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You see this a lot—typically, in medium to large enterprise networks, the various strategies for
security are based on some recipe of internal and perimeter routers plus firewall devices. Internal
routers provide additional security to the network by screening traffic to various parts of the protected
corporate network, and they do this using access lists (access lists are beyond the scope of
this book, but they’re covered in
CCNA: Cisco Certified Network Associate Study Guide, Sixth
Edition
[Sybex, 2007]). You can see where each of these types of devices are found in Figure 9.1.
To protect network device configure files from outside network security threats, use a firewall,
as shown in Figure 9.1, to restrict access from the outside to the network devices and use
SSH instead of Telnet to access device configurations.
Figure 9.1uses the terms
trusted network
and
untrusted network
and you can see where
they are found in a typical secured network as well as the demilitarized zone (DMZ), which
can be global (real) Internet addresses or private addresses, depending on how you configure
your firewall, but this is typically where you’ll find the HTTP, DNS, email, and other Internettype
corporate servers.
Instead of having routers, we can use virtual local area networks (VLANs) with switches on the
inside trusted network. Multilayer switches containing their own security features can sometimes
replace internal (LAN) routers to provide higher performance in VLAN architectures.
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487
FIGURE 9 . 1
A typical secured network
Let’s discuss the security threats a typical secured internetwork faces; then I’ll provide some
ways of protecting the Cisco router and switch.
Recognizing Security Threats
Yes, it’s true: Security attacks vary considerably in their complexity and threat level, and some
even happen because of WUI, or witless user ignorance. (Knowing this term isn’t an exam
objective, but WUI causes security attacks more than you’d think!)
You see, it all comes down to planning, or rather, lack thereof. Basically, the vital tool that
the Internet has become today was absolutely unforeseen by those who brought it into being.
This is a big reason why security is now such an issue—most IP implementations are innately
insecure. No worries though, because Cisco has a few tricks up its sleeve to help us with this.
The following are common trouble spots in the type of enterprise network shown in Figure 9.1:
Wireless LANs (WLANs)
Any user that can connect to the WLAN can access the devices in the
trusted network. People from outside the building may even be able to gain access if the WLAN
access points are left unsecured. Use the highest security you can afford on your WLANs.
Mobile devices
When a mobile user connects their laptop at home or at another remote location,
the laptop may become infected with a virus or a Trojan horse. When the user returns to
the office and connects to the trusted network, the infection may spread. It is critical that all
PCs and laptops are running the latest virus scanning software.
Employees
I am sure this is no shock to you, but some employees may be disgruntled workers.
This may be the worst security problem a company can have. The employees are inside the
trusted network and have access to many, if not all, devices and servers. It is important to grant
each employee the minimum access they need.
Internet
Untrusted
Network
Perimeter
(premises)
Router
Firewall Internal
(local network)
Router
Corporate
(trusted)
Network
DMZ
Web
Server
Mail
Server
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Remember, attacks from inside the network remain as high a threat as those
from outside. Prepare your network security plan accordingly.
Common Attack Profiles
This section covers the most common attacks used on today’s networks.
Application-layer attacks
These attacks commonly zero in on well-known holes in the software
that’s typically found running on servers. Favorite targets include FTP, sendmail, and HTTP.
Because the permission levels granted to these accounts are most often “privileged,” bad guys simply
access and exploit the machine that’s running one of the applications I just mentioned.
Autorooters
You can think of these as a kind of hacker automaton. Bad guys use something
called a rootkit to probe, scan, and then capture data on a strategically positioned computer
that’s poised to give them “eyes” into entire systems—automatically!
Back doors
These are simply paths leading into a computer or network. Through simple invasions,
or via more elaborate Trojan horse code, bad guys can use their implanted inroads into a specific
host or even a network whenever they want to—until you detect and stop them, that is!
Denial of service (DoS) and distributed denial of service (DDoS) attacks
These are bad—
pretty tough to get rid of too! But even hackers don’t respect other hackers that execute them
because, though nasty, they’re really easy to accomplish. (This means that some 10-year-old
could actually bring you to your knees, and that is just wrong!) Basically, a DoS attack is a flood
of packets that are requesting a TCP connection to server. And there are several different flavors:
TCP SYN flood
TCP SYN flood attacks begin with a client-initiated, seemingly run-ofthe-
mill, TCP connection that sends a SYN message to a server. The server predictably
responds by sending a SYN-ACK message back to the client machine, which then establishes
the connection by returning an ACK message. Sounds fine, but it’s actually during
this process—when the connection is only halfway open—that the victim machine is literally
flooded with a deluge of half-open connections and pretty much becomes paralyzed.
