Redhat ENTERPRISE LINUX 3 User Manual
Red Hat Enterprise Linux 3
Security Guide
Red Hat Enterprise Linux 3: Security Guide
Copyright © 2003 by Red Hat, Inc.
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Table of Contents
Introduction.......................................................................................................................................... i
1. Document Conventions.........................................................................................................ii
2. More to Come ...................................................................................................................... iv
2.1. Send in Your Feedback ......................................................................................... iv
I. A General Introduction to Security ................................................................................................ i
1. Security Overview................................................................................................................. 1
1.1. What is Computer Security? .................................................................................. 1
1.2. Security Controls ...................................................................................................5
1.3. Conclusion ............................................................................................................. 6
2. Attackers and Vulnerabilities................................................................................................ 7
2.1. A Quick History of Hackers ..................................................................................7
2.2. Threats to Network Security .................................................................................. 7
2.3. Threats to Server Security...................................................................................... 8
2.4. Threats to Workstation and Home PC Security ................................................... 10
II. Configuring Red Hat Enterprise Linux for Security................................................................ 11
3. Security Updates ................................................................................................................. 13
3.1. Updating Packages...............................................................................................13
4. Workstation Security........................................................................................................... 19
4.1. Evaluating Workstation Security .........................................................................19
4.2. BIOS and Boot Loader Security ..........................................................................19
4.3. Password Security................................................................................................22
4.4. Administrative Controls....................................................................................... 27
4.5. Available Network Services ................................................................................. 33
4.6. Personal Firewalls................................................................................................35
4.7. Security Enhanced Communication Tools........................................................... 36
5. Server Security....................................................................................................................37
5.1. Securing Services With TCP Wrappers and xinetd .......................................... 37
5.2. Securing Portmap.................................................................................................40
5.3. Securing NIS........................................................................................................ 40
5.4. Securing NFS .......................................................................................................42
5.5. Securing the Apache HTTP Server...................................................................... 43
5.6. Securing FTP ....................................................................................................... 44
5.7. Securing Sendmail ............................................................................................... 47
5.8. Verifying Which Ports Are Listening ..................................................................48
6. Virtual Private Networks .....................................................................................................51
6.1. VPNs and Red Hat Enterprise Linux ...................................................................51
6.2. Crypto IP Encapsulation (CIPE) .......................................................................... 51
6.3. Why Use CIPE? ...................................................................................................52
6.4. CIPE Installation..................................................................................................53
6.5. CIPE Server Configuration .................................................................................. 53
6.6. Configuring Clients for CIPE .............................................................................. 54
6.7. Customizing CIPE ...............................................................................................56
6.8. CIPE Key Management ....................................................................................... 57
6.9. IPsec..................................................................................................................... 57
6.10. IPsec Installation................................................................................................ 58
6.11. IPsec Host-to-Host Configuration...................................................................... 58
6.12. IPsec Network-to-Network configuration .......................................................... 60
7. Firewalls..............................................................................................................................65
7.1. Netfilter and IPTables ..........................................................................................66
7.2. Using IPTables..................................................................................................... 66
7.3. Common iptables Filtering.............................................................................. 68
7.4. FORWARD and NAT Rules..................................................................................... 69
7.5. DMZs and iptables .......................................................................................... 70
7.6. Viruses and Spoofed IP Addresses ......................................................................70
7.7. IP6Tables..............................................................................................................70
7.8. Additional Resources ........................................................................................... 71
III. Assessing Your Security............................................................................................................. 73
8. Vulnerability Assessment....................................................................................................75
8.1. Thinking Like the Enemy .................................................................................... 75
8.2. Defining Assessment and Testing........................................................................ 75
8.3. Evaluating the Tools.............................................................................................77
IV. Intrusions and Incident Response.............................................................................................81
9. Intrusion Detection..............................................................................................................83
9.1. Defining Intrusion Detection Systems ................................................................. 83
9.2. Host-based IDS ....................................................................................................83
9.3. Network-based IDS.............................................................................................. 86
10. Incident Response .............................................................................................................89
10.1. Defining Incident Response ............................................................................... 89
10.2. Creating an Incident Response Plan .................................................................. 89
10.3. Implementing the Incident Response Plan......................................................... 90
10.4. Investigating the Incident...................................................................................91
10.5. Restoring and Recovering Resources ................................................................93
10.6. Reporting the Incident........................................................................................ 94
V. Appendixes.................................................................................................................................... 95
A. Hardware and Network Protection..................................................................................... 97
A.1. Secure Network Topologies................................................................................97
A.2. Hardware Security............................................................................................. 100
B. Common Exploits and Attacks.........................................................................................101
C. Common Ports.................................................................................................................. 105
Index.................................................................................................................................................117
Colophon.......................................................................................................................................... 123
Introduction
Welcome to the Red Hat Enterprise Linux Security Guide!
