two machines and not on one of the endpoints itself.

2.7.4. Site-to-Site VPN Using Libreswan

To create a site-to-site IPsec VPN, joining together two networks, an IPsec tunnel is created between two hosts, endpoints, which are configured to permit traffic from one or more subnets to pass through. They can therefore be thought of as gateways to the remote portion of the network. The configuration of the site-to-site VPN only differs from the host-to-host VPN in that one or more networks or subnets must be specified in the configuration file.
To configure Libreswan to create a site-to-site IPsec VPN, first configure a host-to-host IPsec VPN as described in Section 2.7.3, “Host-To-Host VPN Using Libreswan” and then copy or move the file to a file with a suitable name, such as /etc/ipsec.d/my_site-to-site.conf. Using an editor running as root, edit the custom configuration file /etc/ipsec.d/my_site-to-site.conf as follows:
conn mysubnet

conn mysubnet6

conn mytunnel
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
To bring the tunnels up, restart Libreswan or manually load and initiate all the connections using the following commands as root:
~]# ipsec auto --add mysubnet
~]# ipsec auto --add mysubnet6
~]# ipsec auto --add mytunnel
~]# ipsec auto --up mysubnet
104 "mysubnet" #1: STATE_MAIN_I1: initiate
003 "mysubnet" #1: received Vendor ID payload [Dead Peer Detection]
003 "mytunnel" #1: received Vendor ID payload [FRAGMENTATION]
106 "mysubnet" #1: STATE_MAIN_I2: sent MI2, expecting MR2
108 "mysubnet" #1: STATE_MAIN_I3: sent MI3, expecting MR3
003 "mysubnet" #1: received Vendor ID payload [CAN-IKEv2]
004 "mysubnet" #1: STATE_MAIN_I4: ISAKMP SA established {auth=OAKLEY_RSA_SIG cipher=aes_128 prf=oakley_sha group=modp2048}
117 "mysubnet" #2: STATE_QUICK_I1: initiate
004 "mysubnet" #2: STATE_QUICK_I2: sent QI2, IPsec SA established tunnel mode {ESP=>0x9414a615 <0x1a8eb4ef xfrm=AES_128-HMAC_SHA1 NATOA=none NATD=none DPD=none}
~]# ipsec auto --up mysubnet6
003 "mytunnel" #1: received Vendor ID payload [FRAGMENTATION]
117 "mysubnet" #2: STATE_QUICK_I1: initiate
004 "mysubnet" #2: STATE_QUICK_I2: sent QI2, IPsec SA established tunnel mode {ESP=>0x06fe2099 <0x75eaa862 xfrm=AES_128-HMAC_SHA1 NATOA=none NATD=none DPD=none}
~]# ipsec auto --up mytunnel
104 "mytunnel" #1: STATE_MAIN_I1: initiate
003 "mytunnel" #1: received Vendor ID payload [Dead Peer Detection]
003 "mytunnel" #1: received Vendor ID payload [FRAGMENTATION]
106 "mytunnel" #1: STATE_MAIN_I2: sent MI2, expecting MR2
108 "mytunnel" #1: STATE_MAIN_I3: sent MI3, expecting MR3
003 "mytunnel" #1: received Vendor ID payload [CAN-IKEv2]
004 "mytunnel" #1: STATE_MAIN_I4: ISAKMP SA established {auth=OAKLEY_RSA_SIG cipher=aes_128 prf=oakley_sha group=modp2048}
117 "mytunnel" #2: STATE_QUICK_I1: initiate
004 "mytunnel" #2: STATE_QUICK_I2: sent QI2, IPsec SA established tunnel mode {ESP=>0x16bca4f7 >0x9c2ae273 xfrm=AES_128-HMAC_SHA1 NATOA=none NATD=none DPD=none} Verify Site-to-Site VPN Using Libreswan
Verifying that packets are being sent via the VPN tunnel is the same procedure as explained in Section, “Verify Host-To-Host VPN Using Libreswan”.

2.7.5. Site-to-Site Single Tunnel VPN Using Libreswan

Often, when a site-to-site tunnel is built, the gateways need to communicate with each other using their internal IP addresses instead of their public IP addresses. This can be accomplished using a single tunnel. If the left host, with host name west, has internal IP address and the right host, with host name east, has internal IP address, store the following configuration using a single tunnel to the /etc/ipsec.d/myvpn.conf file on both servers:
conn myvpn
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==

2.7.6. Subnet Extrusion Using Libreswan

IPsec is often deployed in a hub-and-spoke architecture. Each leaf node has an IP range that is part of a larger range. Leaves communicate with each other via the hub. This is called subnet extrusion. In the example below, we configure the head office with and two branches that use a smaller /24 subnet.
At the head office:
conn branch1

conn branch2
At the branch1 office, we use the same connection. Additionally, we use a pass-through connection to exclude our local LAN traffic from being sent through the tunnel:
conn branch1

conn passthrough

2.7.7. Road Warrior Access VPN Using Libreswan

Road warriors are traveling users with mobile clients with a dynamically assigned IP address, such as laptops. These are authenticated using certificates.
On the server:
conn roadwarriors
    # if access to the LAN is given, enable this
    # trust our own Certificate Agency
    # allow clients to be behind a NAT router
    # load connection, don't initiate
    # kill vanished roadwarriors
The value specifies the actual IP address or host name of your server.
This option specifies a certificate referring to its friendly name or nickname that has been used to import the certificate. Usually, the name is generated as a part of a PKCS #12 certificate bundle in the form of a .p12 file. See the pkcs12(1) and pk12util(1) man pages for more information.
On the mobile client, the road warrior's device, use a slight variation of the above configuration:
conn roadwarriors
    # pick up our dynamic IP
    # right can also be a DNS hostname
    # if access to the remote LAN is required, enable this
    # trust our own Certificate Agency
    # Initiate connection
This option enables the user to connect to the VPN whenever the ipsec system service is started. Replace it with the auto=add if you want to establish the connection later.

2.7.8. Road Warrior Access VPN Using Libreswan and XAUTH with X.509

Libreswan offers a method to natively assign IP address and DNS information to roaming VPN clients as the connection is established by using the XAUTH IPsec extension. Extended authentication (XAUTH) can be deployed using PSK or X.509 certificates. Deploying using X.509 is more secure. Client certificates can be revoked by a certificate revocation list or by Online Certificate Status Protocol (OCSP). With X.509 certificates, individual clients cannot impersonate the server. With a PSK, also called Group Password, this is theoretically possible.
XAUTH requires the VPN client to additionally identify itself with a user name and password. For One time Passwords (OTP), such as Google Authenticator or RSA SecureID tokens, the one-time token is appended to the user password.
There are three possible backends for XAUTH:
This uses the configuration in /etc/pam.d/pluto to authenticate the user. Pluggable Authentication Modules (PAM) can be configured to use various backends by itself. It can use the system account user-password scheme, an LDAP directory, a RADIUS server or a custom password authentication module. See the Using Pluggable Authentication Modules (PAM) chapter for more information.
This uses the configuration file /etc/ipsec.d/passwd (not to be confused with /etc/ipsec.d/nsspassword). The format of this file is similar to the Apache .htpasswd file and the Apache htpasswd command can be used to create entries in this file. However, after the user name and password, a third column is required with the connection name of the IPsec connection used, for example when using a conn remoteusers to offer VPN to remote users, a password file entry should look as follows:
NOTE: when using the htpasswd command, the connection name has to be manually added after the user:password part on each line.
The server will always pretend the XAUTH user and password combination was correct. The client still has to specify a user name and a password, although the server ignores these. This should only be used when users are already identified by X.509 certificates, or when testing the VPN without needing an XAUTH backend.
An example server configuration with X.509 certificates:
conn xauth-rsa
    modecfgbanner="Authorized access is allowed"
    # for walled-garden on xauth failure
    # xauthfail=soft
When xauthfail is set to soft, instead of hard, authentication failures are ignored, and the VPN is setup as if the user authenticated properly. A custom updown script can be used to check for the environment variable XAUTH_FAILED. Such users can then be redirected, for example, using iptables DNAT, to a walled garden where they can contact the administrator or renew a paid subscription to the service.
VPN clients use the modecfgdomain value and the DNS entries to redirect queries for the specified domain to these specified nameservers. This allows roaming users to access internal-only resources using the internal DNS names.
The modecfgdns options contain a comma-separated list of internal DNS servers for the client to use for DNS resolution. Optionally, to send a banner text to VPN cliens, use the modecfgbanner option.
If leftsubnet is not, split tunneling configuration requests are sent automatically to the client. For example, when using leftsubnet=, the VPN client would only send traffic for through the VPN.
On the client, the user has to input a user password, which depends on the backend used. For example:
The administrator generated the password and stored it in the /etc/ipsec.d/passwd file.
The password is obtained at the location specified in the PAM configuration in the /etc/pam.d/pluto file.
The password is not checked and always accepted. Use this option for testing purposes or if you want to ensure compatibility for xauth-only clients.
For more information about XAUTH, see the Extended Authentication within ISAKMP/Oakley (XAUTH) Internet-Draft document.

2.7.9. Additional Resources

The following sources of information provide additional resources regarding Libreswan and the ipsec daemon. Installed Documentation
  • ipsec(8) man page — Describes command options for ipsec.
  • ipsec.conf(5) man page — Contains information on configuring ipsec.
  • ipsec.secrets(5) man page — Describes the format of the ipsec.secrets file.
  • ipsec_auto(8) man page — Describes the use of the auto command line client for manipulating Libreswan IPsec connections established using automatic exchanges of keys.
  • ipsec_rsasigkey(8) man page — Describes the tool used to generate RSA signature keys.
  • /usr/share/doc/libreswan-version/README.nss — Describes the commands for using raw RSA keys and certificates with the NSS crypto library used with the Libreswan pluto daemon. Online Documentation
The website of the upstream project.
Network Security Services (NSS) project.

2.8. Firewalls

Information security is commonly thought of as a process and not a product. However, standard security implementations usually employ some form of dedicated mechanism to control access privileges and restrict network resources to users who are authorized, identifiable, and traceable. Red Hat Enterprise Linux includes several tools to assist administrators and security engineers with network-level access control issues.
Firewalls are one of the core components of a network security implementation. Several vendors market firewall solutions catering to all levels of the marketplace: from home users protecting one PC to data center solutions safeguarding vital enterprise information. Firewalls can be stand-alone hardware solutions, such as firewall appliances by Cisco, Nokia, and Sonicwall. Vendors such as Checkpoint, McAfee, and Symantec have also developed proprietary software firewall solutions for home and business markets.
Apart from the differences between hardware and software firewalls, there are also differences in the way firewalls function that separate one solution from another. Table 2.6, “Firewall Types” details three common types of firewalls and how they function:
Table 2.6. Firewall Types
Method Description Advantages Disadvantages
NAT Network Address Translation (NAT) places private IP subnetworks behind one or a small pool of public IP addresses, masquerading all requests to one source rather than several. The Linux kernel has built-in NAT functionality through the Netfilter kernel subsystem.
Can be configured transparently to machines on a LAN.
Protection of many machines and services behind one or more external IP addresses simplifies administration duties.
Restriction of user access to and from the LAN can be configured by opening and closing ports on the NAT firewall/gateway.
Cannot prevent malicious activity once users connect to a service outside of the firewall.
Packet Filter A packet filtering firewall reads each data packet that passes through a LAN. It can read and process packets by header information and filters the packet based on sets of programmable rules implemented by the firewall administrator. The Linux kernel has built-in packet filtering functionality through the Netfilter kernel subsystem.
Customizable through the iptables front-end utility.
Does not require any customization on the client side, as all network activity is filtered at the router level rather than the application level.
Since packets are not transmitted through a proxy, network performance is faster due to direct connection from client to remote host.
Cannot filter packets for content like proxy firewalls.
Processes packets at the protocol layer, but cannot filter packets at an application layer.
Complex network architectures can make establishing packet filtering rules difficult, especially if coupled with IP masquerading or local subnets and DMZ networks.
Proxy Proxy firewalls filter all requests of a certain protocol or type from LAN clients to a proxy machine, which then makes those requests to the Internet on behalf of the local client. A proxy machine acts as a buffer between malicious remote users and the internal network client machines.
Gives administrators control over what applications and protocols function outside of the LAN.
Some proxy servers can cache frequently-accessed data locally rather than having to use the Internet connection to request it. This helps to reduce bandwidth consumption.
Proxy services can be logged and monitored closely, allowing tighter control over resource utilization on the network.
Proxies are often application-specific (HTTP, Telnet, etc.), or protocol-restricted (most proxies work with TCP-connected services only).
Application services cannot run behind a proxy, so your application servers must use a separate form of network security.
Proxies can become a network bottleneck, as all requests and transmissions are passed through one source rather than directly from a client to a remote service.

2.8.1. Netfilter and IPTables

The Linux kernel features a powerful networking subsystem called Netfilter. The Netfilter subsystem provides stateful or stateless packet filtering as well as NAT and IP masquerading services. Netfilter also has the ability to mangle IP header information for advanced routing and connection state management. Netfilter is controlled using the iptables tool. IPTables Overview
The power and flexibility of Netfilter is implemented using the iptables administration tool, a command line tool similar in syntax to its predecessor, ipchains, which Netfilter/iptables replaced in the Linux kernel 2.4 and above.
iptables uses the Netfilter subsystem to enhance network connection, inspection, and processing. iptables features advanced logging, pre- and post-routing actions, network address translation, and port forwarding, all in one command line interface.
This section provides an overview of iptables. For more detailed information, see Section 2.8.9, “IPTables”.

2.8.2. Basic Firewall Configuration

Just as a firewall in a building attempts to prevent a fire from spreading, a computer firewall attempts to prevent malicious software from spreading to your computer. It also helps to prevent unauthorized users from accessing your computer.
In a default Red Hat Enterprise Linux installation, a firewall exists between your computer or network and any untrusted networks, for example the Internet. It determines which services on your computer remote users can access. A properly configured firewall can greatly increase the security of your system. It is recommended that you configure a firewall for any Red Hat Enterprise Linux system with an Internet connection. Firewall Configuration Tool
During the Firewall Configuration screen of the Red Hat Enterprise Linux installation, you were given the option to enable a basic firewall as well as to allow specific devices, incoming services, and ports.
After installation, you can change this preference by using the Firewall Configuration Tool.
To start this application, either select SystemAdministrationFirewall from the panel, or type system-config-firewall at a shell prompt.
Firewall Configuration Tool

Figure 2.5. Firewall Configuration Tool


The Firewall Configuration Tool only configures a basic firewall. If the system needs more complex rules, see Section 2.8.9, “IPTables” for details on configuring specific iptables rules.
As of Red Hat Enterprise Linux 6.5, the iptables and ip6tables services now provide the ability to assign a fallback firewall configuration if the default configuration cannot be applied. If application of the firewall rules from /etc/sysconfig/iptables fails, the fallback file is applied if it exists. The fallback file is named /etc/sysconfig/iptables.fallback and uses the same file format as /etc/sysconfig/iptables. If application of the fallback file also fails, there is no further fallback. To create a fallback file, use the standard firewall configuration tool and rename or copy the file to the fallback file.
For the ip6tables service, replace all occurrences of iptables with ip6tables in the above examples.


If you have previously set up some custom packet-filtering rules by directly using the iptables utility (as described in Section 2.8.9, “IPTables”), running the system-config-firewall utility will erase these custom rules immediately. Enabling and Disabling the Firewall
Select one of the following options for the firewall:
  • Disabled — Disabling the firewall provides complete access to your system and does no security checking. This should only be selected if you are running on a trusted network (not the Internet) or need to configure a custom firewall using the iptables command line tool.


    Firewall configurations and any customized firewall rules are stored in the /etc/sysconfig/iptables file. If you choose Disabled and click OK, these configurations and firewall rules will be lost.
  • Enabled — This option configures the system to reject incoming connections that are not in response to outbound requests, such as DNS replies or DHCP requests. If access to services running on this machine is needed, you can choose to allow specific services through the firewall.
    If you are connecting your system to the Internet, but do not plan to run a server, this is the safest choice. Trusted Services
Enabling options in the Trusted services list allows the specified service to pass through the firewall.
The HTTP protocol is used by Apache (and by other Web servers) to serve web pages. If you plan on making your Web server publicly available, select this check box. This option is not required for viewing pages locally or for developing web pages. This service requires that the httpd package be installed.
Enabling WWW (HTTP) will not open a port for HTTPS, the SSL version of HTTP. If this service is required, select the Secure WWW (HTTPS) check box.
The FTP protocol is used to transfer files between machines on a network. If you plan on making your FTP server publicly available, select this check box. This service requires that the vsftpd package be installed.
Secure Shell (SSH) is a suite of tools for logging into and executing commands on a remote machine. To allow remote access to the machine via SSH, select this check box. This service requires that the openssh-server package be installed.
Telnet is a protocol for logging into remote machines. Telnet communications are unencrypted and provide no security from network snooping. Allowing incoming Telnet access is not recommended. To allow remote access to the machine via telnet, select this check box. This service requires that the telnet-server package be installed.
Mail (SMTP)
SMTP is a protocol that allows remote hosts to connect directly to your machine to deliver mail. You do not need to enable this service if you collect your mail from your ISP's server using POP3 or IMAP, or if you use a tool such as fetchmail. To allow delivery of mail to your machine, select this check box. Note that an improperly configured SMTP server can allow remote machines to use your server to send spam.
The Network File System (NFS) is a file sharing protocol commonly used on *NIX systems. Version 4 of this protocol is more secure than its predecessors. If you want to share files or directories on your system with other network users, select this check box.
Samba is an implementation of Microsoft's proprietary SMB networking protocol. If you need to share files, directories, or locally-connected printers with Microsoft Windows machines, select this check box. Other Ports
The Firewall Configuration Tool includes an Other ports section for specifying custom IP ports as being trusted by iptables. For example, to allow IRC and Internet printing protocol (IPP) to pass through the firewall, add the following to the Other ports section:
194:tcp,631:tcp Saving the Settings
Click OK to save the changes and enable or disable the firewall. If Enable firewall was selected, the options selected are translated to iptables commands and written to the /etc/sysconfig/iptables file. The iptables service is also started so that the firewall is activated immediately after saving the selected options. If Disable firewall was selected, the /etc/sysconfig/iptables file is removed and the iptables service is stopped immediately.
The selected options are also written to the /etc/sysconfig/system-config-firewall file so that the settings can be restored the next time the application is started. Do not edit this file manually.
Even though the firewall is activated immediately, the iptables service is not configured to start automatically at boot time. Refer to Section, “Activating the IPTables Service” for more information. Activating the IPTables Service
The firewall rules are only active if the iptables service is running. To manually start the service, use the following command as the root user:
~]# service iptables restart
iptables: Applying firewall rules:                         [  OK  ]
To ensure that iptables starts when the system is booted, use the following command:
~]# chkconfig --level 345 iptables on

2.8.3. Using IPTables

The first step in using iptables is to start the iptables service. Use the following command as the root user to start the iptables service:
~]# service iptables restart
iptables: Applying firewall rules:                         [  OK  ]


The ip6tables service can be turned off if you intend to use the iptables service only. If you deactivate the ip6tables service, remember to deactivate the IPv6 network also. Never leave a network device active without the matching firewall.
To force iptables to start by default when the system is booted, use the following command as the root user:
~]# chkconfig --level 345 iptables on
This forces iptables to start whenever the system is booted into runlevel 3, 4, or 5. IPTables Command Syntax
The following sample iptables command illustrates the basic command syntax:
iptables -A <chain> -j <target>
The -A option specifies that the rule be appended to <chain>. Each chain is comprised of one or more rules, and is therefore also known as a ruleset.
The three built-in chains are INPUT, OUTPUT, and FORWARD. These chains are permanent and cannot be deleted. The chain specifies the point at which a packet is manipulated.
The -j <target> option specifies the target of the rule; i.e., what to do if the packet matches the rule. Examples of built-in targets are ACCEPT, DROP, and REJECT.
Refer to the iptables man page for more information on the available chains, options, and targets. Basic Firewall Policies
Establishing basic firewall policies creates a foundation for building more detailed, user-defined rules.
Each iptables chain is comprised of a default policy, and zero or more rules which work in concert with the default policy to define the overall ruleset for the firewall.
The default policy for a chain can be either DROP or ACCEPT. Security-minded administrators typically implement a default policy of DROP, and only allow specific packets on a case-by-case basis. For example, the following policies block all incoming and outgoing packets on a network gateway:
~]# iptables -P INPUT DROP
~]# iptables -P OUTPUT DROP
It is also recommended that any forwarded packets — network traffic that is to be routed from the firewall to its destination node — be denied as well, to restrict internal clients from inadvertent exposure to the Internet. To do this, use the following rule:
~]# iptables -P FORWARD DROP
When you have established the default policies for each chain, you can create and save further rules for your particular network and security requirements.
The following sections describe how to save iptables rules and outline some of the rules you might implement in the course of building your iptables firewall. Saving and Restoring IPTables Rules
Changes to iptables are transitory; if the system is rebooted or if the iptables service is restarted, the rules are automatically flushed and reset. To save the rules so that they are loaded when the iptables service is started, use the following command as the root user:
~]# service iptables save
iptables: Saving firewall rules to /etc/sysconfig/iptables:[  OK  ]
The rules are stored in the file /etc/sysconfig/iptables and are applied whenever the service is started or the machine is rebooted.