"Ping of death" attacks
You probably know that TCP/IP’s maximum packet size is 65,536
octets. It’s okay if you didn’t know that—just understand that this attack is executed by simply
pinging with oversized packets, causing a device to keep rebooting incessantly, freeze up, or just
totally crash.
Tribe Flood Network (TFN) and Tribe Flood Network 2000 (TFN2K)
These nasty little
numbers are more complex in that they initiate synchronized DoS attacks from multiple sources
and can target multiple devices. This is achieved, in part, by something known as “IP spoofing,”
which I’ll be describing soon.
Stacheldraht
This attack is actually a mélange of methods, and it translates from the German
term for barbed wire. It basically incorporates TFN and adds a dash of encryption. It all begins
with a huge invasion at the root level, followed up with a DoS attack finale.
IP spoofing
This is pretty much what it sounds like it is—a bad guy from within or outside
of your network masquerades as a trusted host machine by doing one of two things: presenting
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with an IP address that’s inside your network’s scope of trusted addresses or using an approved,
trusted external IP address. Because the hacker’s true identity is veiled behind the spoofed
address, this is often just the beginning of your problems.
Keystroke loggers
This is a software program that is actually kinda cool, depending on
which side of the program you are on. The keystroke logger keeps track of all keystrokes a person
types on a keyboard. Some loggers actually can email the information to the attacker.
Man-in-the-middle attacks
Interception! But it’s not a football, it’s a bunch of your network’s
packets—your precious data! A common guilty party could be someone working for
your very own ISP using a tool known as a sniffer (discussed later) and augmenting it with
routing and transport protocols.
Network reconnaissance
Before breaking into a network, hackers often gather all the information
they can about it, because the more they know about the network, the better they can
compromise it. They accomplish their objectives through methods like port scans, DNS queries,
and ping sweeps.
Packet sniffers
This is the tool I mentioned earlier, but I didn’t tell you what it is, and it may
come as a surprise that it’s actually software. Here’s how it works—a network adapter card
is set to promiscuous mode so it will send all packets snagged from the network’s physical
layer through to a special application to be viewed and sorted out. A packet sniffer can nick
some highly valuable, sensitive data including, but not limited to, passwords and usernames,
making them prized among identity thieves.
Password attacks
These come in many flavors, and even though they can be achieved via
more sophisticated types of attacks like IP spoofing, packet sniffing, and Trojan horses, their
sole purpose is to—surprise—discover user passwords so the thief can pretend they’re a valid
user and then access that user’s privileges and resources.
Brute force attack
Another software-oriented attack that employs a program running on a
targeted network that tries to log in to some type of shared network resource like a server. For
the hacker, it’s ideal if the accessed accounts have a lot of privileges because then the bad guys
can form back doors to use for gaining access later and bypass the need for passwords entirely.
Port redirection attacks
This approach requires a host machine the hacker has broken into
and uses to get wonky traffic (that normally wouldn’t be allowed passage) through a firewall.
Trojan horse attacks and viruses
These two are actually pretty similar—both Trojan horses
and viruses infect user machines with malicious code and mess it up with varying degrees of paralysis,
destruction, even death! But they do have their differences—viruses are really just nasty programs
attached to command.com, which just happens to be the main interpreter for all Windows
systems. Viruses then run amok, deleting files and infecting any flavor of command.com it finds on
the now diseased machine. The difference between a virus and a Trojan horse is that Trojans are
actually complete applications encased inside code that makes them appear to be a completely different
entity—say, a simple, innocent game—not the ugly implements of destruction they truly are!
Trust exploitation attacks
These attacks happen when someone exploits a trust relationship
inside your network. For example, a company’s perimeter network connection usually shelters
important things like SMTP, DNS, and HTTP servers, making the servers really vulnerable
because they’re all on the same segment.
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To be honest, I’m not going to go into detail on how to mitigate each and every one of the
security threats I just talked about, not only because that would be outside the scope of this
book, but also because the methods I am going to teach you will truly protect you from being
attacked in general. You will learn enough tricks to make all but the most determined bad guys
give up on you and search for easier prey. So basically, think of this as a chapter on how to
practice “safe networking.”
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