The Red Hat Enterprise Linux Security Guide is designed to assist users of Red Hat Enterprise Linux
in learning the processes and practices of securing workstations and servers against local and remote
intrusion, exploitation, and malicious activity. The Red Hat Enterprise Linux Security 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
Red Hat Enterprise Linux can be both fully functional and secured from most common intrusion and
exploit methods.
This guide discusses several security-related topics in great detail, including:
• Firewalls
• Encryption
• Securing Critical Services
• Virtual Private Networks
• Intrusion Detection
The manual is divided into the following parts:
• General Introduction to Security
• Configuring Red Hat Enterprise Linux for Security
• Assessing Your Security
• Intrusions and Incident Response
• Appendix
We would like to thank Thomas Rude for his generous contributions to this manual. He wrote the
Vulnerability Assessments and Incident Response chapters. Thanks, "farmerdude."
This manual assumes that you have an advanced knowledge of Red Hat Enterprise Linux. If you are
a new user or only have basic to intermediate knowledge of Red Hat Enterprise Linux and need more
information on using the system, refer to the following guides which discuss the fundamental aspects
of Red Hat Enterprise Linux in greater detail than the Red Hat Enterprise Linux Security Guide:
• The Red Hat Enterprise Linux Installation Guide provides information regarding installation.
• The Red Hat Enterprise Linux Introduction to System Administration contains introductory infor-
mation for new Red Hat Enterprise Linux system administrators.
• The Red Hat Enterprise Linux System Administration Guide offers detailed information about con-
figuring Red Hat Enterprise Linux to suit your particular needs as a user. This guide includes some
services that are discussed (from a security standpoint) in the Red Hat Enterprise Linux Security
Guide.
• Red Hat Enterprise Linux Reference Guide provides detailed information suited for more experi-
enced users to refer to when needed, as opposed to step-by-step instructions.
HTML, PDF, and RPM versions of the manuals are available on the Red Hat Enterprise Linux Documentation CD and online at http://www.redhat.com/docs/.
ii Introduction
Note
Although this manual reflects the most current information possible, read the Red Hat Enterprise
Linux Release Notes for information that may not have been available prior to our documenta-
tion being finalized. They can be found on the Red Hat Enterprise Linux CD #1 and online at
http://www.redhat.com/docs/.
1. Document Conventions
When you read this manual, certain words are represented in different fonts, typefaces, sizes, and
weights. This highlighting is systematic; different words are represented in the same style to indicate
their inclusion in a specific category. The types of words that are represented this way include the
following:
command
Linux commands (and other operating system commands, when used) are represented this way.
This style should indicate to you that you can type the word or phrase on the command line
and press [Enter] to invoke a command. Sometimes a command contains words that would be
displayed in a different style on their own (such as file names). In these cases, they are considered
to be part of the command, so the entire phrase is displayed as a command. For example:
Use the cat testfile command to view the contents of a file, named testfile, in the current
working directory.
file name
File names, directory names, paths, and RPM package names are represented this way. This style
should indicate that a particular file or directory exists by that name on your system. Examples:
The .bashrc file in your home directory contains bash shell definitions and aliases for your own
use.
The /etc/fstab file contains information about different system devices and file systems.
Install the webalizer RPM if you want to use a Web server log file analysis program.
application
This style indicates that the program is an end-user application (as opposed to system software).
For example:
Use Mozilla to browse the Web.
[key]
A key on the keyboard is shown in this style. For example:
To use [Tab] completion, type in a character and then press the [Tab] key. Your terminal displays
the list of files in the directory that start with that letter.
[key]-[combination]
A combination of keystrokes is represented in this way. For example:
The [Ctrl]-[Alt]-[Backspace] key combination exits your graphical session and return you to the
graphical login screen or the console.