2.8.4. Common IPTables Filtering

Preventing remote attackers from accessing a LAN is one of the most important aspects of network security. The integrity of a LAN should be protected from malicious remote users through the use of stringent firewall rules.
However, with a default policy set to block all incoming, outgoing, and forwarded packets, it is impossible for the firewall/gateway and internal LAN users to communicate with each other or with external resources.
To allow users to perform network-related functions and to use networking applications, administrators must open certain ports for communication.
For example, to allow access to port 80 on the firewall, append the following rule:
~]# iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
This allows users to browse websites that communicate using the standard port 80. To allow access to secure websites (for example,, you also need to provide access to port 443, as follows:
~]# iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT


When creating an iptables ruleset, order is important.
If a rule specifies that any packets from the subnet be dropped, and this is followed by a rule that allows packets from (which is within the dropped subnet), then the second rule is ignored.
The rule to allow packets from must precede the rule that drops the remainder of the subnet.
To insert a rule in a specific location in an existing chain, use the -I option. For example:
~]# iptables -I INPUT 1 -i lo -p all -j ACCEPT
This rule is inserted as the first rule in the INPUT chain to allow local loopback device traffic.
There may be times when you require remote access to the LAN. Secure services, for example SSH, can be used for encrypted remote connection to LAN services.
Administrators with PPP-based resources (such as modem banks or bulk ISP accounts), dial-up access can be used to securely circumvent firewall barriers. Because they are direct connections, modem connections are typically behind a firewall/gateway.
For remote users with broadband connections, however, special cases can be made. You can configure iptables to accept connections from remote SSH clients. For example, the following rules allow remote SSH access:
~]# iptables -A INPUT -p tcp --dport 22 -j ACCEPT
~]# iptables -A OUTPUT -p tcp --sport 22 -j ACCEPT
These rules allow incoming and outbound access for an individual system, such as a single PC directly connected to the Internet or a firewall/gateway. However, they do not allow nodes behind the firewall/gateway to access these services. To allow LAN access to these services, you can use Network Address Translation (NAT) with iptables filtering rules.

2.8.5. FORWARD and NAT Rules

Most ISPs provide only a limited number of publicly routable IP addresses to the organizations they serve.
Administrators must, therefore, find alternative ways to share access to Internet services without giving public IP addresses to every node on the LAN. Using private IP addresses is the most common way of allowing all nodes on a LAN to properly access internal and external network services.
Edge routers (such as firewalls) can receive incoming transmissions from the Internet and route the packets to the intended LAN node. At the same time, firewalls/gateways can also route outgoing requests from a LAN node to the remote Internet service.
This forwarding of network traffic can become dangerous at times, especially with the availability of modern cracking tools that can spoof internal IP addresses and make the remote attacker's machine act as a node on your LAN.
To prevent this, iptables provides routing and forwarding policies that can be implemented to prevent abnormal usage of network resources.
The FORWARD chain allows an administrator to control where packets can be routed within a LAN. For example, to allow forwarding for the entire LAN (assuming the firewall/gateway is assigned an internal IP address on eth1), use the following rules:
~]# iptables -A FORWARD -i eth1 -j ACCEPT
~]# iptables -A FORWARD -o eth1 -j ACCEPT
This rule gives systems behind the firewall/gateway access to the internal network. The gateway routes packets from one LAN node to its intended destination node, passing all packets through its eth1 device.


By default, the IPv4 policy in Red Hat Enterprise Linux kernels disables support for IP forwarding. This prevents machines that run Red Hat Enterprise Linux from functioning as dedicated edge routers. To enable IP forwarding, use the following command as the root user:
~]# sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
This configuration change is only valid for the current session; it does not persist beyond a reboot or network service restart. To permanently set IP forwarding, edit the /etc/sysctl.conf file as follows:
Locate the following line:
net.ipv4.ip_forward = 0
Edit it to read as follows:
net.ipv4.ip_forward = 1
As the root user, run the following command to enable the change to the sysctl.conf file:
~]# sysctl -p /etc/sysctl.conf
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
[output truncated] Postrouting and IP Masquerading
Accepting forwarded packets via the firewall's internal IP device allows LAN nodes to communicate with each other; however they still cannot communicate externally to the Internet.
To allow LAN nodes with private IP addresses to communicate with external public networks, configure the firewall for IP masquerading, which masks requests from LAN nodes with the IP address of the firewall's external device (in this case, eth0):
~]# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
This rule uses the NAT packet matching table (-t nat) and specifies the built-in POSTROUTING chain for NAT (-A POSTROUTING) on the firewall's external networking device (-o eth0).
POSTROUTING allows packets to be altered as they are leaving the firewall's external device.
The -j MASQUERADE target is specified to mask the private IP address of a node with the external IP address of the firewall/gateway. Prerouting
If you have a server on your internal network that you want make available externally, you can use the -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your internal service can be forwarded.
For example, if you want to forward incoming HTTP requests to your dedicated Apache HTTP Server at, use the following command as the root user:
~]# iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to
This rule specifies that the NAT table use the built-in PREROUTING chain to forward incoming HTTP requests exclusively to the listed destination IP address of


If you have a default policy of DROP in your FORWARD chain, you must append a rule to forward all incoming HTTP requests so that destination NAT routing is possible. To do this, use the following command as the root user:
~]# iptables -A FORWARD -i eth0 -p tcp --dport 80 -d -j ACCEPT
This rule forwards all incoming HTTP requests from the firewall to the intended destination; the Apache HTTP Server behind the firewall. DMZs and IPTables
You can create iptables rules to route traffic to certain machines, such as a dedicated HTTP or FTP server, in a demilitarized zone (DMZ). A DMZ is a special local subnetwork dedicated to providing services on a public carrier, such as the Internet.
For example, to set a rule for routing incoming HTTP requests to a dedicated HTTP server at (outside of the range of the LAN), NAT uses the PREROUTING table to forward the packets to the appropriate destination:
~]# iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT \
With this command, all HTTP connections to port 80 from outside of the LAN are routed to the HTTP server on a network separate from the rest of the internal network. This form of network segmentation can prove safer than allowing HTTP connections to a machine on the network.
If the HTTP server is configured to accept secure connections, then port 443 must be forwarded as well.

2.8.6. Malicious Software and Spoofed IP Addresses

More elaborate rules can be created that control access to specific subnets, or even specific nodes, within a LAN. You can also restrict certain dubious applications or programs such as Trojans, worms, and other client/server viruses from contacting their server.
For example, some Trojans scan networks for services on ports from 31337 to 31340 (called the elite ports in cracking terminology).
Since there are no legitimate services that communicate via these non-standard ports, blocking them can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.
The following rules drop all TCP traffic that attempts to use port 31337:
~]# iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
~]# iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN.
For example, if your LAN uses the range, you can design a rule that instructs the Internet-facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range.
Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically.
~]# iptables -A FORWARD -s -i eth0 -j DROP


There is a distinction between the DROP and REJECT targets when dealing with appended rules.
The REJECT target denies access and returns a connection refused error to users who attempt to connect to the service. The DROP target, as the name implies, drops the packet without any warning.
Administrators can use their own discretion when using these targets.

2.8.7. IPTables and Connection Tracking

You can inspect and restrict connections to services based on their connection state. A module within iptables uses a method called connection tracking to store information about incoming connections. You can allow or deny access based on the following connection states:
  • NEW — A packet requesting a new connection, such as an HTTP request.
  • ESTABLISHED — A packet that is part of an existing connection.
  • RELATED — A packet that is requesting a new connection but is part of an existing connection. For example, FTP uses port 21 to establish a connection, but data is transferred on a different port (typically port 20).
  • INVALID — A packet that is not part of any connections in the connection tracking table.
You can use the stateful functionality of iptables connection tracking with any network protocol, even if the protocol itself is stateless (such as UDP). The following example shows a rule that uses connection tracking to forward only the packets that are associated with an established connection:
~]# iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

2.8.8. IPv6

The introduction of the next-generation Internet Protocol, called IPv6, expands beyond the 32-bit address limit of IPv4 (or IP). IPv6 supports 128-bit addresses, and carrier networks that are IPv6 aware are therefore able to address a larger number of routable addresses than IPv4.
Red Hat Enterprise Linux supports IPv6 firewall rules using the Netfilter 6 subsystem and the ip6tables command. In Red Hat Enterprise Linux 6, both IPv4 and IPv6 services are enabled by default.
The ip6tables command syntax is identical to iptables in every aspect except that it supports 128-bit addresses. For example, use the following command to enable SSH connections on an IPv6-aware network server:
~]# ip6tables -A INPUT -i eth0 -p tcp -s 3ffe:ffff:100::1/128 --dport 22 -j ACCEPT
For more information about IPv6 networking, see the IPv6 Information Page at

2.8.9. IPTables

Included with Red Hat Enterprise Linux are advanced tools for network packet filtering — the process of controlling network packets as they enter, move through, and exit the network stack within the kernel. Kernel versions prior to 2.4 relied on ipchains for packet filtering and used lists of rules applied to packets at each step of the filtering process. The 2.4 kernel introduced iptables (also called netfilter), which is similar to ipchains but greatly expands the scope and control available for filtering network packets.
This chapter focuses on packet filtering basics, explains various options available with iptables commands, and explains how filtering rules can be preserved between system reboots.


The default firewall mechanism in the 2.4 and later kernels is iptables, but iptables cannot be used if ipchains is already running. If ipchains is present at boot time, the kernel issues an error and fails to start iptables.
The functionality of ipchains is not affected by these errors. Packet Filtering
The Linux kernel uses the Netfilter facility to filter packets, allowing some of them to be received by or pass through the system while stopping others. This facility is built in to the Linux kernel, and has five built-in tables or rules lists, as follows:
  • filter — The default table for handling network packets.
  • nat — Used to alter packets that create a new connection and used for Network Address Translation (NAT).
  • mangle — Used for specific types of packet alteration.
  • raw — Used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target.
  • security — Used for Mandatory Access Control (MAC) networking rules, such as those enabled by the SECMARK and CONNSECMARK targets.
Each table has a group of built-in chains, which correspond to the actions performed on the packet by netfilter.
The built-in chains for the filter table are as follows:
  • INPUT — Applies to network packets that are targeted for the host.
  • OUTPUT — Applies to locally-generated network packets.
  • FORWARD — Applies to network packets routed through the host.
The built-in chains for the nat table are as follows:
  • PREROUTING — Applies to network packets when they arrive.
  • OUTPUT — Applies to locally-generated network packets before they are sent out.
  • POSTROUTING — Applies to network packets before they are sent out.
The built-in chains for the mangle table are as follows:
  • INPUT — Applies to network packets targeted for the host.
  • OUTPUT — Applies to locally-generated network packets before they are sent out.
  • FORWARD — Applies to network packets routed through the host.
  • PREROUTING — Applies to incoming network packets before they are routed.
  • POSTROUTING — Applies to network packets before they are sent out.
The built-in chains for the raw table are as follows:
  • OUTPUT — Applies to locally-generated network packets before they are sent out.
  • PREROUTING — Applies to incoming network packets before they are routed.
The built-in chains for the security table are as follows:
  • INPUT — Applies to network packets targeted for the host.
  • OUTPUT — Applies to locally-generated network packets before they are sent out.
  • FORWARD — Applies to network packets routed through the host.
Every network packet received by or sent from a Linux system is subject to at least one table. However, a packet may be subjected to multiple rules within each table before emerging at the end of the chain. The structure and purpose of these rules may vary, but they usually seek to identify a packet coming from or going to a particular IP address, or set of addresses, when using a particular protocol and network service. The following image outlines how the flow of packets is examined by the iptables subsystem:
Packet filtering in IPTables

Figure 2.6. Packet filtering in IPTables


By default, firewall rules are saved in the /etc/sysconfig/iptables or /etc/sysconfig/ip6tables files.
The iptables service starts before any DNS-related services when a Linux system is booted. This means that firewall rules can only reference numeric IP addresses (for example, Domain names (for example, in such rules produce errors.
Regardless of their destination, when packets match a particular rule in one of the tables, a target or action is applied to them. If the rule specifies an ACCEPT target for a matching packet, the packet skips the rest of the rule checks and is allowed to continue to its destination. If a rule specifies a DROP target, that packet is refused access to the system and nothing is sent back to the host that sent the packet. If a rule specifies a QUEUE target, the packet is passed to user-space. If a rule specifies the optional REJECT target, the packet is dropped, but an error packet is sent to the packet's originator.
Every chain has a default policy to ACCEPT, DROP, REJECT, or QUEUE. If none of the rules in the chain apply to the packet, then the packet is dealt with in accordance with the default policy.
The iptables command configures these tables, as well as sets up new tables if necessary.


The netfilter modules are not loaded by default. Therefore a user will not see all of them by looking in the /proc/ directory as it only shows what is being used or has been loaded already. This means that there is no way to see what features of netfilter are available before you attempt to use it. Command Options for IPTables
Rules for filtering packets are created using the iptables command. The following aspects of the packet are most often used as criteria:
  • Packet Type — Specifies the type of packets the command filters.
  • Packet Source/Destination — Specifies which packets the command filters based on the source or destination of the packet.
  • Target — Specifies what action is taken on packets matching the above criteria.
Refer to Section, “IPTables Match Options” and Section, “Target Options” for more information about specific options that address these aspects of a packet.
The options used with specific iptables rules must be grouped logically, based on the purpose and conditions of the overall rule, for the rule to be valid. The remainder of this section explains commonly-used options for the iptables command. Structure of IPTables Command Options
Many iptables commands have the following structure:
iptables [-t <table-name>] <command> <chain-name> \
  <parameter-1> <option-1> \
  <parameter-n> <option-n>
<table-name> — Specifies which table the rule applies to. If omitted, the filter table is used.
<command> — Specifies the action to perform, such as appending or deleting a rule.
<chain-name> — Specifies the chain to edit, create, or delete.
<parameter>-<option> pairs — Parameters and associated options that specify how to process a packet that matches the rule.
The length and complexity of an iptables command can change significantly, based on its purpose.
For example, a command to remove a rule from a chain can be very short:
iptables -D <chain-name> <line-number>
In contrast, a command that adds a rule which filters packets from a particular subnet using a variety of specific parameters and options can be rather long. When constructing iptables commands, it is important to remember that some parameters and options require further parameters and options to construct a valid rule. This can produce a cascading effect, with the further parameters requiring yet more parameters. Until every parameter and option that requires another set of options is satisfied, the rule is not valid.
Type iptables -h to view a comprehensive list of iptables command structures. Command Options
Command options instruct iptables to perform a specific action. Only one command option is allowed per iptables command. With the exception of the help command, all commands are written in upper-case characters.
The iptables command options are as follows:
  • -A — Appends the rule to the end of the specified chain. Unlike the -I option described below, it does not take an integer argument. It always appends the rule to the end of the specified chain.
  • -D <integer> | <rule> — Deletes a rule in a particular chain by number (such as 5 for the fifth rule in a chain), or by rule specification. The rule specification must exactly match an existing rule.
  • -E — Renames a user-defined chain. A user-defined chain is any chain other than the default, pre-existing chains. (Refer to the -N option, below, for information on creating user-defined chains.) This is a cosmetic change and does not affect the structure of the table.


    If you attempt to rename one of the default chains, the system reports a Match not found error. You cannot rename the default chains.
  • -F — Flushes the selected chain, which effectively deletes every rule in the chain. If no chain is specified, this command flushes every rule from every chain.
  • -h — Provides a list of command structures, as well as a quick summary of command parameters and options.
  • -I [<integer>] — Inserts the rule in the specified chain at a point specified by a user-defined integer argument. If no argument is specified, the rule is inserted at the top of the chain.


    As noted above, the order of rules in a chain determines which rules apply to which packets. This is important to remember when adding rules using either the -A or -I option.
    This is especially important when adding rules using the -I with an integer argument. If you specify an existing number when adding a rule to a chain, iptables adds the new rule before (or above) the existing rule.
  • -L — Lists all of the rules in the chain specified after the command. To list all rules in all chains in the default filter table, do not specify a chain or table. Otherwise, the following syntax should be used to list the rules in a specific chain in a particular table:
    iptables -L <chain-name> -t <table-name>
    Additional options for the -L command option, which provide rule numbers and allow more verbose rule descriptions, are described in Section, “Listing Options”.
  • -N — Creates a new chain with a user-specified name. The chain name must be unique, otherwise an error message is displayed.
  • -P — Sets the default policy for the specified chain, so that when packets traverse an entire chain without matching a rule, they are sent to the specified target, such as ACCEPT or DROP.
  • -R — Replaces a rule in the specified chain. The rule's number must be specified after the chain's name. The first rule in a chain corresponds to rule number one.
  • -X — Deletes a user-specified chain. You cannot delete a built-in chain.
  • -Z — Sets the byte and packet counters in all chains for a table to zero. IPTables Parameter Options
Certain iptables commands, including those used to add, append, delete, insert, or replace rules within a particular chain, require various parameters to construct a packet filtering rule.
  • -c — Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options to specify which counter to reset.
  • -d — Sets the destination hostname, IP address, or network of a packet that matches the rule. When matching a network, the following IP address/netmask formats are supported:
    • N.N.N.N/M.M.M.M — Where N.N.N.N is the IP address range and M.M.M.M is the netmask.
    • N.N.N.N/M — Where N.N.N.N is the IP address range and M is the bitmask.
  • -f — Applies this rule only to fragmented packets.
    You can use the exclamation point character (!) option before this parameter to specify that only unfragmented packets are matched.


