Introduction to Network Security
Reprinted with the permission of Kent Information Services, Inc.
(Also available in Postscript
and PDF formats for
those who prefer, and nicer hardcopy.)
Network security is a complicated subject, historically only tackled
by well-trained and experienced experts. However, as more and more
people become ``wired'', an increasing number of people need to
understand the basics of security in a networked world. This
document was written with the basic computer user and information
systems manager in mind, explaining the concepts needed to read
through the hype in the marketplace and understand risks and how to
deal with them.
Some history of networking is included, as well as an introduction
to TCP/IP and internetworking . We go on to consider risk
management, network threats, firewalls, and more special-purpose
secure networking devices.
This is not intended to be a ``frequently asked questions''
reference, nor is it a ``hands-on'' document describing how to
accomplish specific functionality.
It is hoped that the reader will have a wider perspective on
security in general, and better understand how to reduce and manage
risk personally, at home, and in the workplace.
A basic understanding of computer networks is requisite in order to
understand the principles of network security. In this section, we'll
cover some of the foundations of computer networking, then move on to
an overview of some popular networks. Following that, we'll take a
more in-depth look at TCP/IP, the network protocol suite that is used
to run the Internet and many intranets.
Once we've covered this, we'll go back and discuss some of the threats
that managers and administrators of computer networks need to
confront, and then some tools that can be used to reduce the exposure
to the risks of network computing.
A ``network'' has been defined as ``any set of
interlinking lines resembling a net, a network of roads ||
an interconnected system, a network of alliances.'' This
definition suits our purpose well: a computer network is simply a
system of interconnected computers. How they're connected is
irrelevant, and as we'll soon see, there are a number of ways to do
The International Standards Organization (ISO) Open
Systems Interconnect (OSI) Reference Model defines seven layers of
communications types, and the interfaces among them. (See
Figure 1.) Each layer depends on the services provided by
the layer below it, all the way down to the physical network
hardware, such as the computer's network interface card, and the wires
that connect the cards together.
An easy way to look at this is to compare this model with something we
use daily: the telephone. In order for you and I to talk when we're
out of earshot, we need a device like a telephone. (In the ISO/OSI
model, this is at the application layer.) The telephones, of course,
are useless unless they have the ability to translate the sound into
electronic pulses that can be transferred over wire and back again.
(These functions are provided in layers below the application layer.)
Finally, we get down to the physical connection: both must be plugged
into an outlet that is connected to a switch that's part of the
telephone system's network of switches.
If I place a call to you, I pick up the receiver, and dial your
number. This number specifies which central office to which to send
my request, and then which phone from that central office to ring.
Once you answer the phone, we begin talking, and our session has
begun. Conceptually, computer networks function exactly the same
It isn't important for you to memorize the ISO/OSI Reference Model's
layers; but it's useful to know that they exist, and that each layer
cannot work without the services provided by the layer below it.
The ISO/OSI Reference Model
Over the last 25 years or so, a number of networks and network
protocols have been defined and used. We're going to look at two of
these networks, both of which are ``public'' networks. Anyone can
connect to either of these networks, or they can use types of networks
to connect their own hosts (computers) together, without
connecting to the public networks. Each type takes a very different
approach to providing network services.
UUCP (Unix-to-Unix CoPy) was originally developed to connect
Unix (surprise!) hosts together. UUCP has since been ported to
many different architectures, including PCs, Macs, Amigas, Apple IIs,
VMS hosts, everything else you can name, and even some things you
can't. Additionally, a number of systems have been developed around
the same principles as UUCP.
UUCP and similar systems are
batch-oriented systems: everything that they have to do is
added to a queue, and then at some specified time, everything in the
queue is processed.
UUCP networks are commonly
built using dial-up (modem) connections. This doesn't have to
be the case though: UUCP can be used over any sort of connection
between two computers, including an Internet connection.
Building a UUCP network is a simple matter of configuring two hosts
to recognize each other, and know how to get in touch with each other.
Adding on to the network is simple; if hosts called A and
B have a UUCP network between them, and C would like to
join the network, then it must be configured to talk to A
and/or B. Naturally, anything that C talks to must be
made aware of C's existence before any connections will work.
Now, to connect D to the network, a connection must be
established with at least one of the hosts on the network, and so
on. Figure 2 shows a sample UUCP network.
A Sample UUCP Network
In a UUCP network, users are identified in the format
host!userid. The ``!'' character (pronounced ``bang'' in
networking circles) is used to separate hosts and users. A
bangpath is a string of host(s) and a userid like
A!cmcurtin or C!B!A!cmcurtin.
If I am a user on host A and you are a user on host
E, I might be known as A!cmcurtin and you as
E!you. Because there is no direct link between your host
(E) and mine (A), in order for us to communicate, we
need to do so through a host (or hosts!) that has connectivity
to both E and A. In our sample network, C has
the connectivity we need. So, to send me a file, or piece of email,
you would address it to C!A!cmcurtin. Or, if you feel like
taking the long way around, you can address me as
The ``public'' UUCP network is simply a huge worldwide network of
hosts connected to each other.