Introduction iii
text found on a GUI interface
A title, word, or phrase found on a GUI interface screen or window is shown in this style. Text
shown in this style is being used to identify a particular GUI screen or an element on a GUI
screen (such as text associated with a checkbox or field). Example:
Select the Require Password checkbox if you would like your screensaver to require a password
before stopping.
top level of a menu on a GUI screen or window
A word in this style indicates that the word is the top level of a pulldown menu. If you click on
the word on the GUI screen, the rest of the menu should appear. For example:
Under File on a GNOME terminal, the New Tab option allows you to open multiple shell
prompts in the same window.
If you need to type in a sequence of commands from a GUI menu, they are shown like the
following example:
Go to Main Menu Button (on the Panel) => Programming => Emacs to start the Emacs text
editor.
button on a GUI screen or window
This style indicates that the text can be found on a clickable button on a GUI screen. For example:
Click on the Back button to return to the webpage you last viewed.
computer output
Text in this style indicates text displayed to a shell prompt such as error messages and responses
to commands. For example:
The ls command displays the contents of a directory. For example:
Desktop about.html logs paulwesterberg.png
Mail backupfiles mail reports
The output returned in response to the command (in this case, the contents of the directory) is
shown in this style.
prompt
A prompt, which is a computer’s way of signifying that it is ready for you to input something, is
shown in this style. Examples:
$
#
[stephen@maturin stephen]$
leopard login:
user input
Text that the user has to type, either on the command line, or into a text box on a GUI screen, is
displayed in this style. In the following example, text is displayed in this style:
To boot your system into the text based installation program, you must type in the text command at the boot: prompt.
replaceable
Text used for examples which is meant to be replaced with data provided by the user is displayed
in this style. In the following example,
version-numberis displayed in this style:
iv Introduction
The directory for the kernel source is /usr/src/
version-numberis the version of the kernel installed on this system.
version-number/, where
Additionally, we use several different strategies to draw your attention to certain pieces of information. In order of how critical the information is to your system, these items are marked as note, tip,
important, caution, or a warning. For example:
Note
Remember that Linux is case sensitive. In other words, a rose is not a ROSE is not a rOsE.
Tip
The directory /usr/share/doc/ contains additional documentation for packages installed on your
system.
Important
If you modify the DHCP configuration file, the changes will not take effect until you restart the DHCP
daemon.
Caution
Do not perform routine tasks as root — use a regular user account unless you need to use the root
account for system administration tasks.
Warning
Be careful to remove only the necessary Red Hat Enterprise Linux partitions. Removing other partitions could result in data loss or a corrupted system environment.
2. More to Come
The Red Hat Enterprise Linux Security Guide is part of Red Hat’s growing commitment to provide
useful and timely support and information to Red Hat Enterprise Linux users. As new tools and security methodologies are released, this guide will be expanded to include them.
Introduction v
2.1. Send in Your Feedback
If you spot a typo in the Red Hat Enterprise Linux Security Guide, or if you have thought of a
way to make this manual better, we would love to hear from you! Submit a report in Bugzilla
(http://bugzilla.redhat.com/bugzilla/) against the component rhel-sg.
Be sure to mention the manual’s identifier:
rhel-sg(EN)-3-Print-RHI (2003-07-25T17:12)
By mentioning the identifier, we know exactly which version of the guide you have.
If you have a suggestion for improving the documentation, try to be as specific as possible. If you
have found an error, include the section number and some of the surrounding text so we can find it
easily.
vi Introduction
I. A General Introduction to Security
This part defines information security, its history, and the industry that has developed to address it. It
also discusses some of the risks that computer users or administrators face.
Table of Contents
1. Security Overview...........................................................................................................................1
2. Attackers and Vulnerabilities ........................................................................................................ 7
Chapter 1.
Security Overview
Because of the increased reliance on powerful, networked computers to help run businesses and keep
track of our personal information, 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 the organization. Because
most organizations are dynamic in nature, with workers accessing company IT resources locally and
remotely, the need for secure computing environments has become more pronounced.
Unfortunately, most organizations (as well as individual users) regard security as an afterthought, a
process that is overlooked in favor of increased power, productivity, and budgetary concerns. Proper
security implementation is often enacted postmortem — after an unauthorized intrusion has already
occurred. Security experts agree that the right measures taken prior to connecting a site to an untrusted
network such as the Internet is an effective means of thwarting most attempts at intrusion.