    Distinguishing between fragmented and unfragmented packets is desirable, despite fragmented packets being a standard part of the IP protocol.
    Originally designed to allow IP packets to travel over networks with differing frame sizes, these days fragmentation is more commonly used to generate DoS attacks using malformed packets. It's also worth noting that IPv6 disallows fragmentation entirely.
  • -i — Sets the incoming network interface, such as eth0 or ppp0. With iptables, this optional parameter may only be used with the INPUT and FORWARD chains when used with the filter table and the PREROUTING chain with the nat and mangle tables.
    This parameter also supports the following special options:
    • Exclamation point character (!) — Reverses the directive, meaning any specified interfaces are excluded from this rule.
    • Plus character (+) — A wildcard character used to match all interfaces that match the specified string. For example, the parameter -i eth+ would apply this rule to any Ethernet interfaces but exclude any other interfaces, such as ppp0.
    If the -i parameter is used but no interface is specified, then every interface is affected by the rule.
  • -j — Jumps to the specified target when a packet matches a particular rule.
    The standard targets are ACCEPT, DROP, QUEUE, and RETURN.
    Extended options are also available through modules loaded by default with the Red Hat Enterprise Linux iptables RPM package. Valid targets in these modules include LOG, MARK, and REJECT, among others. Refer to the iptables man page for more information about these and other targets.
    This option can also be used to direct a packet matching a particular rule to a user-defined chain outside of the current chain so that other rules can be applied to the packet.
    If no target is specified, the packet moves past the rule with no action taken. The counter for this rule, however, increases by one.
  • -o — Sets the outgoing network interface for a rule. This option is only valid for the OUTPUT and FORWARD chains in the filter table, and the POSTROUTING chain in the nat and mangle tables. This parameter accepts the same options as the incoming network interface parameter (-i).
  • -p <protocol> — Sets the IP protocol affected by the rule. This can be either icmp, tcp, udp, or all, or it can be a numeric value, representing one of these or a different protocol. You can also use any protocols listed in the /etc/protocols file.
    The "all" protocol means the rule applies to every supported protocol. If no protocol is listed with this rule, it defaults to "all".
  • -s — Sets the source for a particular packet using the same syntax as the destination (-d) parameter. IPTables Match Options
Different network protocols provide specialized matching options which can be configured to match a particular packet using that protocol. However, the protocol must first be specified in the iptables command. For example, -p <protocol-name> enables options for the specified protocol. Note that you can also use the protocol ID, instead of the protocol name. Refer to the following examples, each of which have the same effect:
~]# iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
~]# iptables -A INPUT -p 5813 --icmp-type any -j ACCEPT
Service definitions are provided in the /etc/services file. For readability, it is recommended that you use the service names rather than the port numbers.


Secure the /etc/services file to prevent unauthorized editing. If this file is editable, attackers can use it to enable ports on your machine you have otherwise closed. To secure this file, run the following commands as root:
~]# chown root.root /etc/services
~]# chmod 0644 /etc/services
~]# chattr +i /etc/services
This prevents the file from being renamed, deleted or having links made to it. TCP Protocol
These match options are available for the TCP protocol (-p tcp):
  • --dport — Sets the destination port for the packet.
    To configure this option, use a network service name (such as www or smtp); a port number; or a range of port numbers.
    To specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535.
    Use an exclamation point character (!) after the --dport option to match all packets that do not use that network service or port.
    To browse the names and aliases of network services and the port numbers they use, view the /etc/services file.
    The --destination-port match option is synonymous with --dport.
  • --sport — Sets the source port of the packet using the same options as --dport. The --source-port match option is synonymous with --sport.
  • --syn — Applies to all TCP packets designed to initiate communication, commonly called SYN packets. Any packets that carry a data payload are not touched.
    Use an exclamation point character (!) before the --syn option to match all non-SYN packets.
  • --tcp-flags <tested flag list> <set flag list> — Allows TCP packets that have specific bits (flags) set, to match a rule.
    The --tcp-flags match option accepts two parameters. The first parameter is the mask; a comma-separated list of flags to be examined in the packet. The second parameter is a comma-separated list of flags that must be set for the rule to match.
    The possible flags are:
    • ACK
    • FIN
    • PSH
    • RST
    • SYN
    • URG
    • ALL
    • NONE
    For example, an iptables rule that contains the following specification only matches TCP packets that have the SYN flag set and the ACK and FIN flags not set:
    --tcp-flags ACK,FIN,SYN SYN
    Use the exclamation point character (!) after the --tcp-flags to reverse the effect of the match option.
  • --tcp-option — Attempts to match with TCP-specific options that can be set within a particular packet. This match option can also be reversed by using the exclamation point character (!) after the option. UDP Protocol
These match options are available for the UDP protocol (-p udp):
  • --dport — Specifies the destination port of the UDP packet, using the service name, port number, or range of port numbers. The --destination-port match option is synonymous with --dport.
  • --sport — Specifies the source port of the UDP packet, using the service name, port number, or range of port numbers. The --source-port match option is synonymous with --sport.
For the --dport and --sport options, to specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535. ICMP Protocol
The following match option is available for the Internet Control Message Protocol (ICMP) (-p icmp):
  • --icmp-type — Sets the name or number of the ICMP type to match with the rule. A list of valid ICMP names can be retrieved by typing the iptables -p icmp -h command. Additional Match Option Modules
Additional match options are available through modules loaded by the iptables command.
To use a match option module, load the module by name using the -m <module-name>, where <module-name> is the name of the module.
Many modules are available by default. You can also create modules to provide additional functionality.
The following is a partial list of the most commonly used modules:
  • limit module — Places limits on how many packets are matched to a particular rule.
    When used in conjunction with the LOG target, the limit module can prevent a flood of matching packets from filling up the system log with repetitive messages or using up system resources.
    Refer to Section, “Target Options” for more information about the LOG target.
    The limit module enables the following options:
    • --limit — Sets the maximum number of matches for a particular time period, specified as a <value>/<period> pair. For example, using --limit 5/hour allows five rule matches per hour.
      Periods can be specified in seconds, minutes, hours, or days.
      If a number and time modifier are not used, the default value of 3/hour is assumed.
    • --limit-burst — Sets a limit on the number of packets able to match a rule at one time.
      This option is specified as an integer and should be used in conjunction with the --limit option.
      If no value is specified, the default value of five (5) is assumed.
  • state module — Enables state matching.
    The state module enables the following options:
    • --state — match a packet with the following connection states:
      • ESTABLISHED — The matching packet is associated with other packets in an established connection. You need to accept this state if you want to maintain a connection between a client and a server.
      • INVALID — The matching packet cannot be tied to a known connection.
      • NEW — The matching packet is either creating a new connection or is part of a two-way connection not previously seen. You need to accept this state if you want to allow new connections to a service.
      • RELATED — The matching packet is starting a new connection related in some way to an existing connection. An example of this is FTP, which uses one connection for control traffic (port 21), and a separate connection for data transfer (port 20).
      These connection states can be used in combination with one another by separating them with commas, such as -m state --state INVALID,NEW.
  • mac module — Enables hardware MAC address matching.
    The mac module enables the following option:
    • --mac-source — Matches a MAC address of the network interface card that sent the packet. To exclude a MAC address from a rule, place an exclamation point character (!) after the --mac-source match option.
Refer to the iptables man page for more match options available through modules. Target Options
When a packet has matched a particular rule, the rule can direct the packet to a number of different targets which determine the appropriate action. Each chain has a default target, which is used if none of the rules on that chain match a packet or if none of the rules which match the packet specify a target.
The following are the standard targets:
  • <user-defined-chain> — A user-defined chain within the table. User-defined chain names must be unique. This target passes the packet to the specified chain.
  • ACCEPT — Allows the packet through to its destination or to another chain.
  • DROP — Drops the packet without responding to the requester. The system that sent the packet is not notified of the failure.
  • QUEUE — The packet is queued for handling by a user-space application.
  • RETURN — Stops checking the packet against rules in the current chain. If the packet with a RETURN target matches a rule in a chain called from another chain, the packet is returned to the first chain to resume rule checking where it left off. If the RETURN rule is used on a built-in chain and the packet cannot move up to its previous chain, the default target for the current chain is used.
In addition, extensions are available which allow other targets to be specified. These extensions are called target modules or match option modules and most only apply to specific tables and situations. Refer to Section, “Additional Match Option Modules” for more information about match option modules.
Many extended target modules exist, most of which only apply to specific tables or situations. Some of the most popular target modules included by default in Red Hat Enterprise Linux are:
  • LOG — Logs all packets that match this rule. Because the packets are logged by the kernel, the /etc/syslog.conf file determines where these log entries are written. By default, they are placed in the /var/log/messages file.
    Additional options can be used after the LOG target to specify the way in which logging occurs:
    • --log-level — Sets the priority level of a logging event. Refer to the syslog.conf man page for a list of priority levels.
    • --log-ip-options — Logs any options set in the header of an IP packet.
    • --log-prefix — Places a string of up to 29 characters before the log line when it is written. This is useful for writing syslog filters for use in conjunction with packet logging.


      Due to an issue with this option, you should add a trailing space to the log-prefix value.
    • --log-tcp-options — Logs any options set in the header of a TCP packet.
    • --log-tcp-sequence — Writes the TCP sequence number for the packet in the log.
  • REJECT — Sends an error packet back to the remote system and drops the packet.
    The REJECT target accepts --reject-with <type> (where <type> is the rejection type) allowing more detailed information to be returned with the error packet. The message port-unreachable is the default error type given if no other option is used. Refer to the iptables man page for a full list of <type> options.
Other target extensions, including several that are useful for IP masquerading using the nat table, or with packet alteration using the mangle table, can be found in the iptables man page. Listing Options
The default list command, iptables -L [<chain-name>], provides a very basic overview of the default filter table's current chains. Additional options provide more information:
  • -v — Displays verbose output, such as the number of packets and bytes each chain has processed, the number of packets and bytes each rule has matched, and which interfaces apply to a particular rule.
  • -x — Expands numbers into their exact values. On a busy system, the number of packets and bytes processed by a particular chain or rule may be abbreviated to Kilobytes, Megabytes, or Gigabytes. This option forces the full number to be displayed.
  • -n — Displays IP addresses and port numbers in numeric format, rather than the default hostname and network service format.
  • --line-numbers — Lists rules in each chain next to their numeric order in the chain. This option is useful when attempting to delete the specific rule in a chain or to locate where to insert a rule within a chain.
  • -t <table-name> — Specifies a table name. If omitted, defaults to the filter table. Saving IPTables Rules
Rules created with the iptables command are stored in memory. If the system is restarted before saving the iptables rule set, all rules are lost. For netfilter rules to persist through a system reboot, they need to be saved. To save netfilter rules, type the following command as root:
~]# /sbin/service iptables save
iptables: Saving firewall rules to /etc/sysconfig/iptables:[  OK  ]
This executes the iptables init script, which runs the /sbin/iptables-save program and writes the current iptables configuration to /etc/sysconfig/iptables. The existing /etc/sysconfig/iptables file is saved as /etc/sysconfig/
The next time the system boots, the iptables init script reapplies the rules saved in /etc/sysconfig/iptables by using the /sbin/iptables-restore command.
While it is always a good idea to test a new iptables rule before committing it to the /etc/sysconfig/iptables file, it is possible to copy iptables rules into this file from another system's version of this file. This provides a quick way to distribute sets of iptables rules to multiple machines.
You can also save the iptables rules to a separate file for distribution, backup, or other purposes. To do so, run the following command as root:
iptables-save > <filename>
… where <filename> is a user-defined name for your ruleset.


If distributing the /etc/sysconfig/iptables file to other machines, type /sbin/service iptables reload or /sbin/service iptables restart for the new rules to take effect. It is better to use the reload command because there is no period of time without a firewall in place. See the description of the reload command in Section, “IPTables Control Scripts”. For IPv6, substitute ip6tables for iptables in the /sbin/service commands listed in this section. For more information about IPv6 and netfilter, see Section, “IPTables and IPv6”.


Note the difference between the iptables command (/sbin/iptables), which is used to manipulate the tables and chains that constitute the iptables functionality, and the iptables service (/sbin/service iptables), which is used to enable and disable the iptables service itself. IPTables Control Scripts
There are two basic methods for controlling iptables in Red Hat Enterprise Linux:
  • Firewall Configuration Tool (system-config-firewall) — A graphical interface for creating, activating, and saving basic firewall rules. Refer to Section 2.8.2, “Basic Firewall Configuration” for more information.
  • /sbin/service iptables <option> — Used to manipulate various functions of iptables using its initscript. The following options are available:
    • start — If a firewall is configured (that is, /etc/sysconfig/iptables exists), all running iptables are stopped completely and then started using the /sbin/iptables-restore command. This option only works if the ipchains kernel module is not loaded. To check if this module is loaded, type the following command as root:
      ~]# lsmod | grep ipchains
      If this command returns no output, it means the module is not loaded. If necessary, use the /sbin/rmmod command to remove the module.
    • stop — If a firewall is running, the firewall rules in memory are flushed, and all iptables modules and helpers are unloaded.
      If the IPTABLES_SAVE_ON_STOP directive in the /etc/sysconfig/iptables-config configuration file is changed from its default value to yes, current rules are saved to /etc/sysconfig/iptables and any existing rules are moved to the file /etc/sysconfig/
      Refer to Section, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • reload — If a firewall is running, the firewall rules are reloaded from the configuration file. The reload command does not unload helpers that have been in use before, but will add new helpers that have been added to IPTABLES_MODULES (for IPv4) and IP6TABLES_MODULES (for IPv6). The advantage of not flushing the current firewall rules is that if the new rules cannot be applied, because of an error in the rules, the old rules are still in place.
    • restart — If a firewall is running, the firewall rules in memory are flushed, and the firewall is started again if it is configured in /etc/sysconfig/iptables. This option only works if the ipchains kernel module is not loaded.
      If the IPTABLES_SAVE_ON_RESTART directive in the /etc/sysconfig/iptables-config configuration file is changed from its default value to yes, current rules are saved to /etc/sysconfig/iptables and any existing rules are moved to the file /etc/sysconfig/
      Refer to Section, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • status — Displays the status of the firewall and lists all active rules.
      The default configuration for this option displays IP addresses in each rule. To display domain and hostname information, edit the /etc/sysconfig/iptables-config file and change the value of IPTABLES_STATUS_NUMERIC to no. Refer to Section, “IPTables Control Scripts Configuration File” for more information about the iptables-config file.
    • panic — Flushes all firewall rules. The policy of all configured tables is set to DROP.
      This option could be useful if a server is known to be compromised. Rather than physically disconnecting from the network or shutting down the system, you can use this option to stop all further network traffic but leave the machine in a state ready for analysis or other forensics.
    • save — Saves firewall rules to /etc/sysconfig/iptables using iptables-save. Refer to Section, “Saving IPTables Rules” for more information.


To use the same initscript commands to control netfilter for IPv6, substitute ip6tables for iptables in the /sbin/service commands listed in this section. For more information about IPv6 and netfilter, see Section, “IPTables and IPv6”. IPTables Control Scripts Configuration File
The behavior of the iptables initscripts is controlled by the /etc/sysconfig/iptables-config configuration file. The following is a list of directives contained in this file:
  • IPTABLES_MODULES — Specifies a space-separated list of additional iptables modules to load when a firewall is activated. These can include connection tracking and NAT helpers.
  • IPTABLES_MODULES_UNLOAD — Unloads modules on restart and stop. This directive accepts the following values:
    • yes — The default value. This option must be set to achieve a correct state for a firewall restart or stop.
    • no — This option should only be set if there are problems unloading the netfilter modules.
  • IPTABLES_SAVE_ON_STOP — Saves current firewall rules to /etc/sysconfig/iptables when the firewall is stopped. This directive accepts the following values:
    • yes — Saves existing rules to /etc/sysconfig/iptables when the firewall is stopped, moving the previous version to the /etc/sysconfig/ file.
    • no — The default value. Does not save existing rules when the firewall is stopped.
  • IPTABLES_SAVE_ON_RESTART — Saves current firewall rules when the firewall is restarted. This directive accepts the following values:
    • yes — Saves existing rules to /etc/sysconfig/iptables when the firewall is restarted, moving the previous version to the /etc/sysconfig/ file.
    • no — The default value. Does not save existing rules when the firewall is restarted.
  • IPTABLES_SAVE_COUNTER — Saves and restores all packet and byte counters in all chains and rules. This directive accepts the following values:
    • yes — Saves the counter values.
    • no — The default value. Does not save the counter values.
  • IPTABLES_STATUS_NUMERIC — Outputs IP addresses in numeric form instead of domain or hostnames. This directive accepts the following values:
    • yes — The default value. Returns only IP addresses within a status output.
    • no — Returns domain or hostnames within a status output. IPTables and IP Sets
The ipset utility is used to administer IP sets in the Linux kernel. An IP set is a framework for storing IP addresses, port numbers, IP and MAC address pairs, or IP address and port number pairs. The sets are indexed in such a way that very fast matching can be made against a set even when the sets are very large. IP sets enable simpler and more manageable configurations as well as providing performance advantages when using iptables. The iptables matches and targets referring to sets create references which protect the given sets in the kernel. A set cannot be destroyed while there is a single reference pointing to it.
The use of ipset enables iptables commands, such as those below, to be replaced by a set:
~]# iptables -A INPUT -s -j DROP
~]# iptables -A INPUT -s -j DROP
~]# iptables -A INPUT -s -j DROP
The set is created as follows:
~]# ipset create my-block-set hash:net
~]# ipset add my-block-set
~]# ipset add my-block-set
~]# ipset add my-block-set
The set is then referenced in an iptables command as follows:
~]# iptables -A INPUT -m set --set my-block-set src -j DROP
If the set is used more than once a saving in configuration time is made. If the set contains many entries a saving in processing time is made. Installing ipset
To install the ipset utility, issue the following command as root:
~]# yum install ipset
To see the usage message:
~]$ ipset -h
ipset v6.11

Usage: ipset [options] COMMAND ipset Commands
The format of the ipset command is as follows:
ipset [options] command [command-options]
Where command is one of:
create | add | del | test | destroy | list | save | restore | flush | rename | swap | help | version | - 
Allowed options are:
-exist | -output [ plain | save | xml ] | -quiet | -resolve | -sorted | -name | -terse
The create command is used to create a new data structure to store a set of IP data. The add command adds new data to the set, the data added is referred to as an element of the set.
The -exist option suppresses error message if the element already exists, and it has a special role in updating a time out value. To change a time out, use the ipset add command and specify all the data for the element again, changing only the time out value as required, and using the -exist option.
The test option is for testing if the element already exists within a set.
The format of the create command is as follows:
ipset create set-name type-name [create-options]
The set-name is a suitable name chosen by the user, the type-name is the name of the data structure used to store the data comprising the set. The format of the type-name is as follows:
The allowed methods for storing data are:
 bitmap | hash | list 
The allowed data types are:
ip | net | mac | port | iface 
When adding, deleting, or testing entries in a set, the same comma separated data syntax must be used for the data that makes up one entry, or element, in the set. For example:
ipset add set-name ipaddr,portnum,ipaddr


A set cannot contain IPv4 and IPv6 addresses at the same time. When a set is created it is bound to a family, inet for IPv4 or inet6 for IPv6, and the default is inet.

