NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
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
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.
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.
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 to be 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. Mitnick 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 cases (refer
to Section 1.1.2
Computer Security Timeline for 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.
Several key events contributed to the birth and rise of computer
security. The following timeline lists some of the more important
events that brought attention to computer and information security
and its importance today.
Polish cryptographers invent the Enigma machine in 1918, an
electro-mechanical rotor cypher device which converts plain-text
messages to an encrypted result. Originally developed to secure
banking communications, the German military finds the potential of
the device by securing communications during World War II. A
brilliant mathematician named Alan Turing develops a method for
breaking the codes of Enigma, enabling Allied forces to develop
Colossus, a machine often credited to ending the war a year
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
coined the term "hacker" in the context it is known today.
The DoD creates the Advanced Research Projects Agency Network
(ARPANet), which gains popularity 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
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 for
the public use of data networks which were 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 communication between disparate users. USENET
quickly becomes one of the most popular forums for the exchange of
ideas in computing, networking, and, of course, cracking.
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 authorities 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
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 is 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 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
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
Possibly the most heralded of all crackers is Kevin Mitnick, who
hacked into several corporate systems, 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
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
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.
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
945 Million people use or have used the Internet worldwide
(Computer Industry Almanac, 2004). 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.
In 2003, the number of CERT reported incidences jumped to
137,529 from 82,094 in 2002 and from 52,658 in 2001.
The worldwide economic impact of the three most dangerous
Internet Viruses of the last three years was estimated at US$13.2
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. Falling
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.
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
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
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