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 crucial 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)
and quality of service (QoS). In these metrics, industries calculate aspects such as data integrity and
high-availability as part of their planning and process management costs. In some industries, such
as electronic commerce, the availability and trustworthiness of data can be the difference between
success and failure.
1.1.1. How did Computer Security Come about?
Many readers may recall the movie "Wargames," starring Matthew Broderick in his portrayal of a
high school student who breaks into the United States Department of Defense (DoD) supercomputer
and inadvertently causes a nuclear war threat. In this movie, Broderick uses his modem to dial into the
DoD computer (called WOPR) and plays games with the artificially intelligent software controlling
all of the nuclear missile silos. The movie was released during the "cold war" between the former
Soviet Union and the United States and was considered a success in its theatrical release in 1983.
The popularity of the movie inspired many individuals and groups to begin implementing some of
the methods that the young protagonist used to crack restricted systems, including what is known as
war dialing — a method of searching phone numbers for analog modem connections in a defined area
code and phone prefix combination.
More than 10 years later, after a four-year, multi-jurisdictional pursuit involving the Federal Bureau
of Investigation (FBI) and the aid of computer professionals across the country, infamous computer
cracker Kevin Mitnick was arrested and charged with 25 counts of computer and access device fraud
that resulted in an estimated US$80 Million in losses of intellectual property and source code from
Nokia, NEC, Sun Microsystems, Novell, Fujitsu, and Motorola. At the time, the FBI considered it the
largest computer-related criminal offense in U.S. history. He was convicted and sentenced to a combined 68 months in prison for his crimes, of which he served 60 months before his parole on January
21, 2000. He was further barred from using computers or doing any computer-related consulting until
2003. Investigators say that Mitnick was an expert in social engineering — using human beings to
gain access to passwords and systems using falsified credentials.
Information security has evolved over the years due to the increasing reliance on public networks
to disclose personal, financial, and other restricted information. There are numerous instances such
as the Mitnick and the Vladimir Levin case (refer to Section 1.1.2 Computer Security Timeline for
2 Chapter 1. Security Overview
more information) that prompted organizations across all industries to rethink the way they handle
information 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
transaction, 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 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.
1.1.2. Computer Security Timeline
Several key events contributed to the birth and rise of computer security. The following lists some of
the most important events that brought attention to computer and information security and its importance today.
1.1.2.1. The 1960s
• Students at the Massachusetts Institute of Technology (MIT) form the Tech Model Railroad Club
(TMRC) begin exploring and programming the school’s PDP-1 mainframe computer system. The
group eventually use the term "hacker" in the context it is known today.
• The DoD creates the Advanced Research Projects Agency Network (ARPANet), which gains pop-
ularity in research and academic circles as a conduit for the electronic exchange of data and information. This paves the way for the creation of the carrier network known today as the Internet.
• Ken Thompson develops the UNIX operating system, widely hailed as the most "hacker-friendly"
OS because of its accessible developer tools and compilers, and its supportive user community.
Around the same time, Dennis Ritchie develops the C programming language, arguably the most
popular hacking language in computer history.
1.1.2.2. The 1970s
• Bolt, Beranek, and Newman, a computing research and development contractor for government
and industry, develops the Telnet protocol, a public extension of the ARPANet. This opens doors
to public use of data networks once restricted to government contractors and academic researchers.
Telnet, though, is also arguably the most insecure protocol for public networks, according to several
security researchers.
• Steve Jobs and Steve Wozniak found Apple Computer and begin marketing the Personal Computer
(PC). The PC is the springboard for several malicious users to learn the craft of cracking systems
remotely using common PC communication hardware such as analog modems and war dialers.
• Jim Ellis and Tom Truscott create USENET, a bulletin-board style system for electronic commu-
nication between disparate users. USENET quickly becomes one the most popular forums for the
exchange of ideas in computing, networking, and, of course, cracking.
Chapter 1. Security Overview 3
1.1.2.3. The 1980s
• IBM develops and markets PCs based on the Intel 8086 microprocessor, a relatively inexpensive
architecture that brought computing from the office to the home. This serves to commodify the PC
as a common and accessible tool that was fairly powerful and easy to use, aiding in the proliferation
of such hardware in the homes and offices of malicious users.