Example 2.3. Create an IP Set

To create an IP set consisting of a source IP address, a port, and destination IP address, issue a command as follows:
~]# ipset create my-set hash:ip,port,ip
Once the set is created, entries can be added as follows:
~]# ipset add my-set,80,
~]# ipset add my-set,443,
The set types have the following optional parameters in common. They must be specified when the set is created in order for them to be used:
  • timeout — The value given with the create command will be the default value for the set created. If a value is given with the add command, it will be the initial non-default value for the element.

Example 2.4. List an IP Set

To list the contents of a specific IP Set, my-set, issue a command as follows:
~]# ipset list my-set
Name: my-set
Type: hash:ip,port,ip
Header: family inet hashsize 1024 maxelem 65536 
Size in memory: 8360
References: 0
Omit the set name to list all sets.

Example 2.5. Test the Elements of an IP Set

Listing the contents of large sets is time consuming. You can test for the existence of an element as follows:
~]# ipset test my-set,80,,tcp:80, is in set my-set. IP Set Types
Stores an IPv4 host address, a network range, or an IPv4 network addresses with the prefix-length in CIDR notation if the netmask option is used when the set is created. It can optionally store a timeout value, a counter value, and a comment. It can store up to 65536 entries. The command to create the bitmap:ip set has the following format:
ipset create set-name range start_ipaddr-end_ipaddr |ipaddr/prefix-length [netmask prefix-length] [timeout value] [counters] [comment]

Example 2.6. Create an IP Set for a Range of Addresses Using a Prefix Length

To create an IP set for a range of addresses using a prefix length, make use of the bitmap:ip set type as follows:
~]# ipset create my-range bitmap:ip range
Once the set is created, entries can be added as follows:
~]# ipset add my-range
Review the members of the list:
~]# ipset list my-range
Name: my-range
Type: bitmap:ip
Header: range 
Size in memory: 84
References: 0
To add a range of addresses:
~]# ipset add my-range
Review the members of the list:
~]# ipset list my-range
Name: my-range
Type: bitmap:ip
Header: range 
Size in memory: 84
References: 0

Example 2.7. Create an IP Set for a Range of Addresses Using a Netmask

To create an IP set for a range of address using a netmask, make use of the bitmap:ip set type as follows:
~]# ipset create my-big-range bitmap:ip range netmask 24
Once the set is created, entries can be added as follows:
~]# ipset add my-big-range
If you attempt to add an address, the range containing that address will be added:
~]# ipset add my-big-range
~]# ipset list my-big-range
Name: my-big-range
Type: bitmap:ip
Header: range netmask 24 
Size in memory: 84
References: 0
Stores an IPv4 address and a MAC address as a pair. It can store up to 65536 entries.
ipset create my-range bitmap:ip,mac range start_ipaddr-end_ipaddr | ipaddr/prefix-length [timeout value ] [counters] [comment]

Example 2.8. Create an IP Set for a Range of IPv4 MAC Address Pairs

To create an IP set for a range of IPv4 MAC address pairs, make use of the bitmap:ip,mac set type as follows:
~]# ipset create my-range bitmap:ip,mac range
It is not necessary to specify a MAC address when creating the set.
Once the set is created, entries can be added as follows:
~]# ipset add my-range,12:34:56:78:9A:BC
Stores a range of ports. It can store up to 65536 entries.
ipset create my-port-range bitmap:port range start_port-end_port [timeout value ] [counters] [comment]
The set match and SET target netfilter kernel modules interpret the stored numbers as TCP or UDP port numbers. The protocol can optionally be specified together with the port. The proto only needs to be specified if a service name is used, and that name does not exist as a TCP service.

Example 2.9. Create an IP Set for a Range of Ports

To create an IP set for a range of ports, make use of the bitmap:port set type as follows:
~]# ipset create my-permitted-port-range bitmap:port range 1024-49151
Once the set is created, entries can be added as follows:
~]# ipset add my-permitted-port-range 5060-5061
Stores a host or network address in the form of a hash. By default, an address specified without a network prefix length is a host address. The all-zero IP address cannot be stored.
ipset create my-addresses hash:ip [family[ inet | inet6 ]] [hashsize value] [maxelem value ] [netmask prefix-length] [timeout value ]
The inet family is the default, if family is omitted addresses will be interpreted as IPv4 addresses. The hashsize value is the initial hash size to use and defaults to 1024. The maxelem value is the maximum number of elements which can be stored in the set, it defaults to 65536.
The netfilter tool searches for a network prefix which is the most specific, it tries to find the smallest block of addresses that match.

Example 2.10. Create an IP Set for IP Addresses

To create an IP set for IP addresses, make use of the hash:ip set type as follows:
~]# ipset create my-addresses hash:ip
Once the set is created, entries can be added as follows:
~]# ipset add my-addresses
If additional options such as netmask and timeout are required, they must be specified when the set is created. For example:
~]# ipset create my-busy-addresses hash:ip maxelem 24 netmask 28 timeout 100
The maxelem option restricts to total number of elements in the set, thus conserving memory space.
The timeout option means that elements will only exist in the set for the number of seconds specified. For example:
~]# ipset add my-busy-addresses timeout 100
The following output shows the time counting down:
[root@rhel6 ~]# ipset add my-busy-addresses timeout 100
[root@rhel6 ~]# ipset list my-busy-addresses
Name: my-busy-addresses
Type: hash:ip
Header: family inet hashsize 1024 maxelem 24 netmask 28 timeout 100 
Size in memory: 8300
References: 0
Members: timeout 90
[root@rhel6 ~]# ipset list my-busy-addresses
Name: my-busy-addresses
Type: hash:ip
Header: family inet hashsize 1024 maxelem 24 netmask 28 timeout 100 
Size in memory: 8300
References: 0
Members: timeout 83
The element will be removed from the set when the timeout period ends.
See the ipset(8) manual page for more examples. IPTables and IPv6
If the iptables-ipv6 package is installed, netfilter in Red Hat Enterprise Linux can filter the next-generation IPv6 Internet protocol. The command used to manipulate the IPv6 netfilter is ip6tables.
Most directives for this command are identical to those used for iptables, except the nat table is not yet supported. This means that it is not yet possible to perform IPv6 network address translation tasks, such as masquerading and port forwarding.
Rules for ip6tables are saved in the /etc/sysconfig/ip6tables file. Previous rules saved by the ip6tables initscripts are saved in the /etc/sysconfig/ file.
Configuration options for the ip6tables init script are stored in /etc/sysconfig/ip6tables-config, and the names for each directive vary slightly from their iptables counterparts.
For example, for the iptables-config directive IPTABLES_MODULES the equivalent in the ip6tables-config file is IP6TABLES_MODULES. Additional Resources
There are several aspects to firewalls and the Linux Netfilter subsystem that could not be covered in this chapter. For more information, see the following resources. Useful Firewall Websites
  • — The home of the netfilter/iptables project. Contains assorted information about iptables, including a FAQ addressing specific problems and various helpful guides by Rusty Russell, the Linux IP firewall maintainer. The HOWTO documents on the site cover subjects such as basic networking concepts, kernel packet filtering, and NAT configurations.
  • — The Linux Documentation Project contains several useful guides relating to firewall creation and administration.
  • — The official list of registered and common service ports as assigned by the Internet Assigned Numbers Authority. Installed IP Tables Documentation
  • man iptables — Contains a description of iptables as well as a comprehensive list of targets, options, and match extensions.

[3] Since system BIOSes differ between manufacturers, some may not support password protection of either type, while others may support one type but not the other.
[4] GRUB also accepts unencrypted passwords, but it is recommended that an MD5 hash be used for added security.

Chapter 3. Encryption

There are two main types of data that must be protected: data at rest and data in motion. These different types of data are protected in similar ways using similar technology but the implementations can be completely different. No single protective implementation can prevent all possible methods of compromise as the same information may be at rest and in motion at different points in time.

3.1. Data at Rest

Data at rest is data that is stored on a hard drive, tape, CD, DVD, disk, or other media. This information's biggest threat comes from being physically stolen. Laptops in airports, CDs going through the mail, and backup tapes that get left in the wrong places are all examples of events where data can be compromised through theft. If the data is encrypted on the media, it lowers the chances of the data being accessed.

3.1.1. Full Disk Encryption

Full disk or partition encryption is one of the best ways of protecting your data. Not only is each file protected but also the temporary storage that may contain parts of these files is also protected. Full disk encryption will protect all of your files so you do not have to worry about selecting what you want to protect and possibly missing a file.
Red Hat Enterprise Linux 6 natively supports LUKS Encryption. LUKS bulk encrypts your hard drive partitions so that while your computer is off, your data is protected. This will also protect your computer from attackers attempting to use single-user-mode to login to your computer or otherwise gain access.
Full disk encryption solutions like LUKS only protect the data when your computer is off. Once the computer is on and LUKS has decrypted the disk, the files on that disk are available to anyone who would normally have access to them. To protect your files when the computer is on, use full disk encryption in combination with another solution such as file based encryption. Also remember to lock your computer whenever you are away from it. A passphrase protected screen saver set to activate after a few minutes of inactivity is a good way to keep intruders out. For more information on LUKS, see Section 3.1.3, “LUKS Disk Encryption”.

3.1.2. File-Based Encryption

File-based encryption is used to protect the contents of files on mobile storage devices, such as CDs, flash drives, or external hard drives. Some file-based encryption solutions may leave remnants of the encrypted files that an attacker who has physical access to your computer can recover under some circumstances. To protect the contents of these files from attackers who may have access to your computer, use file-based encryption combined with another solution, such as full disk encryption.

3.1.3. LUKS Disk Encryption

Linux Unified Key Setup-on-disk-format (or LUKS) allows you to encrypt partitions on your Linux computer. This is particularly important when it comes to mobile computers and removable media. LUKS allows multiple user keys to decrypt a master key which is used for the bulk encryption of the partition.
Overview of LUKS
What LUKS does
  • LUKS encrypts entire block devices and is therefore well-suited for protecting the contents of mobile devices such as removable storage media or laptop disk drives.
  • The underlying contents of the encrypted block device are arbitrary. This makes it useful for encrypting swap devices. This can also be useful with certain databases that use specially formatted block devices for data storage.
  • LUKS uses the existing device mapper kernel subsystem.
  • LUKS provides passphrase strengthening which protects against dictionary attacks.
  • LUKS devices contain multiple key slots, allowing users to add backup keys/passphrases.
What LUKS does not do:
  • LUKS is not well-suited for applications requiring many (more than eight) users to have distinct access keys to the same device.
  • LUKS is not well-suited for applications requiring file-level encryption. LUKS Implementation in Red Hat Enterprise Linux
Red Hat Enterprise Linux 6 utilizes LUKS to perform file system encryption. By default, the option to encrypt the file system is unchecked during the installation. If you select the option to encrypt your hard drive, you will be prompted for a passphrase that will be asked every time you boot the computer. This passphrase "unlocks" the bulk encryption key that is used to decrypt your partition. If you choose to modify the default partition table you can choose which partitions you want to encrypt. This is set in the partition table settings.
The default cipher used for LUKS (refer to cryptsetup --help) is aes-cbc-essiv:sha256. Note that the installation program, Anaconda, uses by default the AES cipher in XTS mode, aes-xts-plain64. The default key size for LUKS is 256 bits. The default key size for LUKS with Anaconda (XTS mode) is 512 bits.


Changing the default cryptographic attributes can affect your system's performance and expose your system to various security risks. You should not change the default cryptographic attributes of your system without good knowledge of cryptography and understanding to the capabilities of the used cipher combinations.
Red Hat strongly recommends using the default ciphers. If you need to use any other cipher than the cipher that is configured as the default, you can initialize your partition with the --cipher and --key-size options. The syntax of the command is the following:
cryptsetup --verify-passphrase --cipher <cipher>-<mode>-<iv> --key-size <key-size> luksFormat <device>
where <cipher>-<mode>-<iv> is a string representing the used cipher. The string consists of three parts: a block cipher, block cipher mode, and an initial vector (IV).
A block cipher is a deterministic algorithm that operates on data blocks and allows encryption and decryption of bulk data. Block ciphers that are available on Red Hat Enterprise Linux are:
  • AES — Advanced Encryption Standard, a 128-bit symmetric block cipher using encryption keys with lengths of 128, 192, and 256 bits; for more information, see the FIPS PUB 197.
  • Twofish — A 128-bit block cipher operating with encryption keys of the range from 128 bits to 256 bits.
  • Serpent — A 128-bit block cipher operating with 128-bit, 192-bit and 256-bit encryption keys.
  • cast5 — A 64-bit Feistel cipher supporting encryption keys of the range from 40 to 128 bits; for more information, see the RFC 2144.
  • cast6 — A 128-bit Feistel cipher using 128-bit, 160-bit, 192-bit, 224-bit, or 256-bit encryption keys; for more information, see the RFC 2612.
Block cipher mode describes a way the block cipher is repeatedly applied on bulk data in order to encrypt or decrypt the data securely. The following modes can be used:
  • CBC — Cipher Block Chaining; for more information, see the NIST SP 800-38A.
  • XTS — XEX Tweakable Block Cipher with Ciphertext Stealing; for more information, see the IEEE 1619, or NIST SP 800-38E.
  • CTR — Counter; for more information, see the NIST SP 800-38A.
  • ECB — Electronic Codebook; for more information, see the NIST SP 800-38A.
  • CFB — Cipher Feedback; for more information, see the NIST SP 800-38A.
  • OFB — Output Feedback; for more information, see the NIST SP 800-38A.
An initial vector is a block of data used for ciphertext randomization. IV ensures that repeated encryption of the same plain text provides different ciphertext output. IV must not be reused with the same encryption key. For ciphers in CBC mode, IV must be unpredictable, otherwise the system could become vulnerable to certain watermarking attacks (see LUKS/cryptsetup FAQ for more information). Red Hat recommends using the following IV with AES:
  • ESSIV — Encrypted Salt-Sector Initialization Vector - This IV should be used for ciphers in CBC mode. You should use the default hash: sha256.
  • plain64 (or plain) — IV sector offset - This IV should be used for ciphers in XTS mode.
You may also specify the length of the used encryption key. The size of the key depends on the used combination of the block cipher and block cipher mode. If you do not specify the key length, LUKS will use the default value for the given combination. For example: if you decide to use a 128-bit key for AES in CBC mode, LUKS will encrypt your partition using the AES-128 implementation, while specifying a 512-bit key for AES in XTS mode means that the AES-256 implementation will be used. Note that XTS mode operates with two keys, the first is determined for tweakable encryption and the second for regular encryption. Manually Encrypting Directories


Following this procedure will remove all data on the partition that you are encrypting. You WILL lose all your information! Make sure you backup your data to an external source before beginning this procedure!
  1. Enter runlevel 1 by typing the following at a shell prompt as root:
    telinit 1
  2. Unmount your existing /home:
    umount /home
  3. If the command in the previous step fails, use fuser to find processes hogging /home and kill them:
    fuser -mvk /home
  4. Verify /home is no longer mounted:
    grep home /proc/mounts
  5. Fill your partition with random data:
    shred -v --iterations=1 /dev/VG00/LV_home
    This command proceeds at the sequential write speed of your device and may take some time to complete. It is an important step to ensure no unencrypted data is left on a used device, and to obfuscate the parts of the device that contain encrypted data as opposed to just random data.
  6. Initialize your partition:
    cryptsetup --verbose --verify-passphrase luksFormat /dev/VG00/LV_home
  7. Open the newly encrypted device:
    cryptsetup luksOpen /dev/VG00/LV_home home
  8. Make sure the device is present:
    ls -l /dev/mapper | grep home
  9. Create a file system:
    mkfs.ext3 /dev/mapper/home
  10. Mount the file system:
    mount /dev/mapper/home /home
  11. Make sure the file system is visible:
    df -h | grep home
  12. Add the following to the /etc/crypttab file:
    home /dev/VG00/LV_home none
  13. Edit the /etc/fstab file, removing the old entry for /home and adding the following line:
    /dev/mapper/home /home ext3 defaults 1 2
  14. Restore default SELinux security contexts:
    /sbin/restorecon -v -R /home
  15. Reboot the machine:
    shutdown -r now
  16. The entry in the /etc/crypttab makes your computer ask your luks passphrase on boot.
  17. Log in as root and restore your backup.
You now have an encrypted partition for all of your data to safely rest while the computer is off. Adding a New Passphrase to an Existing Device
Use the following command to add a new passphrase to an existing device:
cryptsetup luksAddKey <device>
After being prompted for any one of the existing passprases for authentication, you will be prompted to enter the new passphrase. Removing a Passphrase from an Existing Device
Use the following command to remove a passphrase from an existing device:
cryptsetup luksRemoveKey <device>
You will be prompted for the passphrase you want to remove and then for any one of the remaining passphrases for authentication. Creating Encrypted Block Devices in Anaconda
You can create encrypted devices during system installation. This allows you to easily configure a system with encrypted partitions.
To enable block device encryption, check the Encrypt System check box when selecting automatic partitioning or the Encrypt check box when creating an individual partition, software RAID array, or logical volume. After you finish partitioning, you will be prompted for an encryption passphrase. This passphrase will be required to access the encrypted devices. If you have pre-existing LUKS devices and provided correct passphrases for them earlier in the install process the passphrase entry dialog will also contain a check box. Checking this check box indicates that you would like the new passphrase to be added to an available slot in each of the pre-existing encrypted block devices.


Checking the Encrypt System check box on the Automatic Partitioning screen and then choosing Create custom layout does not cause any block devices to be encrypted automatically.


You can use a kickstart file to set a separate passphrase for each new encrypted block device. Also, kickstart allows you to specify a different type of encryption if the Anaconda default cipher, aes-xts-plain64, does not suit you. In dependencies on a device you want to encrypt, you can specify the --cipher=<cipher-string> along with the autopart, part, partition, logvol, and raid directives. This option has to be used together with the --encrypted option, otherwise it has no effect. For more information about the <cipher-string> format and possible cipher combinations, see Section, “LUKS Implementation in Red Hat Enterprise Linux”. For more information about kickstart configuration, see the Red Hat Enterprise Linux 6 Installation Guide.

3.2. Data in Motion

Data in motion is data that is being transmitted over a network. The biggest threats to data in motion are interception and alteration. Your user name and password should never be transmitted over a network without protection as it could be intercepted and used by someone else to impersonate you or gain access to sensitive information. Encrypting the network session ensures a higher security level for data in motion.
Data in motion is particularly vulnerable to attackers because the attacker does not have to be near the computer in which the data is being stored rather they only have to be somewhere along the path. Encryption tunnels can protect data along the path of communications.

3.2.1. Virtual Private Networks

Virtual Private Networks (VPN) provide encrypted tunnels between computers or networks of computers across all ports. With a VPN in place, all network traffic from the client is forwarded to the server through the encrypted tunnel. This means that the client is logically on the same network as the server it is connected to via the VPN. VPNs are very common and are simple to use and setup.

3.2.2. Secure Shell

Secure Shell (SSH) is a powerful network protocol used to communicate with another system over a secure channel. The transmissions over SSH are encrypted and protected from interception. Cryptographic login can also be utilized to provide a better authentication method over traditional user names and passwords. See Section, “Cryptographic Login”.
SSH is very easy to activate. By starting the sshd daemon, the system begins to accept connections and will allow access to the system when a correct user name and password is provided during the connection process. The standard TCP port for the SSH service is 22. However, this can be changed by modifying the /etc/ssh/sshd_config configuration file and restarting the service. This file also contains other configuration options for SSH.
By default, the sshd service starts automatically at boot time. Run the following command as root to query the status of the daemon:
~]# service sshd status
If you need to restart the sshd service, issue the following command as root:
~]# service sshd restart
Refer to the Services and Daemons chapter of the Red Hat Enterprise Linux 6 Deployment Guide for more information regarding the management of system services.
Secure Shell (SSH) also provides encrypted tunnels between computers but only using a single port. Port forwarding can be done over an SSH tunnel and traffic will be encrypted as it passes through that tunnel, but using port forwarding is not as fluid as a VPN (Section 3.2.1, “Virtual Private Networks”). Cryptographic Login
SSH supports the use of cryptographic keys for logging in to computers. This is much more secure than using only a password. If you combine this method with other authentication methods, it can be considered a multi-factor authentication. See Section, “Multiple Authentication Methods” for more information about using multiple authentication methods.
In order to enable the use of cryptographic keys for authentication, the PubkeyAuthentication configuration directive in the /etc/ssh/sshd_config file needs to be set to yes. Note that this is the default setting. Set the PasswordAuthentication directive to no to disable the possibility of using passwords for logging in.
SSH keys can be generated using the ssh-keygen command. If invoked without additional arguments, it creates a 2048-bit RSA key set. The keys are stored, by default, in the ~/.ssh directory. You can utilize the -b switch to modify the bit-strength of the key. Using 2048-bit keys is normally sufficient. See the Generating Key Pairs chapter of the Red Hat Enterprise Linux 6 Deployment Guide for more detailed information about generating SSH keys.
You should see the two keys in your ~/.ssh directory. If you accepted the defaults when running the ssh-keygen command, then the generated files are named id_rsa and and contain the private and public key respectively. You should always protect the private key from exposure by making it unreadable by anyone else but the file's owner. The public key, however, needs to be transferred to the system you are going to log in to. You can use the ssh-copy-id command to transfer the key to the server:
~]$ ssh-copy-id -i [user@]server
This command will also automatically append the public key to the ~/.ssh/authorized_key file on the server. The sshd daemon will check this file when you attempt to log in to the server.
Similarly to passwords and any other authentication mechanism, you should change your SSH keys regularly. When you do, make sure you remove any unused keys from the authorized_key file. Multiple Authentication Methods
Using multiple authentication methods, or multi-factor authentication, increases the level of protection against unauthorized access, and as such should be considered when hardening a system to prevent it from being compromised. Users attempting to log in to a system that uses multi-factor authentication must successfully complete all specified authentication methods in order to be granted access.
Use the AuthenticationMethods configuration directive in the /etc/ssh/sshd_config file to specify which authentication methods are to be utilized. Note that it is possible to define more than one list of required authentication methods using this directive. If that is the case, the user must complete every method in at least one of the lists. The lists need to be separated by blank spaces, and the individual authentication-method names within the lists must be comma-separated. For example:
AuthenticationMethods publickey,gssapi-with-mic publickey,keyboard-interactive
An sshd daemon configured using the above AuthenticationMethods directive only grants access if the user attempting to log in successfully completes either publickey authentication followed by gssapi-with-mic or by keyboard-interactive authentication. Note that each of the requested authentication methods needs to be explicitly enabled using a corresponding configuration directive (such as PubkeyAuthentication) in the /etc/ssh/sshd_config file. Refer to the AUTHENTICATION section of ssh(1) for a general list of available authentication methods. Other Ways of Securing SSH
Protocol Version
Even though the implementation of the SSH protocol supplied with Red Hat Enterprise Linux supports both the SSH-1 and SSH-2 versions of the protocol, only the latter should be used whenever possible. The SSH-2 version contains a number of improvements over the older SSH-1, and the majority of advanced configuration options is only available when using SSH-2.
Users are encouraged to make use of SSH-2 in order to maximize the extent to which the SSH protocol protects the authentication and communication for which it is used. The version or versions of the protocol supported by the sshd daemon can be specified using the Protocol configuration directive in the /etc/ssh/sshd_config file. The default setting is 2.
Key Types
While the ssh-keygen command generates a pair of SSH-2 RSA keys by default, using the -t option, it can be instructed to generate DSA or ECDSA keys as well. The ECDSA (Elliptic Curve Digital Signature Algorithm) offers better performance at the same symmetric key length. It also generates shorter keys.
Non-Default Port
By default, the sshd daemon listens on the 22 network port. Changing the port reduces the exposure of the system to attacks based on automated network scanning, thus increasing security through obscurity. The port can be specified using the Port directive in the /etc/ssh/sshd_config configuration file. Note also that the default SELinux policy must be changed to allow for the use of a non-default port. You can do this by modifying the ssh_port_t SELinux type by typing the following command as root:
~]# semanage -a -t ssh_port_t -p tcp port_number
In the above command, replace port_number with the new port number specified using the Port directive.
No Root Login
Provided that your particular use case does not require the possibility of logging in as the root user, you should consider setting the PermitRootLogin configuration directive to no in the /etc/ssh/sshd_config file. By disabling the possibility of logging in as the root user, the administrator can audit which user runs what privileged command after they log in as regular users and then gain root rights.


This section draws attention to the most common ways of securing an SSH setup. By no means should this list of suggested measures be considered exhaustive or definitive. Refer to sshd_config(5) for a description of all configuration directives available for modifying the behavior of the sshd daemon and to ssh(1) for an explanation of basic SSH concepts.

3.3. OpenSSL Intel AES-NI Engine

The Intel Advanced Encryption Standard (AES) New Instructions (AES-NI) engine is available for certain Intel processors, and allows for extremely fast hardware encryption and decryption.


For a list of Intel processors that support the AES-NI engine, see: Intel's ARK.
The AES-NI engine is automatically enabled if the detected processor is among the supported ones. To check that the processor is supported, follow the steps below:
  1. Ensure that the processor has the AES instruction set:
    ~]# grep -m1 -o aes /proc/cpuinfo
  2. As root, run the following commands and compare their outputs. Significantly better performance of the latter command indicates that AES-NI is enabled. Note that the outputs below are shortened for brevity:
    ~]# openssl speed aes-128-cbc
    The 'numbers' are in 1000s of bytes per second processed.
    type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
    aes-128 cbc      99696.17k   107792.98k   109961.22k   110559.91k   110742.19k
    ~]# openssl speed -evp aes-128-cbc
    The 'numbers' are in 1000s of bytes per second processed.
    type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
    aes-128-cbc     800450.23k   873269.82k   896864.85k   903446.19k   902752.94k
To test the speed of OpenSSH you can run a command like the following:
~]# dd if=/dev/zero count=100 bs=1M | ssh -c aes128-cbc localhost "cat >/dev/null"
root@localhost's password: 
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 4.81868 s, 21.8 MB/s
See Intel® Advanced Encryption Standard Instructions (AES-NI) for details about the AES-NI engine.

3.4. Using the Random Number Generator

In order to be able to generate secure cryptographic keys that cannot be easily broken, a source of random numbers is required. Generally, the more random the numbers are, the better the chance of obtaining unique keys. Entropy for generating random numbers is usually obtained from computing environmental “noise” or using a hardware random number generator.
The rngd daemon, which is a part of the rng-tools package, is capable of using both environmental noise and hardware random number generators for extracting entropy. The daemon checks whether the data supplied by the source of randomness is sufficiently random and then stores it in the kernel's random-number entropy pool. The random numbers it generates are made available through the /dev/random and /dev/urandom character devices.
The difference between /dev/random and /dev/urandom is that the former is a blocking device, which means it stops supplying numbers when it determines that the amount of entropy is insufficient for generating a properly random output. Conversely, /dev/urandom is a non-blocking source, which reuses the kernel's entropy pool and is thus able to provide an unlimited supply of pseudo-random numbers, albeit with less entropy. As such, /dev/urandom should not be used for creating long-term cryptographic keys.
To install the rng-tools package, issue the following command as the root user:
~]# yum install rng-tools
To start the rngd daemon, execute the following command as root:
~]# service rngd start
To query the status of the daemon, use the following command:
~]# service rngd status
To start the rngd daemon with optional parameters, execute it directly. For example, to specify an alternative source of random-number input (other than /dev/hwrandom), use the following command:
~]# rngd --rng-device=/dev/hwrng
The above command starts the rngd daemon with /dev/hwrng as the device from which random numbers are read. Similarly, you can use the -o (or --random-device) option to choose the kernel device for random-number output (other than the default /dev/random). See the rngd(8) manual page for a list of all available options.
The rng-tools package also contains the rngtest utility, which can be used to check the randomness of data. To test the level of randomness of the output of /dev/random, use the rngtest tool as follows:
~]$ cat /dev/random | rngtest -c 1000
rngtest 2
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

rngtest: starting FIPS tests...
rngtest: bits received from input: 20000032
rngtest: FIPS 140-2 successes: 1000
rngtest: FIPS 140-2 failures: 0
rngtest: FIPS 140-2(2001-10-10) Monobit: 0
rngtest: FIPS 140-2(2001-10-10) Poker: 0
rngtest: FIPS 140-2(2001-10-10) Runs: 0
rngtest: FIPS 140-2(2001-10-10) Long run: 1
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=308.697; avg=623.670; max=730.823)Kibits/s
rngtest: FIPS tests speed: (min=51.971; avg=137.737; max=167.311)Mibits/s
rngtest: Program run time: 31461595 microseconds
A high number of failures shown in the output of the rngtest tool indicates that the randomness of the tested data is sub-optimal and should not be relied upon. See the rngtest(1) manual page for a list of options available for the rngtest utility.

3.5. GNU Privacy Guard (GPG)

GnuPG (GPG) is an open source version of PGP that allows you to sign and and also encrypt a file or an email message. This is useful to maintain integrity of the message or file and also protects the confidentiality of the information contained within the file or email. In the case of email, GPG provides dual protection. Not only can it provide Data at Rest protection but also Data in Motion protection once the message has been sent across the network. Refer to Section 3.1, “Data at Rest” and Section 3.2, “Data in Motion” for more information about these concepts.
GPG is used to identify yourself and authenticate your communications, including those with people you do not know. GPG allows anyone reading a GPG-signed email to verify its authenticity. In other words, GPG allows someone to be reasonably certain that communications signed by you actually are from you. GPG is useful because it helps prevent third parties from altering code or intercepting conversations and altering the message.

3.5.1. Creating GPG Keys in GNOME

To create a GPG Key in GNOME, follow these steps:
  1. Install the Seahorse utility, which makes GPG key management easier:
    ~]# yum install seahorse
  2. To create a key, from the ApplicationsAccessories menu select Passwords and Encryption Keys, which starts the application Seahorse.
  3. From the File menu select New and then PGP Key. Then click Continue.
  4. Type your full name, email address, and an optional comment describing who you are (for example: John C. Smith,, Software Engineer). Click Create. A dialog is displayed asking for a passphrase for the key. Choose a strong passphrase but also easy to remember. Click OK and the key is created.


If you forget your passphrase, you will not be able to decrypt the data.
To find your GPG key ID, look in the Key ID column next to the newly created key. In most cases, if you are asked for the key ID, prepend 0x to the key ID, as in 0x6789ABCD. You should make a backup of your private key and store it somewhere secure.

3.5.2. Creating GPG Keys in KDE

To create a GPG Key in KDE, follow these steps:
  1. Start the KGpg program from the main menu by selecting ApplicationsUtilitiesEncryption Tool. If you have never used KGpg before, the program walks you through the process of creating your own GPG keypair.
  2. A dialog box appears prompting you to create a new key pair. Enter your name, email address, and an optional comment. You can also choose an expiration time for your key, as well as the key strength (number of bits) and algorithms.
  3. Enter your passphrase in the next dialog box. At this point, your key appears in the main KGpg window.


If you forget your passphrase, you will not be able to decrypt the data.
To find your GPG key ID, look in the Key ID column next to the newly created key. In most cases, if you are asked for the key ID, prepend 0x to the key ID, as in 0x6789ABCD. You should make a backup of your private key and store it somewhere secure.

3.5.3. Creating GPG Keys Using the Command Line

  1. Use the following shell command:
    ~]$ gpg2 --gen-key
    This command generates a key pair that consists of a public and a private key. Other people use your public key to authenticate and/or decrypt your communications. Distribute your public key as widely as possible, especially to people who you know will want to receive authentic communications from you, such as a mailing list.
  2. A series of prompts directs you through the process. Press the Enter key to assign a default value if desired. The first prompt asks you to select what kind of key you prefer:
    Please select what kind of key you want:
    (1) RSA and RSA (default)
    (2) DSA and Elgamal
    (3) DSA (sign only)
    (4) RSA (sign only)
    Your selection?
    In almost all cases, the default is the correct choice. An RSA/RSA key allows you not only to sign communications, but also to encrypt files.
  3. Choose the key size:
    RSA keys may be between 1024 and 4096 bits long.
    What keysize do you want? (2048)
    Again, the default, 2048, is sufficient for almost all users and represents an extremely strong level of security.
  4. Choose when the key will expire. It is a good idea to choose an expiration date instead of using the default, which is none. If, for example, the email address on the key becomes invalid, an expiration date will remind others to stop using that public key.
    Please specify how long the key should be valid.
    0 = key does not expire
    d = key expires in n days
    w = key expires in n weeks
    m = key expires in n months
    y = key expires in n years
    key is valid for? (0)
    Entering a value of 1y, for example, makes the key valid for one year. (You may change this expiration date after the key is generated, if you change your mind.)
  5. Before the gpg2 application asks for signature information, the following prompt appears:
    Is this correct (y/N)?
    Enter y to finish the process.
  6. Enter your name and email address for your GPG key. Remember this process is about authenticating you as a real individual. For this reason, include your real name. If you choose a bogus email address, it will be more difficult for others to find your public key. This makes authenticating your communications difficult. If you are using this GPG key for self-introduction on a mailing list, for example, enter the email address you use on that list.
    Use the comment field to include aliases or other information. (Some people use different keys for different purposes and identify each key with a comment, such as "Office" or "Open Source Projects.")
  7. At the confirmation prompt, enter the letter O to continue if all entries are correct, or use the other options to fix any problems. Finally, enter a passphrase for your secret key. The gpg2 program asks you to enter your passphrase twice to ensure you made no typing errors.
  8. Finally, gpg2 generates random data to make your key as unique as possible. Move your mouse, type random keys, or perform other tasks on the system during this step to speed up the process. Once this step is finished, your keys are complete and ready to use:
    	  pub  1024D/1B2AFA1C 2005-03-31 John Q. Doe <>
    	  Key fingerprint = 117C FE83 22EA B843 3E86  6486 4320 545E 1B2A FA1C
    	  sub  1024g/CEA4B22E 2005-03-31 [expires: 2006-03-31]
  9. The key fingerprint is a shorthand "signature" for your key. It allows you to confirm to others that they have received your actual public key without any tampering. You do not need to write this fingerprint down. To display the fingerprint at any time, use this command, substituting your email address:
    ~]$ gpg2 --fingerprint
    Your "GPG key ID" consists of 8 hex digits identifying the public key. In the example above, the GPG key ID is 1B2AFA1C. In most cases, if you are asked for the key ID, prepend 0x to the key ID, as in 0x6789ABCD.


If you forget your passphrase, the key cannot be used and any data encrypted using that key will be lost.

3.6. Using stunnel

The stunnel program is an encryption wrapper between a client and a server. It listens on the port specified in its configuration file, encrypts the communication with the client, and forwards the data to the original daemon listening on its usual port. This way, you can secure any service that itself does not support any type of encryption, or improve the security of a service that uses a type of encryption that you want to avoid for security reasons, such as SSL versions 2 and 3, affected by the POODLE SSL vulnerability (CVE-2014-3566). See Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings. OpenLDAP older than 2.4.39 (before Red Hat Enterprise Linux 6.6) and CUPS are examples of components that do not provide a way to disable SSL in their own configuration.

3.6.1. Installing stunnel

Install the stunnel package by running the following command as root:
~]# yum install stunnel

3.6.2. Configuring stunnel as a TLS Wrapper

To configure stunnel, follow these steps:
  1. You need a valid certificate for stunnel regardless of what service you use it with. If you do not have a suitable certificate, you can apply to a Certificate Authority to obtain one, or you can create a self-signed cerfiticate.


    Always use certificates signed by a Certificate Authority for servers running in a production environment. Self-signed certificates are only appropriate for testing purposes or private networks.
    To create a self-signed certificate for stunnel, enter the /etc/pki/tls/certs/ directory and type the following command as root:
    certs]# make stunnel.pem
    Answer all of the questions to complete the process.
  2. When you have a certificate, create a configuration file for stunnel. It is a text file in which every line specifies an option or the beginning of a service definition. You can also keep comments and empty lines in the file to improve its legibility, where comments start with a semicolon.
    The stunnel RPM package contains the /etc/stunnel/ directory, in which you can store the configuration file. Although stunnel does not require any special format of the file name or its extension, use /etc/stunnel/stunnel.conf. The following content configures stunnel as a TLS wrapper:
    cert = /etc/pki/tls/certs/stunnel.pem
    ; Allow only TLS, thus avoiding SSL
    sslVersion = TLSv1
    chroot = /var/run/stunnel
    setuid = nobody
    setgid = nobody
    pid = /
    socket = l:TCP_NODELAY=1
    socket = r:TCP_NODELAY=1
    accept = port
    connect = port
    TIMEOUTclose = 0
    Alternatively, you can avoid SSL by replacing the line containing sslVersion = TLSv1 with the following lines:
    options = NO_SSLv2
    options = NO_SSLv3
    The purpose of the options is as follows:
    • cert — the path to your certificate
    • sslVersion — the version of SSL; note that you can use TLS here even though SSL and TLS are two independent cryptographic protocols
    • chroot — the changed root directory in which the stunnel process runs, for greater security
    • setuid, setgid — the user and group that the stunnel process runs as; nobody is a restricted system account
    • pid — the file in which stunnel saves its process ID, relative to chroot
    • socket — local and remote socket options; in this case, disable Nagle's algorithm to improve network latency
    • [service_name] — the beginning of the service definition; the options used below this line apply to the given service only, whereas the options above affect stunnel globally
    • accept — the port to listen on
    • connect — the port to connect to; this must be the port that the service you are securing uses
    • TIMEOUTclose — how many seconds to wait for the close_notify alert from the client; 0 instructs stunnel not to wait at all
    • options — OpenSSL library options

    Example 3.1. Securing OpenLDAP

    To configure stunnel as a TLS wrapper for OpenLDAP older than 2.4.39, use the following values:
    accept = 636
    connect = 389
    636 is the standard port for secure LDAP. 389 is the port that the OpenLDAP daemon listens on.