• The Transmission Control Protocol, developed by Vint Cerf, is split into two separate parts. The
Internet Protocol is born from this split, and the combined TCP/IP protocol becomes the standard
for all Internet communication today.
• Based on developments in the area of phreaking, or exploring and hacking the telephone system, the
magazine 2600: The Hacker Quarterly is created and begins discussion on topics such as cracking
computers and computer networks to a broad audience.
• The 414 gang (named after the area code where they lived and hacked from) are raided by author-
ities after a nine-day cracking spree where they break into systems from such top-secret locations
as the Los Alamos National Laboratory, a nuclear weapons research facility.
• The Legion of Doom and the Chaos Computer Club are two pioneering cracker groups that begin
exploiting vulnerabilities in computers and electronic data networks.
• The Computer Fraud and Abuse Act of 1986 was voted into law by congress based on the exploits of
Ian Murphy, also known as Captain Zap, who broke into military computers, stole information from
company merchandise order databases, and used restricted government telephone switchboards to
make phone calls.
• Based on the Computer Fraud and Abuse Act, the courts were able to convict Robert Morris, a
graduate student, for unleashing the Morris Worm to over 6,000 vulnerable computers connected
to the Internet. The next most prominent case ruled under this act was Herbert Zinn, a high-school
dropout who cracked and misused systems belonging to AT&T and the DoD.
• Based on concerns that the Morris Worm ordeal could be replicated, the Computer Emergency
Response Team (CERT) is created to alert computer users of network security issues.
• Clifford Stoll writes The Cuckoo’s Egg, Stoll’s account of investigating crackers who exploit his
system.
1.1.2.4. The 1990s
• ARPANet is decommissioned. Traffic from that network is transferred to the Internet.
• Linus Torvalds develops the Linux kernel for use with the GNU operating system; the widespread
development and adoption of Linux is largely due to the collaboration of users and developers communicating via the Internet. Because of its roots in UNIX, Linux is most popular among hackers and
administrators who found it quite useful for building secure alternatives to legacy servers running
proprietary (closed-source) operating systems.
• The graphical Web browser is created and sparks an exponentially higher demand for public Internet
access.
• Vladimir Levin and accomplices illegally transfer US$10 Million in funds to several accounts by
cracking into the CitiBank central database. Levin is arrested by Interpol and almost all of the
money is recovered.
• Possibly the most heralded of all crackers is Kevin Mitnick, who hacked into several corporate sys-
tems, stealing everything from personal information of celebrities to over 20,000 credit card numbers and source code for proprietary software. He is arrested and convicted of wire fraud charges
and serves 5 years in prison.
4 Chapter 1. Security Overview
• Kevin Poulsen and an unknown accomplice rig radio station phone systems to win cars and cash
prizes. He is convicted for computer and wire fraud and is sentenced to 5 years in prison.
• The stories of cracking and phreaking become legend, and several prospective crackers convene at
the annual DefCon convention to celebrate cracking and exchange ideas between peers.
• A 19-year-old Israeli student is arrested and convicted for coordinating numerous break-ins to US
government systems during the Persian-Gulf conflict. Military officials call it "the most organized
and systematic attack" on government systems in US history.
• US Attorney General Janet Reno, in response to escalated security breaches in government systems,
establishes the National Infrastructure Protection Center.
• British communications satellites are taken over and ransomed by unknown offenders. The British
government eventually seizes control of the satellites.
1.1.3. 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 yahoo.com, cnn.com, amazon.com,
fbi.gov, 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 assailants using specially created, widely available programs that scanned vulnerable
network servers, installed client applications called trojans on the servers, and 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, no matter where or for what purpose the packets are sent.
This brings us to the new millennium, a time where an estimated 400 Million people use or have used
the Internet worldwide. At the same time:
• On any given day, there are approximately 225 major incidences of security breach reported to the
CERT Coordination Center at Carnegie Mellon University. [source: http://www.cert.org]
• In 2002, the number of CERT reported incidences jumped to 82,094 from 52,658 in 2001. As of
this writing, the number of incidences reported in only the first quarter of 2003 is 42,586. [source:
http://www.cert.org]
• The worldwide economic impact of the three most dangerous Internet Viruses of the last two years
was estimated at US$13.2 Billion. [source: http://www.newsfactor.com/perl/story/16407.html]
Computer security has become a quantifiable and justifiable expense for all IT budgets. Organizations
that require data integrity and high availability elicit the skills of system administrators, developers,
and engineers to ensure 24x7 reliability of their systems, services, and information. To fall victim to
malicious users, processes, or coordinated attacks is a direct threat to the success of the organization.