    Example 3.2. Securing CUPS

    Similarly, to configure stunnel as a TLS wrapper for CUPS, use the following values:
    accept = 632
    connect = 631
    Instead of 632, you can use any free port that you prefer. 631 is the port that CUPS normally uses.
  3. Create the chroot directory and give the user specified by the setuid option write access to it. To do so, run the following commands as root:
    ~]# mkdir /var/run/stunnel
    ~]# chown nobody:nobody /var/run/stunnel
    This allows stunnel to create the PID file.
  4. If your system is using firewall settings that disallow access to the new port, change them accordingly. See Section, “Other Ports” in Section 2.8, “Firewalls” for details.
  5. When you have created the configuration file and the chroot directory, and when you are sure that the specified port is accessible, you are ready to start using stunnel.

3.6.3. Starting, Stopping and Restarting stunnel

To start stunnel, run the following command as root:
~]# stunnel /etc/stunnel/stunnel.conf
By default, stunnel uses /var/log/secure to log its output.
To terminate stunnel, kill the process by running the following command as root:
~]# kill `cat /var/run/stunnel/`
If you edit the configuration file while stunnel is running, terminate stunnel and start it again for your changes to take effect.

3.7. Hardening TLS Configuration

TLS (Transport Layer Security) is a cryptographic protocol used to secure network communications. When hardening system security settings by configuring preferred key-exchange protocols, authentication methods, and encryption algorithms, it is necessary to bear in mind that the broader the range of supported clients, the lower the resulting security. Conversely, strict security settings lead to a limited compatibility with clients, which can result in some users being locked out of the system. Be sure to target the strictest available configuration and only relax it when it is required for compatibility reasons.
Note that the default settings provided by libraries included in Red Hat Enterprise Linux are secure enough for most deployments. The TLS implementations use secure algorithms where possible while not preventing connections from or to legacy clients or servers. Apply the hardened settings described in this section in environments with strict security requirements where legacy clients or servers that do not support secure algorithms or protocols are not expected or allowed to connect.

3.7.1. Choosing Algorithms to Enable

There are several components that need to be selected and configured. Each of the following directly influences the robustness of the resulting configuration (and, consequently, the level of support in clients) or the computational demands that the solution has on the system.

Protocol Versions

The latest version of TLS provides the best security mechanism. Unless you have a compelling reason to include support for older versions of TLS (or even SSL), allow your systems to negotiate connections using only the latest version of TLS.
Do not allow negotiation using SSL version 2 or 3. Both of those versions have serious security vulnerabilities. Only allow negotiation using TLS version 1.0 or higher. The current version of TLS, 1.2, should always be preferred.


Please note that currently, the security of all versions of TLS depends on the use of TLS extensions, specific ciphers (see below), and other workarounds. All TLS connection peers need to implement secure renegotiation indication (RFC 5746), must not support compression, and must implement mitigating measures for timing attacks against CBC-mode ciphers (the Lucky Thirteen attack). TLS v1.0 clients need to additionally implement record splitting (a workaround against the BEAST attack). TLS v1.2 supports Authenticated Encryption with Associated Data (AEAD) mode ciphers like AES-GCM, AES-CCM, or Camellia-GCM, which have no known issues. All the mentioned mitigations are implemented in cryptographic libraries included in Red Hat Enterprise Linux.
See Table 3.1, “Protocol Versions” for a quick overview of protocol versions and recommended usage.
Table 3.1. Protocol Versions
Protocol VersionUsage Recommendation
SSL v2
Do not use. Has serious security vulnerabilities.
SSL v3
Do not use. Has serious security vulnerabilities.
TLS v1.0
Use for interoperability purposes where needed. Has known issues that cannot be mitigated in a way that guarantees interoperability, and thus mitigations are not enabled by default. Does not support modern cipher suites.
TLS v1.1
Use for interoperability purposes where needed. Has no known issues but relies on protocol fixes that are included in all the TLS implementations in Red Hat Enterprise Linux. Does not support modern cipher suites.
TLS v1.2
Recommended version. Supports the modern AEAD cipher suites.
Some components in Red Hat Enterprise Linux are configured to use TLS v1.0 even though they provide support for TLS v1.1 or even v1.2. This is motivated by an attempt to achieve the highest level of interoperability with external services that may not support the latest versions of TLS. Depending on your interoperability requirements, enable the highest available version of TLS.


SSL v3 is not recommended for use. However, if, despite the fact that it is considered insecure and unsuitable for general use, you absolutely must leave SSL v3 enabled, see Section 3.6, “Using stunnel” for instructions on how to use stunnel to securely encrypt communications even when using services that do not support encryption or are only capable of using obsolete and insecure modes of encryption.
While not immediately insecure, cipher suites that offer less than 128 bits of security should not be considered for their short useful life. Algorithms that use 128 bit of security or more can be expected to be unbreakable for at least several years, and are thus strongly recommended. Note that while 3DES ciphers advertise the use of 168 bits, they actually offer 112 bits of security.
Always give preference to cipher suites that support (perfect) forward secrecy (PFS), which ensures the confidentiality of encrypted data even in case the server key is compromised. This rules out the fast RSA key exchange, but allows for the use of ECDHE and DHE. Of the two, ECDHE is the faster and therefore the preferred choice.
Note also that when using the ECDHE key exchange with ECDSA certificates, the transaction is even faster than pure RSA key exchange. To provide support for legacy clients, you can install two pairs of certificates and keys on a server: one with ECDSA keys (for new clients) and one with RSA keys (for legacy ones).

Public Key Length

When using RSA keys, always prefer key lengths of at least 3072 bits signed by at least SHA-256, which is sufficiently large for true 128 bits of security.


Keep in mind that the security of your system is only as strong as the weakest link in the chain. For example, a strong cipher alone does not guarantee good security. The keys and the certificates are just as important, as well as the hash functions and keys used by the Certification Authority (CA) to sign your keys.

3.7.2. Using Implementations of TLS

Red Hat Enterprise Linux is distributed with several full-featured implementations of TLS. In this section, the configuration of OpenSSL and GnuTLS is described. See Section 3.7.3, “Configuring Specific Applications” for instructions on how to configure TLS support in individual applications.
The available TLS implementations offer support for various cipher suites that define all the elements that come together when establishing and using TLS-secured communications.
Use the tools included with the different implementations to list and specify cipher suites that provide the best possible security for your use case while considering the recommendations outlined in Section 3.7.1, “Choosing Algorithms to Enable”. The resulting cipher suites can then be used to configure the way individual applications negotiate and secure connections.


Be sure to check your settings following every update or upgrade of the TLS implementation you use or the applications that utilize that implementation. New versions may introduce new cipher suites that you do not want to have enabled and that your current configuration does not disable. Working with Cipher Suites in OpenSSL
OpenSSL is a toolkit and a cryptography library that support the SSL and TLS protocols. On Red Hat Enterprise Linux, a configuration file is provided at /etc/pki/tls/openssl.cnf. The format of this configuration file is described in config(1).
To get a list of all cipher suites supported by your installation of OpenSSL, use the openssl command with the ciphers subcommand as follows:
~]$ openssl ciphers -v 'ALL:COMPLEMENTOFALL'
Pass other parameters (referred to as cipher strings and keywords in OpenSSL documentation) to the ciphers subcommand to narrow the output. Special keywords can be used to only list suites that satisfy a certain condition. For example, to only list suites that are defined as belonging to the HIGH group, use the following command:
~]$ openssl ciphers -v 'HIGH'
See the ciphers(1) manual page for a list of available keywords and cipher strings.
To obtain a list of cipher suites that satisfy the recommendations outlined in Section 3.7.1, “Choosing Algorithms to Enable”, use a command similar to the following:
~]$ openssl ciphers -v 'kEECDH+aECDSA+AES:kEECDH+AES+aRSA:kEDH+aRSA+AES' | column -t
ECDHE-ECDSA-AES256-SHA384      TLSv1.2  Kx=ECDH  Au=ECDSA  Enc=AES(256)     Mac=SHA384
ECDHE-ECDSA-AES256-SHA         SSLv3    Kx=ECDH  Au=ECDSA  Enc=AES(256)     Mac=SHA1
ECDHE-ECDSA-AES128-SHA256      TLSv1.2  Kx=ECDH  Au=ECDSA  Enc=AES(128)     Mac=SHA256
ECDHE-ECDSA-AES128-SHA         SSLv3    Kx=ECDH  Au=ECDSA  Enc=AES(128)     Mac=SHA1
ECDHE-RSA-AES256-GCM-SHA384    TLSv1.2  Kx=ECDH  Au=RSA    Enc=AESGCM(256)  Mac=AEAD
ECDHE-RSA-AES256-SHA384        TLSv1.2  Kx=ECDH  Au=RSA    Enc=AES(256)     Mac=SHA384
ECDHE-RSA-AES256-SHA           SSLv3    Kx=ECDH  Au=RSA    Enc=AES(256)     Mac=SHA1
ECDHE-RSA-AES128-GCM-SHA256    TLSv1.2  Kx=ECDH  Au=RSA    Enc=AESGCM(128)  Mac=AEAD
ECDHE-RSA-AES128-SHA256        TLSv1.2  Kx=ECDH  Au=RSA    Enc=AES(128)     Mac=SHA256
ECDHE-RSA-AES128-SHA           SSLv3    Kx=ECDH  Au=RSA    Enc=AES(128)     Mac=SHA1
DHE-RSA-AES256-GCM-SHA384      TLSv1.2  Kx=DH    Au=RSA    Enc=AESGCM(256)  Mac=AEAD
DHE-RSA-AES256-SHA256          TLSv1.2  Kx=DH    Au=RSA    Enc=AES(256)     Mac=SHA256
DHE-RSA-AES256-SHA             SSLv3    Kx=DH    Au=RSA    Enc=AES(256)     Mac=SHA1
DHE-RSA-AES128-GCM-SHA256      TLSv1.2  Kx=DH    Au=RSA    Enc=AESGCM(128)  Mac=AEAD
DHE-RSA-AES128-SHA256          TLSv1.2  Kx=DH    Au=RSA    Enc=AES(128)     Mac=SHA256
DHE-RSA-AES128-SHA             SSLv3    Kx=DH    Au=RSA    Enc=AES(128)     Mac=SHA1
The above command omits all insecure ciphers, gives preference to ephemeral elliptic curve Diffie-Hellman key exchange and ECDSA ciphers, and omits RSA key exchange (thus ensuring perfect forward secrecy).
Note that this is a rather strict configuration, and it might be necessary to relax the conditions in real-world scenarios to allow for a compatibility with a broader range of clients. Working with Cipher Suites in GnuTLS
GnuTLS is a communications library that implements the SSL and TLS protocols and related technologies.


The GnuTLS installation on Red Hat Enterprise Linux offers optimal default configuration values that provide sufficient security for the majority of use cases. Unless you need to satisfy special security requirements, it is recommended to use the supplied defaults.
Use the gnutls-cli command with the -l (or --list) option to list all supported cipher suites:
~]$ gnutls-cli -l
To narrow the list of cipher suites displayed by the -l option, pass one or more parameters (referred to as priority strings and keywords in GnuTLS documentation) to the --priority option. See the GnuTLS documentation at for a list of all available priority strings. For example, issue the following command to get a list of cipher suites that offer at least 128 bits of security:
~]$ gnutls-cli --priority SECURE128 -l
To obtain a list of cipher suites that satisfy the recommendations outlined in Section 3.7.1, “Choosing Algorithms to Enable”, use a command similar to the following:
~]$ gnutls-cli --priority SECURE256:+SECURE128:-VERS-TLS-ALL:+VERS-TLS1.2:-RSA:-DHE-DSS:-CAMELLIA-128-CBC:-CAMELLIA-256-CBC -l
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384                      0xc0, 0x2c      TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA384                      0xc0, 0x24      TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA1                        0xc0, 0x0a      SSL3.0
TLS_ECDHE_ECDSA_AES_128_GCM_SHA256                      0xc0, 0x2b      TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA256                      0xc0, 0x23      TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA1                        0xc0, 0x09      SSL3.0
TLS_ECDHE_RSA_AES_256_GCM_SHA384                        0xc0, 0x30      TLS1.2
TLS_ECDHE_RSA_AES_256_CBC_SHA1                          0xc0, 0x14      SSL3.0
TLS_ECDHE_RSA_AES_128_GCM_SHA256                        0xc0, 0x2f      TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA256                        0xc0, 0x27      TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA1                          0xc0, 0x13      SSL3.0
TLS_DHE_RSA_AES_256_CBC_SHA256                          0x00, 0x6b      TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA1                            0x00, 0x39      SSL3.0
TLS_DHE_RSA_AES_128_GCM_SHA256                          0x00, 0x9e      TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA256                          0x00, 0x67      TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA1                            0x00, 0x33      SSL3.0

Certificate types: CTYPE-X.509
Protocols: VERS-TLS1.2
Compression: COMP-NULL
Elliptic curves: CURVE-SECP384R1, CURVE-SECP521R1, CURVE-SECP256R1
The above command limits the output to ciphers with at least 128 bits of security while giving preference to the stronger ones. It also forbids RSA key exchange and DSS authentication.
Note that this is a rather strict configuration, and it might be necessary to relax the conditions in real-world scenarios to allow for a compatibility with a broader range of clients.

3.7.3. Configuring Specific Applications

Different applications provide their own configuration mechanisms for TLS. This section describes the TLS-related configuration files employed by the most commonly used server applications and offers examples of typical configurations.
Regardless of the configuration you choose to use, always make sure to mandate that your server application enforces server-side cipher order, so that the cipher suite to be used is determined by the order you configure. Configuring the Apache HTTP Server
The Apache HTTP Server can use both OpenSSL and NSS libraries for its TLS needs. Depending on your choice of the TLS library, you need to install either the mod_ssl or the mod_nss module (provided by eponymous packages). For example, to install the package that provides the OpenSSL mod_ssl module, issue the following command as root:
~]# yum install mod_ssl
The mod_ssl package installs the /etc/httpd/conf.d/ssl.conf configuration file, which can be used to modify the TLS-related settings of the Apache HTTP Server. Similarly, the mod_nss package installs the /etc/httpd/conf.d/nss.conf configuration file.
Install the httpd-manual package to obtain a complete documentation for the Apache HTTP Server, including TLS configuration. The directives available in the /etc/httpd/conf.d/ssl.conf configuration file are described in detail in /usr/share/httpd/manual/mod/mod_ssl.html. Examples of various settings are in /usr/share/httpd/manual/ssl/ssl_howto.html.
When modifying the settings in the /etc/httpd/conf.d/ssl.conf configuration file, be sure to consider the following three directives at the minimum:
Use this directive to specify the version of TLS (or SSL) you want to allow.
Use this directive to specify your preferred cipher suite or disable the ones you want to disallow.
Uncomment and set this directive to on to ensure that the connecting clients adhere to the order of ciphers you specified.
For example:
SSLProtocol all -SSLv2 -SSLv3
SSLCipherSuite HIGH:!aNULL:!MD5
SSLHonorCipherOrder on
Note that the above configuration is the bare minimum, and it can be hardened significantly by following the recommendations outlined in Section 3.7.1, “Choosing Algorithms to Enable”.
To configure and use the mod_nss module, modify the /etc/httpd/conf.d/nss.conf configuration file. The mod_nss module is derived from mod_ssl, and as such it shares many features with it, not least the structure of the configuration file, and the directives that are available. Note that the mod_nss directives have a prefix of NSS instead of SSL. See for an overview of information about mod_nss, including a list of mod_ssl configuration directives that are not applicable to mod_nss.

3.7.4. Additional Information

For more information about TLS configuration and related topics, see the resources listed below.

Installed Documentation

  • config(1) — Describes the format of the /etc/ssl/openssl.conf configuration file.
  • ciphers(1) — Includes a list of available OpenSSL keywords and cipher strings.
  • /usr/share/httpd/manual/mod/mod_ssl.html — Contains detailed descriptions of the directives available in the /etc/httpd/conf.d/ssl.conf configuration file used by the mod_ssl module for the Apache HTTP Server.
  • /usr/share/httpd/manual/ssl/ssl_howto.html — Contains practical examples of real-world settings in the /etc/httpd/conf.d/ssl.conf configuration file used by the mod_ssl module for the Apache HTTP Server.

Online Documentation

Chapter 4. General Principles of Information Security

The following general principles provide an overview of good security practices:
  • Encrypt all data transmitted over networks to help prevent man-in-the-middle attacks and eavesdropping. It is important to encrypt authentication information, such as passwords.
  • Minimize the amount of software installed and running services.
  • Use security-enhancing software and tools, for example, Security-Enhanced Linux (SELinux) for Mandatory Access Control (MAC), Netfilter iptables for packet filtering (firewall), and the GNU Privacy Guard (GPG) for encrypting files.
  • If possible, run each network service on a separate system to minimize the risk of one compromised service being used to compromise other services.
  • Maintain user accounts: create and enforce a strong password policy; delete unused user accounts.
  • Routinely review system and application logs. By default, security-relevant system logs are written to /var/log/secure and /var/log/audit/audit.log. Note: sending logs to a dedicated log server helps prevent attackers from easily modifying local logs to avoid detection.
  • Never log in as the root user unless absolutely necessary. It is recommended that administrators use sudo to execute commands as root when required. Users capable of running sudo are specified in /etc/sudoers. Use the visudo utility to edit /etc/sudoers.

Chapter 5. Secure Installation

Security begins with the first time you put that CD or DVD into your disk drive to install Red Hat Enterprise Linux. Configuring your system securely from the beginning makes it easier to implement additional security settings later.