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 that make up the organization) conducts business is paramount
to implementing a proper security plan.
1.1.4. 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.
Chapter 1. Security Overview 5
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. Unau-
thorized 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.
1.2. 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.
1.2.1. Physical Controls
The 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
1.2.2. Technical Controls
The Technical control uses 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)
6 Chapter 1. Security Overview
• File integrity auditing software
1.2.3. Administrative Controls
Administrative controls define the human factors of security. It involves all levels of personnel within
an organization and determines 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
1.3. Conclusion
Now that you have learned about the origins, reasons, and aspects of security, you can determine the
appropriate course of action with regards 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.
Chapter 2.
Attackers and Vulnerabilities
To plan and implement a good security strategy, first be aware of some of the issues which determined,
motivated attackers exploit to compromise systems. But before detailing these issues, the terminology
used when identifying an attacker must be defined.
2.1. A Quick History of Hackers
The modern meaning of the term hacker has origins dating back to the 1960s and the Massachusetts
Institute of Technology (MIT) Tech Model Railroad Club, which designed train sets of large scale
and intricate detail. Hacker was a name used for club members who discovered a clever trick or
workaround for a problem.
The term hacker has since come to describe everything from computer buffs to gifted programmers.
A common trait among most hackers is a willingness to explore in detail how computer systems and
networks function with little or no outside motivation. Open source software developers often consider
themselves and their colleagues to be hackers, and use the word as a term of respect.
Typically, hackers follow a form of the hacker ethic which dictates that the quest for information and
expertise is essential, and that sharing this knowledge is the hackers duty to the community. During
this quest for knowledge, some hackers enjoy the academic challenges of circumventing security
controls on computer systems. For this reason, the press often uses the term hacker to describe those
who illicitly access systems and networks with unscrupulous, malicious, or criminal intent. The more
accurate term for this type of computer hacker is cracker — a term created by hackers in the mid1980s to differentiate the two communities.
2.1.1. Shades of Grey
Within the community of individuals who find and exploit vulnerabilities in systems and networks are
several distinct groups. These groups are often described by the shade of hat that they "wear" when
performing their security investigations and this shade is indicative of their intent.
The white hat hacker is one who tests networks and systems to examine their performance and determine how vulnerable they are to intrusion. Usually, white hat hackers crack their own systems or
the systems of a client who has specifically employed them for the purposes of security auditing.
Academic researchers and professional security consultants are two examples of white hat hackers.
A black hat hacker is synonymous with a cracker. In general, crackers are less focused on programming and the academic side of breaking into systems. They often rely on available cracking programs
and exploit well known vulnerabilities in systems to uncover sensitive information for personal gain
or to inflict damage on the target system or network.
The grey hat hacker, on the other hand, has the skills and intent of a white hat hacker in most situations
but uses his knowledge for less than noble purposes on occasion. A grey hat hacker can be thought of
as a white hat hacker who wears a black hat at times to accomplish his own agenda.
Grey hat hackers typically subscribe to another form of the hacker ethic, which says it is acceptable to
break into systems as long as the hacker does not commit theft or breach confidentiality. Some would
argue, however that the act of breaking into a system is in itself unethical.
Regardless of the intent of the intruder, it is important to know the weaknesses a cracker may likely
attempt to exploit. The remainder of the chapter focuses on these issues.
8 Chapter 2. Attackers and Vulnerabilities
2.2. Threats to Network Security
Bad practices when configuring the following aspects of a network can increase the risk of attack.
2.2.1. 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 crimeridden neighborhood — nothing may happen for an arbitrary amount of time, but eventually someone
will exploit the opportunity.
2.2.1.1. 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 nodes.
2.2.1.2. 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, allowing access to the entire network.
2.3. 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 cracker to steal or manipulate at will. The following sections detail some of the main issues.
2.3.1. Unused Services and Open Ports
A full installation of Red Hat Enterprise Linux contains up to 1200 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 crackers. Refer To Chapter 5 Server Security for information on closing
ports and disabling unused services.