5.1. Disk Partitions

Red Hat recommends creating separate partitions for /boot, /, /home, /tmp/, and /var/tmp/. If the root partition (/) becomes corrupt, your data could be lost forever. By using separate partitions, the data is slightly more protected. You can also target these partition for frequent backups. The purpose for each partition is different and we will address each partition.
/boot - This partition is the first partition that is read by the system during the boot. The boot loader and kernel images that are used to boot your system into Red Hat Enterprise Linux are stored in this partition. This partition should not be encrypted. If this partition is included in / and that partition is encrypted or otherwise becomes unavailable, your system will not be able to boot.
/home - When user data (/home) is stored in / instead of a separate partition, the partition can fill up causing the operating system to become unstable. Also, when upgrading your system to the next version of Red Hat Enterprise Linux, it is a lot easier if you can keep your data in the /home partition as it will not be overwritten during installation.
/tmp and /var/tmp/ - Both the /tmp and the /var/tmp/ directories are used to store data that does not need to be stored for a long period of time. However, if a lot of data floods one of these directories, it can consume all of your storage space. If this happens and these directories are stored within /, your system could become unstable and crash. For this reason, moving these directories into their own partitions is a good idea.

5.2. Utilize LUKS Partition Encryption

During the installation process, an option to encrypt partitions is presented to the user. The user must supply a passphrase. This passphrase is used as a key to unlock the bulk encryption key, which is used to secure the partition's data.

Chapter 6. Software Maintenance

Software maintenance is extremely important to maintaining a secure system. It is vital to patch software as soon as it becomes available in order to prevent attackers from using known holes to infiltrate your system.

6.1. Install Minimal Software

It is a recommended practice to install only the packages you will use because each piece of software on your computer could possibly contain a vulnerability. If you are installing from the DVD media take the opportunity to select exactly what packages you want to install during the installation. When you find you need another package, you can always add it to the system later.
For more information on minimal installation, see the "Package Group Selection" section of the Red Hat Enterprise Linux 6 Installation Guide. A minimal installation can also be performed via a kickstart file using the --nobase option. For more information, see the "Package Selection" section of the Red Hat Enterprise Linux 6 Installation Guide.

6.2. Plan and Configure Security Updates

All software contains bugs. Often, these bugs can result in a vulnerability that can expose your system to malicious users. Unpatched systems are a common cause of computer intrusions. You should have a plan to install security patches in a timely manner to close those vulnerabilities so they cannot be exploited.
For home users, security updates should be installed as soon as possible. Configuring automatic installation of security updates is one way to avoid having to remember, but does carry a slight risk that something can cause a conflict with your configuration or with other software on the system.
For business or advanced home users, security updates should be tested and scheduled for installation. Additional controls will need to be used to protect the system during the time between the patch release and its installation on the system. These controls would depend on the exact vulnerability, but could include additional firewall rules, the use of external firewalls, or changes in software settings.

6.3. Adjusting Automatic Updates

Red Hat Enterprise Linux is configured to apply all updates on a daily schedule. If you want to change how your system installs updates, you must do so via Software Update Preferences. You can change the schedule, the type of updates to apply, or to notify you of available updates.
In GNOME, you can find controls for your updates at: SystemPreferencesSoftware Updates. In KDE, it is located at: ApplicationsSettingsSoftware Updates.

6.4. Install Signed Packages from Well Known Repositories

Software packages are published through repositories. All well known repositories support package signing. Package signing uses public key technology to prove that the package that was published by the repository has not been changed since the signature was applied. This provides some protection against installing software that may have been maliciously altered after the package was created but before you downloaded it.
Using too many repositories, untrustworthy repositories, or repositories with unsigned packages has a higher risk of introducing malicious or vulnerable code into your system. Use caution when adding repositories to yum/software update.

Chapter 7. System Auditing

The Linux Audit system provides a way to track security-relevant information on your system. Based on pre-configured rules, Audit generates log entries to record as much information about the events that are happening on your system as possible. This information is crucial for mission-critical environments to determine the violator of the security policy and the actions they performed. Audit does not provide additional security to your system; rather, it can be used to discover violations of security policies used on your system. These violations can further be prevented by additional security measures such as SELinux.
The following list summarizes some of the information that Audit is capable of recording in its log files:
  • Date and time, type, and outcome of an event.
  • Sensitivity labels of subjects and objects.
  • Association of an event with the identity of the user who triggered the event.
  • All modifications to Audit configuration and attempts to access Audit log files.
  • All uses of authentication mechanisms, such as SSH, Kerberos, and others.
  • Changes to any trusted database, such as /etc/passwd.
  • Attempts to import or export information into or from the system.
  • Include or exclude events based on user identity, subject and object labels, and other attributes.
The use of the Audit system is also a requirement for a number of security-related certifications. Audit is designed to meet or exceed the requirements of the following certifications or compliance guides:
  • Controlled Access Protection Profile (CAPP)
  • Labeled Security Protection Profile (LSPP)
  • Rule Set Base Access Control (RSBAC)
  • National Industrial Security Program Operating Manual (NISPOM)
  • Federal Information Security Management Act (FISMA)
  • Payment Card Industry — Data Security Standard (PCI-DSS)
  • Security Technical Implementation Guides (STIG)
Audit has also been:
  • Evaluated by National Information Assurance Partnership (NIAP) and Best Security Industries (BSI).
  • Certified to LSPP/CAPP/RSBAC/EAL4+ on Red Hat Enterprise Linux 5.
  • Certified to Operating System Protection Profile / Evaluation Assurance Level 4+ (OSPP/EAL4+) on Red Hat Enterprise Linux 6.

Use Cases

Watching file access
Audit can track whether a file or a directory has been accessed, modified, executed, or the file's attributes have been changed. This is useful, for example, to detect access to important files and have an Audit trail available in case one of these files is corrupted.
Monitoring system calls
Audit can be configured to generate a log entry every time a particular system call is used. This can be used, for example, to track changes to the system time by monitoring the settimeofday, clock_adjtime, and other time-related system calls.
Recording commands run by a user
Because Audit can track whether a file has been executed, a number of rules can be defined to record every execution of a particular command. For example, a rule can be defined for every executable in the /bin directory. The resulting log entries can then be searched by user ID to generate an audit trail of executed commands per user.
Recording security events
The pam_faillock authentication module is capable of recording failed login attempts. Audit can be set up to record failed login attempts as well, and provides additional information about the user who attempted to log in.
Searching for events
Audit provides the ausearch utility, which can be used to filter the log entries and provide a complete audit trail based on a number of conditions.
Running summary reports
The aureport utility can be used to generate, among other things, daily reports of recorded events. A system administrator can then analyze these reports and investigate suspicious activity furthermore.
Monitoring network access
The iptables and ebtables utilities can be configured to trigger Audit events, allowing system administrators to monitor network access.


System performance may be affected depending on the amount of information that is collected by Audit.

7.1. Audit System Architecture

The Audit system consists of two main parts: the user-space applications and utilities, and the kernel-side system call processing. The kernel component receives system calls from user-space applications and filters them through one of the three filters: user, task, or exit. Once a system call passes through one of these filters, it is sent through the exclude filter, which, based on the Audit rule configuration, sends it to the Audit daemon for further processing. Figure 7.1, “Audit system architecture” illustrates this process.
Audit system architecture

Figure 7.1. Audit system architecture

The user-space Audit daemon collects the information from the kernel and creates log file entries in a log file. Other Audit user-space utilities interact with the Audit daemon, the kernel Audit component, or the Audit log files:
  • audisp — the Audit dispatcher daemon interacts with the Audit daemon and sends events to other applications for further processing. The purpose of this daemon is to provide a plug-in mechanism so that real-time analytical programs can interact with Audit events.
  • auditctl — the Audit control utility interacts with the kernel Audit component to control a number of settings and parameters of the event generation process.
  • The remaining Audit utilities take the contents of the Audit log files as input and generate output based on user's requirements. For example, the aureport utility generates a report of all recorded events.

7.2. Installing the audit Packages

In order to use the Audit system, you must have the audit packages installed on your system. The audit packages (audit and audit-libs) are installed by default on Red Hat Enterprise Linux 6. If you do not have these packages installed, execute the following command as the root user to install them:
~]# yum install audit

7.3. Configuring the audit Service

The Audit daemon can be configured in the /etc/audit/auditd.conf configuration file. This file consists of configuration parameters that modify the behavior of the Audit daemon. Any empty lines or any text following a hash sign (#) is ignored. See the auditd.conf(5) man page for a complete listing of all configuration parameters and their explanation.

7.3.1. Configuring auditd for a CAPP Environment

The default auditd configuration should be suitable for most environments. However, if your environment has to meet the criteria set by the Controlled Access Protection Profile (CAPP), which is a part of the Common Criteria certification, the Audit daemon must be configured with the following settings:
  • The directory that holds the Audit log files (usually /var/log/audit/) should reside on a separate partition. This prevents other processes from consuming space in this directory, and provides accurate detection of the remaining space for the Audit daemon.
  • The max_log_file parameter, which specifies the maximum size of a single Audit log file, must be set to make full use of the available space on the partition that holds the Audit log files.
  • The max_log_file_action parameter, which decides what action is taken once the limit set in max_log_file is reached, should be set to keep_logs to prevent Audit log files from being overwritten.
  • The space_left parameter, which specifies the amount of free space left on the disk for which an action that is set in the space_left_action parameter is triggered, must be set to a number that gives the administrator enough time to respond and free up disk space. The space_left value depends on the rate at which the Audit log files are generated.
  • It is recommended to set the space_left_action parameter to email or exec with an appropriate notification method.
  • The admin_space_left parameter, which specifies the absolute minimum amount of free space for which an action that is set in the admin_space_left_action parameter is triggered, must be set to a value that leaves enough space to log actions performed by the administrator.
  • The admin_space_left_action parameter must be set to single to put the system into single-user mode and allow the administrator to free up some disk space.
  • The disk_full_action parameter, which specifies an action that is triggered when no free space is available on the partition that holds the Audit log files, must be set to halt or single. This ensures that the system is either shut down or operating in single-user mode when Audit can no longer log events.
  • The disk_error_action, which specifies an action that is triggered in case an error is detected on the partition that holds the Audit log files, must be set to syslog, single, or halt, depending on your local security policies regarding the handling of hardware malfunctions.
  • The flush configuration parameter must be set to sync or data. These parameters assure that all Audit event data is fully synchronized with the log files on the disk.
The remaining configuration options should be set according to your local security policy.

7.4. Starting the audit Service

Once auditd is properly configured, start the service to collect Audit information and store it in the log files. Execute the following command as the root user to start auditd:
~]# service auditd start
Optionally, you can configure auditd to start at boot time using the following command as the root user:
~]# chkconfig auditd on
A number of other actions can be performed on auditd using the service auditd action command, where action can be one of the following:
  • stop — stops auditd.
  • restart — restarts auditd.
  • reload or force-reload — reloads the configuration of auditd from the /etc/audit/auditd.conf file.
  • rotate — rotates the log files in the /var/log/audit/ directory.
  • resume — resumes logging of Audit events after it has been previously suspended, for example, when there is not enough free space on the disk partition that holds the Audit log files.
  • condrestart or try-restart — restarts auditd only if it is already running.
  • status — displays the running status of auditd.

7.5. Defining Audit Rules

The Audit system operates on a set of rules that define what is to be captured in the log files. There are three types of Audit rules that can be specified:
  • Control rules — allow the Audit system's behavior and some of its configuration to be modified.
  • File system rules — also known as file watches, allow the auditing of access to a particular file or a directory.
  • System call rules — allow logging of system calls that any specified program makes.
Audit rules can be specified on the command line with the auditctl utility (note that these rules are not persistent across reboots), or written in the /etc/audit/audit.rules file. The following two sections summarize both approaches to defining Audit rules.

7.5.1. Defining Audit Rules with the auditctl Utility


All commands which interact with the Audit service and the Audit log files require root privileges. Ensure you execute these commands as the root user.
The auditctl command allows you to control the basic functionality of the Audit system and to define rules that decide which Audit events are logged.

Defining Control Rules

The following are some of the control rules that allow you to modify the behavior of the Audit system:
sets the maximum amount of existing Audit buffers in the kernel, for example:
~]# auditctl -b 8192
sets the action that is performed when a critical error is detected, for example:
~]# auditctl -f 2
The above configuration triggers a kernel panic in case of a critical error.
enables and disables the Audit system or locks its configuration, for example:
~]# auditctl -e 2
The above command locks the Audit configuration.
sets the rate of generated messages per second, for example:
~]# auditctl -r 0
The above configuration sets no rate limit on generated messages.
reports the status of the Audit system, for example:
~]# auditctl -s
AUDIT_STATUS: enabled=1 flag=2 pid=0 rate_limit=0 backlog_limit=8192 lost=259 backlog=0
lists all currently loaded Audit rules, for example:
~]# auditctl -l
LIST_RULES: exit,always watch=/etc/localtime perm=wa key=time-change
LIST_RULES: exit,always watch=/etc/group perm=wa key=identity
LIST_RULES: exit,always watch=/etc/passwd perm=wa key=identity
LIST_RULES: exit,always watch=/etc/gshadow perm=wa key=identity
deletes all currently loaded Audit rules, for example:
~]# auditctl -D
No rules

Defining File System Rules

To define a file system rule, use the following syntax:
auditctl -w path_to_file -p permissions -k key_name
  • path_to_file is the file or directory that is audited.
  • permissions are the permissions that are logged:
    • r — read access to a file or a directory.
    • w — write access to a file or a directory.
    • x — execute access to a file or a directory.
    • a — change in the file's or directory's attribute.
  • key_name is an optional string that helps you identify which rule or a set of rules generated a particular log entry.

Example 7.1. File System Rules

To define a rule that logs all write access to, and every attribute change of, the /etc/passwd file, execute the following command:
~]# auditctl -w /etc/passwd -p wa -k passwd_changes
Note that the string following the -k option is arbitrary.
To define a rule that logs all write access to, and every attribute change of, all the files in the /etc/selinux/ directory, execute the following command:
~]# auditctl -w /etc/selinux/ -p wa -k selinux_changes
To define a rule that logs the execution of the /sbin/insmod command, which inserts a module into the Linux kernel, execute the following command:
~]# auditctl -w /sbin/insmod -p x -k module_insertion

Defining System Call Rules

To define a system call rule, use the following syntax:
auditctl -a action,filter -S system_call -F field=value -k key_name
  • action and filter specify when a certain event is logged. action can be either always or never. filter specifies which kernel rule-matching filter is applied to the event. The rule-matching filter can be one of the following: task, exit, user, and exclude. For more information about these filters, see the beginning of Section 7.1, “Audit System Architecture”.
  • system_call specifies the system call by its name. A list of all system calls can be found in the /usr/include/asm/unistd_64.h file. Several system calls can be grouped into one rule, each specified after the -S option.
  • field=value specifies additional options that furthermore modify the rule to match events based on a specified architecture, group ID, process ID, and others. For a full listing of all available field types and their values, see the auditctl(8) man page.
  • key_name is an optional string that helps you identify which rule or a set of rules generated a particular log entry.

Example 7.2. System Call Rules

To define a rule that creates a log entry every time the adjtimex or settimeofday system calls are used by a program, and the system uses the 64-bit architecture, execute the following command:
~]# auditctl -a always,exit -F arch=b64 -S adjtimex -S settimeofday -k time_change
To define a rule that creates a log entry every time a file is deleted or renamed by a system user whose ID is 500 or larger (the -F auid!=4294967295 option is used to exclude users whose login UID is not set), execute the following command:
~]# auditctl -a always,exit -S unlink -S unlinkat -S rename -S renameat -F auid>=500 -F auid!=4294967295 -k delete
It is also possible to define a file system rule using the system call rule syntax. The following command creates a rule for system calls that is analogous to the -w /etc/shadow -p wa file system rule:
~]# auditctl -a always,exit -F path=/etc/shadow -F perm=wa

7.5.2. Defining Persistent Audit Rules and Controls in the /etc/audit/audit.rules File

To define Audit rules that are persistent across reboots, you must include them in the /etc/audit/audit.rules file. This file uses the same auditctl command line syntax to specify the rules. Any empty lines or any text following a hash sign (#) is ignored.
The auditctl command can also be used to read rules from a specified file with the -R option, for example:
~]# auditctl -R /usr/share/doc/audit-version/stig.rules

Defining Control Rules

A file can contain only the following control rules that modify the behavior of the Audit system: -b, -D, -e, -f, and -r. For more information on these options, see the section called “Defining Control Rules”.

Example 7.3. Control rules in audit.rules

# Delete all previous rules

# Set buffer size
-b 8192

# Make the configuration immutable -- reboot is required to change audit rules
-e 2

# Panic when a failure occurs
-f 2

# Generate at most 100 audit messages per second
-r 100

Defining File System and System Call Rules

File system and system call rules are defined using the auditctl syntax. The examples in Section 7.5.1, “Defining Audit Rules with the auditctl Utility” can be represented with the following rules file:

Example 7.4. File system and system call rules in audit.rules

-w /etc/passwd -p wa -k passwd_changes
-w /etc/selinux/ -p wa -k selinux_changes
-w /sbin/insmod -p x -k module_insertion

-a always,exit -F arch=b64 -S adjtimex -S settimeofday -k time_change
-a always,exit -S unlink -S unlinkat -S rename -S renameat -F auid>=500 -F auid!=4294967295 -k delete

Preconfigured Rules Files

In the /usr/share/doc/audit-version/ directory, the audit package provides a set of pre-configured rules files according to various certification standards:
  • nispom.rules — Audit rule configuration that meets the requirements specified in Chapter 8 of the National Industrial Security Program Operating Manual.
  • capp.rules — Audit rule configuration that meets the requirements set by Controlled Access Protection Profile (CAPP), which is a part of the Common Criteria certification.
  • lspp.rules — Audit rule configuration that meets the requirements set by Labeled Security Protection Profile (LSPP), which is a part of the Common Criteria certification.
  • stig.rules — Audit rule configuration that meets the requirements set by Security Technical Implementation Guides (STIG).
To use these configuration files, create a backup of your original /etc/audit/audit.rules file and copy the configuration file of your choice over the /etc/audit/audit.rules file:
~]# cp /etc/audit/audit.rules /etc/audit/audit.rules_backup
~]# cp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rules

7.6. Understanding Audit Log Files

By default, the Audit system stores log entries in the /var/log/audit/audit.log file; if log rotation is enabled, rotated audit.log files are stored in the same directory.
The following Audit rule logs every attempt to read or modify the /etc/ssh/sshd_config file:
-w /etc/ssh/sshd_config -p warx -k sshd_config
If the auditd daemon is running, running the following command creates a new event in the Audit log file:
~]# cat /etc/ssh/sshd_config
This event in the audit.log file looks as follows:
type=SYSCALL msg=audit(1364481363.243:24287): arch=c000003e syscall=2 success=no exit=-13 a0=7fffd19c5592 a1=0 a2=7fffd19c4b50 a3=a items=1 ppid=2686 pid=3538 auid=500 uid=500 gid=500 euid=500 suid=500 fsuid=500 egid=500 sgid=500 fsgid=500 tty=pts0 ses=1 comm="cat" exe="/bin/cat" subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key="sshd_config"
type=CWD msg=audit(1364481363.243:24287):  cwd="/home/shadowman"
type=PATH msg=audit(1364481363.243:24287): item=0 name="/etc/ssh/sshd_config" inode=409248 dev=fd:00 mode=0100600 ouid=0 ogid=0 rdev=00:00 obj=system_u:object_r:etc_t:s0
The above event consists of three records (each starting with the type= keyword), which share the same time stamp and serial number. Each record consists of several name=value pairs separated by a white space or a comma. A detailed analysis of the above event follows:

First Record

The type field contains the type of the record. In this example, the SYSCALL value specifies that this record was triggered by a system call to the kernel.
For a list of all possible type values and their explanations, see Section B.2, “Audit Record Types”.
The msg field records:
  • a time stamp and a unique ID of the record in the form audit(time_stamp:ID). Multiple records can share the same time stamp and ID if they were generated as part of the same Audit event.
  • various event-specific name=value pairs provided by the kernel or user space applications.
The arch field contains information about the CPU architecture of the system. The value, c000003e, is encoded in hexadecimal notation. When searching Audit records with the ausearch command, use the -i or --interpret option to automatically convert hexadecimal values into their human-readable equivalents. The c000003e value is interpreted as x86_64.
The syscall field records the type of the system call that was sent to the kernel. The value, 2, can be matched with its human-readable equivalent in the /usr/include/asm/unistd_64.h file. In this case, 2 is the open system call. Note that the ausyscall utility allows you to convert system call numbers to their human-readable equivalents. Use the ausyscall --dump command to display a listing of all system calls along with their numbers. For more information, see the ausyscall(8) man page.
The success field records whether the system call recorded in that particular event succeeded or failed. In this case, the call did not succeed.
The exit field contains a value that specifies the exit code returned by the system call. This value varies for different system call. You can interpret the value to its human-readable equivalent with the following command: ausearch --interpret --exit -13 (assuming your Audit log contains an event that failed with exit code -13).
a0=7fffd19c5592, a1=0, a2=7fffd19c5592, a3=a
The a0 to a3 fields record the first four arguments, encoded in hexadecimal notation, of the system call in this event. These arguments depend on the system call that is used; they can be interpreted by the ausearch utility.
The items field contains the number of path records in the event.
The ppid field records the Parent Process ID (PPID). In this case, 2686 was the PPID of the bash process.
The pid field records the Process ID (PID). In this case, 3538 was the PID of the cat process.
The auid field records the Audit user ID, that is the loginuid. This ID is assigned to a user upon login and is inherited by every process even when the user's identity changes (for example, by switching user accounts with the su - john command).
The uid field records the user ID of the user who started the analyzed process. The user ID can be interpreted into user names with the following command: ausearch -i --uid UID. In this case, 500 is the user ID of user shadowman.
The gid field records the group ID of the user who started the analyzed process.
The euid field records the effective user ID of the user who started the analyzed process.
The suid field records the set user ID of the user who started the analyzed process.
The fsuid field records the file system user ID of the user who started the analyzed process.
The egid field records the effective group ID of the user who started the analyzed process.
The sgid field records the set group ID of the user who started the analyzed process.
The fsgid field records the file system group ID of the user who started the analyzed process.
The tty field records the terminal from which the analyzed process was invoked.
The ses field records the session ID of the session from which the analyzed process was invoked.
The comm field records the command-line name of the command that was used to invoke the analyzed process. In this case, the cat command was used to trigger this Audit event.
The exe field records the path to the executable that was used to invoke the analyzed process.
The subj field records the SELinux context with which the analyzed process was labeled at the time of execution.
The key field records the administrator-defined string associated with the rule that generated this event in the Audit log.

Second Record

In the second record, the type field value is CWD — current working directory. This type is used to record the working directory from which the process that invoked the system call specified in the first record was executed.
The purpose of this record is to record the current process's location in case a relative path is captured in the associated PATH record. This way the absolute path can be reconstructed.
The msg field holds the same time stamp and ID value as the value in the first record.
The cwd field contains the path to the directory in which the system call was invoked.

Third Record

In the third record, the type field value is PATH. An Audit event contains a PATH-type record for every path that is passed to the system call as an argument. In this Audit event, only one path (/etc/ssh/sshd_config) was used as an argument.
The msg field holds the same time stamp and ID value as the value in the first and second record.
The item field indicates which item, of the total number of items referenced in the SYSCALL type record, the current record is. This number is zero-based; a value of 0 means it is the first item.
The name field records the path of the file or directory that was passed to the system call as an argument. In this case, it was the /etc/ssh/sshd_config file.
The inode field contains the inode number associated with the file or directory recorded in this event. The following command displays the file or directory that is associated with the 409248 inode number:
~]# find / -inum 409248 -print
The dev field specifies the minor and major ID of the device that contains the file or directory recorded in this event. In this case, the value represents the /dev/fd/0 device.
The mode field records the file or directory permissions, encoded in numerical notation. In this case, 0100600 can be interpreted as -rw-------, meaning that only the root user has read and write permissions to the /etc/ssh/sshd_config file.
The ouid field records the object owner's user ID.
The ogid field records the object owner's group ID.
The rdev field contains a recorded device identifier for special files only. In this case, it is not used as the recorded file is a regular file.
The obj field records the SELinux context with which the recorded file or directory was labeled at the time of execution.
The Audit event analyzed above contains only a subset of all possible fields that an event can contain. For a list of all event fields and their explanation, see Section B.1, “Audit Event Fields”. For a list of all event types and their explanation, see Section B.2, “Audit Record Types”.

Example 7.5. Additional audit.log events

The following Audit event records a successful start of the auditd daemon. The ver field shows the version of the Audit daemon that was started.
type=DAEMON_START msg=audit(1363713609.192:5426): auditd start, ver=2.2 format=raw kernel=2.6.32-358.2.1.el6.x86_64 auid=500 pid=4979 subj=unconfined_u:system_r:auditd_t:s0 res=success
The following Audit event records a failed attempt of user with UID of 500 to log in as the root user.
type=USER_AUTH msg=audit(1364475353.159:24270): user pid=3280 uid=500 auid=500 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 msg='op=PAM:authentication acct="root" exe="/bin/su" hostname=? addr=? terminal=pts/0 res=failed'

7.7. Searching the Audit Log Files

The ausearch utility allows you to search Audit log files for specific events. By default, ausearch searches the /var/log/audit/audit.log file. You can specify a different file using the ausearch options -if file_name command. Supplying multiple options in one ausearch command is equivalent to using the AND operator.

Example 7.6. Using ausearch to search Audit log files

To search the /var/log/audit/audit.log file for failed login attempts, use the following command:
~]# ausearch --message USER_LOGIN --success no --interpret
To search for all account, group, and role changes, use the following command:
To search for all logged actions performed by a certain user, using the user's login ID (auid), use the following command:
~]# ausearch -ua 500 -i
To search for all failed system calls from yesterday up until now, use the following command:
~]# ausearch --start yesterday --end now -m SYSCALL -sv no -i
For a full listing of all ausearch options, see the ausearch(8) man page.

7.8. Creating Audit Reports

The aureport utility allows you to generate summary and columnar reports on the events recorded in Audit log files. By default, all audit.log files in the /var/log/audit/ directory are queried to create the report. You can specify a different file to run the report against using the aureport options -if file_name command.

Example 7.7. Using aureport to generate Audit reports

To generate a report for logged events in the past three days excluding the current example day, use the following command:
~]# aureport --start 04/08/2013 00:00:00 --end 04/11/2013 00:00:00
To generate a report of all executable file events, use the following command:
~]# aureport -x
To generate a summary of the executable file event report above, use the following command:
~]# aureport -x --summary
To generate a summary report of failed events for all users, use the following command:
~]# aureport -u --failed --summary -i
To generate a summary report of all failed login attempts per each system user, use the following command:
~]# aureport --login --summary -i
To generate a report from an ausearch query that searches all file access events for user 500, use the following command:
~]# ausearch --start today --loginuid 500 --raw | aureport -f --summary
To generate a report of all Audit files that are queried and the time range of events they include, use the following command:
~]# aureport -t
For a full listing of all aureport options, see the aureport(8) man page.

7.9. Configuring PAM for Auditing

7.9.1. Configuring pam_tty_audit

The audit system in Red Hat Enterprise Linux uses the pam_tty_audit PAM module to enable or disable auditing of TTY input for specified users. When the audited user logs in, pam_tty_audit records the exact keystrokes the user makes into the /var/log/audit/audit.log file. The module works with the auditd daemon, so make sure it is enabled before configuring pam_tty_audit. See Section 7.4, “Starting the audit Service” for more information.
When you want to specify user names for TTY auditing, modify the /etc/pam.d/system-auth and /etc/pam.d/password-auth files using the disable and enable options in the following format:
 session required disable=username,username2 enable=username 
You can specify one or more user names separated by commas in the options. Any disable or enable option overrides the previous opposite option which matches the same user name. When TTY auditing is enabled, it is inherited by all processes started by that user. In particular, daemons restarted by a user will still have TTY auditing enabled, and will audit TTY input even by other users, unless auditing for these users is explicitly disabled. Therefore, it is recommended to use disable=* as the first option for most daemons using PAM.


By default, pam_tty_audit does NOT log keystrokes when the TTY is in password entry mode. Logging can be re-enabled by adding the log_passwd option along with the other options in the following way:
 session required disable=username,username2 enable=username log_passwd 
When you enable the module, the input is logged in the /var/log/audit/audit.log file, written by the auditd daemon. Note that the input is not logged immediately, because TTY auditing first stores the keystrokes in a buffer and writes the record periodically, or once the audited user logs out. The audit.log file contains all keystrokes entered by the specified user, including backspaces, delete and return keys, the control key and others. Although the contents of audit.log are human-readable it might be easier to use the aureport utility, which provides a TTY report in a format which is easy to read. You can use the following command as root:
~]# aureport --tty
The following is an example of how to configure pam_tty_audit to track the actions of the root user across all terminals and then review the input.

Example 7.8. Configuring pam_tty_audit to log root actions

Enter the following line in the session section of the /etc/pam.d/system-auth and /etc/pam.d/password-auth files:
session    required disable=* enable=root
Use the aureport --tty command to view the log. If the root user has logged in a TTY console at around 11:00 o'clock and tried to issue the pwd command, but then deleted it and issued ls instead, the report will look like this:
~]# aureport --tty -ts today | tail			
40. 08/28/2014 11:00:27 901 0 ? 76 bash "pwd",<backspace>,<backspace><backspace>,"ls",<ret>
41. 08/28/2014 11:00:29 903 0 ? 76 bash <^D>
For more information, see the pam_tty_audit(8) manual page.

7.10. Additional Resources

For more information about the Audit system, see the following sources.

Online Sources

Installed Documentation

Documentation provided by the audit package can be found in the /usr/share/doc/audit-version/ directory.

Manual Pages

  • audispd.conf(5)
  • auditd.conf(5)
  • ausearch-expression(5)
  • audit.rules(7)
  • audispd(8)
  • auditctl(8)
  • auditd(8)
  • aulast(8)
  • aulastlog(8)
  • aureport(8)
  • ausearch(8)
  • ausyscall(8)
  • autrace(8)
  • auvirt(8)

Chapter 8. Compliance and Vulnerability Scanning with OpenSCAP

8.1. Security Compliance in Red Hat Enterprise Linux

A compliance audit is a process of figuring out whether a given object follows all the rules written out in a compliance policy. The compliance policy is defined by security professionals who specify desired settings, often in the form of a checklist, that are to be used in the computing environment.
The compliance policy can vary substantially across organizations and even across different systems within the same organization. Differences among these policies are based on the purpose of these systems and its importance for the organization. The custom software settings and deployment characteristics also raise a need for custom policy checklists.
Red Hat Enterprise Linux provides tools that allow for fully automated compliance audit. These tools are based on the Security Content Automation Protocol (SCAP) standard and are designed for automated tailoring of compliance policies.

Security Compliance Tools Supported on Red Hat Enterprise Linux 6

  • OpenSCAP — The oscap command-line utility is designed to perform configuration and vulnerability scans on a local system, to validate security compliance content, and to generate reports and guides based on these scans and evaluations.
  • Script Check Engine (SCE) — SCE is an extension to SCAP protocol that allows content authors to write their security content using a scripting language, such as Bash, Python or Ruby. The SCE extension is provided with the openscap-engine-sce package.
  • SCAP Security Guide (SSG) — The scap-security-guide package provides the latest collection of security polices for Linux systems.
If you require performing automated compliance audits on multiple systems remotely, you can utilize OpenSCAP solution for Red Hat Satellite. For more information see Section 8.5, “Using OpenSCAP with Red Hat Satellite” and Section 8.8, “Additional Resources”.


Note that Red Hat does not provide any default compliance policy along with the Red Hat Enterprise Linux 6 distribution. The reasons for that are explained in Section 8.2, “Defining Compliance Policy”.

8.2. Defining Compliance Policy

The security or compliance policy is rarely written from scratch. ISO 27000 standard series, derivative works, and other sources provide security policy templates and practice recommendations that should be helpful to start with. However, organizations building theirs information security program need to amend the policy templates to align with their needs. The policy template should be chosen on the basis of its relevancy to the company environment and then the template has to be adjusted because either the template contains build-in assumptions which cannot be applied to the organization, or the template explicitly requires that certain decisions have to be made.
Red Hat Enterprise Linux auditing capabilities are based on the Security Content Automation Protocol (SCAP) standard. SCAP is a synthesis of interoperable specifications that standardize the format and nomenclature by which software flaw and security configuration information is communicated, both to machines and humans. SCAP is a multi-purpose framework of specifications that supports automated configuration, vulnerability and patch checking, technical control compliance activities, and security measurement.
In other words, SCAP is a vendor-neutral way of expressing security policy, and as such it is widely used in modern enterprises. SCAP specifications create an ecosystem where the format of security content is well known and standardized while the implementation of the scanner or policy editor is not mandated. Such a status enables organizations to build their security policy (SCAP content) once, no matter how many security vendors do they employ.
The latest version of SCAP includes several underlying standards. These components are organized into groups according to their function within SCAP as follows:

SCAP Components

  • Languages — This group consists of SCAP languages that define standard vocabularies and conventions for expressing compliance policy.
    • The eXtensible Configuration Checklist Description Format (XCCDF) — A language designed to express, organize, and manage security guidance.
    • Open Vulnerability and Assessment Language (OVAL) — A language developed to perform logical assertion about the state of the scanned system.
    • Open Checklist Interactive Language (OCIL) — A language designed to provide a standard way to query users and interpret user responses to the given questions.
    • Asset Identification (AI) — A language developed to provide a data model, methods, and guidance for identifying security assets.
    • Asset Reporting Format (ARF) — A language designed to express the transport format of information about collected security assets and the relationship between assets and security reports.
  • Enumerations — This group includes SCAP standards that define naming format and an official list or dictionary of items from certain security-related areas of interest.
    • Common Configuration Enumeration (CCE) — An enumeration of security-relevant configuration elements for applications and operating systems.
    • Common Platform Enumeration (CPE) — A structured naming scheme used to identify information technology (IT) systems, platforms, and software packages.
    • Common Vulnerabilities and Exposures (CVE) — A reference method to a collection of publicly known software vulnerabilities and exposures.
  • Metrics — This group comprises of frameworks to identify and evaluate security risks.
    • Common Configuration Scoring System (CCSS) — A metric system to evaluate security-relevant configuration elements and assign them scores in order to help users to prioritize appropriate response steps.
    • Common Vulnerability Scoring System (CVSS) — A metric system to evaluate software vulnerabilities and assign them scores in order to help users prioritize their security risks.
  • Integrity — An SCAP specification to maintain integrity of SCAP content and scan results.
    • Trust Model for Security Automation Data (TMSAD) — A set of recommendations explaining usage of existing specification to represent signatures, hashes, key information, and identity information in context of an XML file within a security automation domain.
Each of the SCAP components has its own XML-based document format and its XML name space. A compliance policy expressed in SCAP can either take a form of a single OVAL definition XML file, data stream file, single zip archive, or a set of separate XML files containing an XCCDF file that represents a policy checklist.

8.2.1. The XCCDF File Format

The XCCDF language is designed to support information interchange, document generation, organizational and situational tailoring, automated compliance testing, and compliance scoring. The language is mostly descriptive and does not contain any commands to perform security scans. However, an XCCDF document can refer to other SCAP components, and as such it can be used to craft a compliance policy that is portable among all the target platforms with the exception of the related assessment documents (OVAL, OCIL).
The common way to represent a compliance policy is a set of XML files where one of the files is an XCCDF checklist. This XCCDF file usually points to the assessment resources, multiple OVAL, OCIL and the Script Check Engine (SCE) files. Furthermore, the file set can contain a CPE dictionary file and an OVAL file defining objects for this dictionary.
Being an XML-based language, the XCCDF defines and uses a vast selection of XML elements and attributes. The following list briefly introduces the main XCCDF elements; for more details about XCCDF, consult the NIST Interagency Report 7275 Revision 4.

Main XML Elements of the XCCDF Document

  • <xccdf:Benchmark> — This is a root element that encloses the whole XCCDF document. It may also contain checklist metadata, such as a title, description, list of authors, date of the latest modification, and status of the checklist acceptance.
  • <xccdf:Rule> — This is a key element that represents a checklist requirement and holds its description. It may contain child elements that define actions verifying or enforcing compliance with the given rule or modify the rule itself.
  • <xccdf:Value> — This key element is used for expressing properties of other XCCDF elements within the benchmark.
  • <xccdf:Group> — This element is used to organize an XCCDF document to structures with the same context or requirement domains by gathering the <xccdf:Rule>, <xccdf:Value>, and <xccdf:Group> elements.
  • <xccdf:Profile> — This element serves for a named tailoring of the XCCDF benchmark. It allows the benchmark to hold several different tailorings. <xccdf:Profile> utilizes several selector elements, such as <xccdf:select> or <xccdf:refine-rule>, to determine which elements are going to be modified and processed while it is in effect.
  • <xccdf:Tailoring> — This element allows defining the benchmark profiles outside the benchmark, which is sometimes desirable for manual tailoring of the compliance policy.
  • <xccdf:TestResult> — This element serves for keeping the scan results for the given benchmark on the target system. Each <xccdf:TestResult> should refer to the profile that was used to define the compliance policy for the particular scan and it should also contain important information about the target system that is relevant for the scan.
  • <xccdf:rule-result> — This is a child element of <xccdf:TestResult> that is used to hold the result of applying a specific rule from the benchmark to the target system.
  • <xccdf:fix> — This is a child element of <xccdf:Rule> that serves for remediation of the target system that is not compliant with the given rule. It can contain a command or script that is run on the target system in order to bring the system into compliance the rule.
  • <xccdf:check> — This is a child element of <xccdf:Rule> that refers to an external source which defines how to evaluate the given rule.
  • <xccdf:select> — This is a selector element that is used for including or excluding the chosen rules or groups of rules from the policy.
  • <xccdf:set-value> — This is a selector element that is used for overwriting the current value of the specified <xccdf:Value> element without modifying any of its other properties.
  • <xccdf:refine-value> — This is a selector element that is used for specifying constraints of the particular <xccdf:Value> element during policy tailoring.
  • <xccdf:refine-rule> — This selector element allows overwriting properties of the selected rules.