Chapter 2. Attackers and Vulnerabilities 9
2.3.2. Unpatched Services
Most server applications that are included in a default installation are solid, thoroughly tested pieces
of software. Having been in use in production environments for many years, their code has been
thoroughly refined and many of the bugs have been found and fixed.
However, there is no such thing as perfect software and there is always room for further refinement.
Moreover, newer software is often not as rigorously tested as one might expect, because of its recent
arrival to production environments or because it may not be as popular as other server software.
Developers and system administrators often find exploitable bugs in server applications and publish
the information on bug tracking and security-related websites such as the Bugtraq mailing list
(http://www.securityfocus.com) or the Computer Emergency Response Team (CERT) website
(http://www.cert.org). Although these mechanisms are an effective way of alerting the community to
security vulnerabilities, it is up to system administrators to patch their systems promptly. This is
particularly true because crackers have access to these same vulnerability tracking services and will
use the information to crack unpatched systems whenever they can. Good system administration
requires vigilance, constant bug tracking, and proper system maintenance to ensure a more secure
computing environment.
Refer to Chapter 3 Security Updates for more information about keeping a system up-to-date.
2.3.3. Inattentive Administration
Administrators who fail to patch their systems are one of the greatest threats to server security. According to the System Administration Network and 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."1This 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 to leave unchanged default
passwords or keys to services. 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 cracker 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.
2.3.4. 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 usernames 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 usernames 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, a cracker 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
1. Source: http://www.sans.org/newlook/resources/errors.html
10 Chapter 2. Attackers and Vulnerabilities
between the remote service and the unsuspecting user capturing information. In this way a cracker
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 a cracker 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 ACSII or DBM (ASCII-derived) database.
A cracker whogains access to this database can then access every user account on a network, including
the administrator’s account.
By default, Red Hat turns off such services. However, since administrators often find themselves
forced to use these services, careful configuration is critical. Refer to Chapter 5 Server Security for
more information about setting up services in a safe manner.
2.4. 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 crackers.
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.
2.4.1. 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, Refer to Section 4.3 Password Security.
2.4.2. 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 usernames 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.
Chapter 4 Workstation Security discusses in more detail what steps administrators and home users
should take to limit the vulnerability of computer workstations.
II. Configuring Red Hat Enterprise Linux for Security
This part informs and instructs administrators on proper techniques and tools to use when securing
Red Hat Enterprise Linux workstations, Red Hat Enterprise Linux servers, and network resources. It
also discusses how to make secure connections, lock down ports and services, and implement active
filtering to prevent network intrusion.
Table of Contents
3. Security Updates ........................................................................................................................... 13
4. Workstation Security ....................................................................................................................19
5. Server Security ..............................................................................................................................37
6. Virtual Private Networks .............................................................................................................51
7. Firewalls......................................................................................................................................... 65
Chapter 3.
Security Updates
As security vulnerabilities are discovered, the effected software must be updated in order to limit
any potential security risks. If the software is part of a package within an Red Hat Enterprise Linux
distribution that is currently supported, Red Hat, Inc. is committed to releasing updated packages that
fix the vulnerability as soon as 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, tested by the Red Hat quality assurance team, and released as an
errata update. However, if an announcement does not include a patch, a Red Hat developer 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 effected packages as soon as possible to minimize the amount of time system is potentially
vulnerable.
3.1. 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, Inc.
and check the signature of the package to verify its integrity.
Red Hat offers two ways to retrieve security errata updates:
1. Download them from Red Hat Network.
2. Download them from the Red Hat Errata website.
3.1.1. Using Red Hat Network
Red Hat Network allows the majority of the update process to be automated. It determines which
RPM packages are necessary for the system, downloads them from a secure repository, verifies the
RPM signature to make sure they have not been tampered with, and updates them. The package install
can occur immediately or can be scheduled during a certain time period.
Red Hat Network requires a System Profile for each machine to be updated. The System Profile
contains hardware and software information about the system. This information is kept confidential
and not given to anyone else. It is only used to determine which errata updates are applicable to each
system, and, without it, Red Hat Network can not determine whether a given system needs updates.
When a security errata (or any type of errata) is released, Red Hat Network sends an email with a
description of the errata as well as a list of systems which are affected. To apply the update, use the Red
Hat Update Agent or schedule the package to be updated through the website http://rhn.redhat.com.
Tip
Red Hat Enterprise Linux includes the Red Hat Network Alert Notification Tool, a
convenient panel icon that displays visible alerts when there is an update for a Red Hat
Enterprise Linux system. Refer to the following URL for more information about the applet:
http://rhn.redhat.com/help/basic/applet.html
14 Chapter 3. Security Updates
To learn more about the benefits of Red Hat Network, refer to the Red Hat Network Reference Guide
available at http://www.redhat.com/docs/manuals/RHNetwork/ or visit http://rhn.redhat.com.
Important
Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 3.1.3 Applying the Changes for general
instructions about applying the changes made by an errata update.
3.1.2. Using the Red Hat Errata Website
When security errata reports are released, they are published on the Red Hat Errata website available at
http://www.redhat.com/apps/support/errata/. From this page, select the product and version for your
system, and then select security at the top of the page to display only Red Hat Enterprise Linux
Security Advisories. If the synopsis of one of the advisories describes a package used on your system,
click on the synopsis for more details.
The details page describes the security exploit and any special instructions that must be performed in
addition to updating the package to fix the security hole.
To download the updated package(s), click on the package name(s) and save to the hard drive. It
is highly recommended that you create a new directory, such as /tmp/updates, and save all the
downloaded packages to it.
All Red Hat Enterprise Linux packages are signed with the Red Hat, Inc. GPG key. The RPM utility
within Red Hat Enterprise Linux automatically tries to verify the GPG signature of an RPM package
before installing it. If the Red Hat, Inc. GPG key is not installed, install it from a secure, static location
such as an Red Hat Enterprise Linux installation CD-ROM.
Assuming the CD-ROM is mounted in /mnt/cdrom, use the following command to import it into the
keyring:
rpm --import /mnt/cdrom/RPM-GPG-KEY
To display a list of all keys installed for RPM verification, execute the following command:
rpm -qa gpg-pubkey*
For the Red Hat, Inc. key, the output includes the following:
gpg-pubkey-db42a60e-37ea5438
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
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 Red Hat, Inc. release of the packages. To verify all the downloaded
packages at once, issue the following command:
rpm -K /tmp/updates/*.rpm
For each package, if the GPG key verifies successfully, the command returns gpg OK.
Chapter 3. Security Updates 15
After verifying the GPG key and downloading all the packages associated with the errata report, install
the packages as root at a shell prompt.
This can be done safely for most packages (except kernel packages) by issuing the following command:
rpm -Uvh /tmp/updates/*.rpm
For kernel packages it is advised that the following command be used:
rpm -ivh /tmp/updates/
kernel-package
Replacekernel-packagein the previous example with the name of the kernel RPM.
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
Replaceold-kernel-packagein the previous example with the name of the older kernel
RPM.
Note
It is not a requirement that the old kernel be removed.
Important
Before installing any security errata, be sure to read any special instructions contained in the errata report and execute them accordingly. Refer to Section 3.1.3 Applying the Changes for general
instructions about applying the changes made by an errata update.
3.1.3. Applying the Changes
After downloading and installing security errata via Red Hat Network or the Red Hat errata website, 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.
Note
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 available to the system administrator.
16 Chapter 3. Security Updates
Applications
User-space applications are any programs which can be initiated by a system user. Typically,
such applications are used only when a user, script, or automated task utility launches them and
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.
Kernel
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.
Todetermine which running applications link against a particular library, use the lsof command,
as in the following example:
lsof /usr/lib/libwrap.so*
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 as in the following example:
/sbin/service
service-namerestart
In the previous example, replaceservice-namewith the name of the service, such as
sshd.
Refer to the chapter titled Controlling Access to Services in the Red Hat Enterprise Linux System
Administration Guide for more information regarding the Services Configuration Tool.
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 by first determining if the process is running
using the ps command and then using 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:
Chapter 3. Security Updates 17
ps -aux | grep imap
This command returns all active IMAP sessions. Individual sessions can then be terminated by
issuing the following command:
kill -9
PID
In the previous example, replacePIDwith the process identification number for an IMAP
session.
To kill all active IMAP sessions, issue the following command:
killall imapd
Refer to the chapter titled TCP Wrappers and xinetd in the Red Hat Enterprise Linux Reference
Guide for general information regarding xinetd.
18 Chapter 3. Security Updates
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