The public UUCP network has been shrinking
in size over the years, with the rise of the availability of
inexpensive Internet connections. Additionally, since UUCP
connections are typically made hourly, daily, or weekly, there is a
fair bit of delay in getting data from one user on a UUCP network to a
user on the other end of the network. UUCP isn't very flexible, as
it's used for simply copying files (which can be netnews, email,
documents, etc.) Interactive protocols (that make applications such
as the World Wide Web possible) have become much more the norm, and
are preferred in most cases.
However, there are still many people whose needs for email and netnews
are served quite well by UUCP, and its integration into the Internet
has greatly reduced the amount of cumbersome addressing that had to be
accomplished in times past.
UUCP, like any other application, has security
tradeoffs. Some strong points for its security is that it is fairly
limited in what it can do, and it's therefore more difficult to trick
into doing something it shouldn't; it's been around a long
time, and most its bugs have been discovered, analyzed, and fixed; and
because UUCP networks are made up of occasional connections to other
hosts, it isn't possible for someone on host E to directly make
contact with host B, and take advantage of that connection to do
On the other hand, UUCP typically works by having a system-wide UUCP
user account and password. Any system that has a UUCP connection with
another must know the appropriate password for the uucp or
nuucp account. Identifying a host beyond that point has
traditionally been little more than a matter of trusting that the
host is who it claims to be, and that a connection is allowed at
that time. More recently, there has been an additional layer of
authentication, whereby both hosts must have the same sequence
number , that is a number that is incremented each time a connection
Hence, if I run host B, I know the uucp password on
host A. If, though, I want to impersonate host C, I'll
need to connect, identify myself as C, hope that I've done so
at a time that A will allow it, and try to guess the correct
sequence number for the session. While this might not be a trivial
attack, it isn't considered very secure.
Internet: This is a word that I've heard way too often in the
last few years. Movies, books, newspapers, magazines, television
programs, and practically every other sort of media imaginable has
dealt with the Internet recently.
The Internet is the world's largest network of networks . When
you want to access the resources offered by the Internet, you don't
really connect to the Internet; you connect to a network that
is eventually connected to the Internet backbone , a network of
extremely fast (and incredibly overloaded!) network components. This
is an important point: the Internet is a network of
networks -- not a network of hosts.
A simple network can be constructed using the same protocols and such
that the Internet uses without actually connecting it to
anything else. Such a basic network is shown in Figure 3.
A Simple Local Area Network
I might be allowed to put one of my hosts on one of my employer's
networks. We have a number of networks, which are all connected
together on a backbone , that is a network of our networks. Our
backbone is then connected to other networks, one of which is to an
Internet Service Provider (ISP) whose backbone is connected to
other networks, one of which is the Internet backbone.
If you have a connection ``to the Internet'' through a local ISP, you
are actually connecting your computer to one of their networks, which
is connected to another, and so on. To use a service from my host,
such as a web server, you would tell your web browser to connect to my
host. Underlying services and protocols would send packets
(small datagrams) with your query to your ISP's network, and then a
network they're connected to, and so on, until it found a path to my
employer's backbone, and to the exact network my host is on. My host
would then respond appropriately, and the same would happen in
reverse: packets would traverse all of the connections until they
found their way back to your computer, and you were looking at my web
In Figure 4, the network shown in Figure 3
is designated ``LAN 1'' and shown in the bottom-right of the picture.
This shows how the hosts on that network are provided connectivity to
other hosts on the same LAN, within the same company, outside of the
company, but in the same ISP cloud , and then from another ISP
somewhere on the Internet.
A Wider View of Internet-connected Networks
The Internet is made up of a wide variety of hosts, from
supercomputers to personal computers, including every imaginable type
of hardware and software. How do all of these computers understand
each other and work together?
TCP/IP (Transport Control Protocol/Internet Protocol) is the
``language'' of the Internet. Anything that can learn to ``speak
TCP/IP'' can play on the Internet. This is functionality that occurs
at the Network (IP) and Transport (TCP) layers in the ISO/OSI
Reference Model. Consequently, a host that has TCP/IP
functionality (such as Unix, OS/2, MacOS, or Windows NT) can easily
support applications (such as Netscape's Navigator) that uses the
One of the most important features of TCP/IP isn't a technological
one: The protocol is an ``open'' protocol, and anyone who wishes to
implement it may do so freely. Engineers and scientists from all over
the world participate in the IETF (Internet Engineering Task
Force) working groups that design the protocols that make the Internet
work. Their time is typically donated by their companies, and the
result is work that benefits everyone.
As noted, IP is a ``network layer'' protocol. This is the layer that
allows the hosts to actually ``talk'' to each other. Such things as
carrying datagrams, mapping the Internet address (such as 10.2.3.4) to
a physical network address (such as 08:00:69:0a:ca:8f), and routing,
which takes care of making sure that all of the devices that have
Internet connectivity can find the way to each other.
IP has a number of very important features which make it an extremely
robust and flexible protocol. For our purposes, though, we're going
to focus on the security of IP, or more specifically, the lack
A number of attacks against IP are possible. Typically, these exploit
the fact that IP does not perform a robust mechanism for
authentication , which is proving that a packet came from where
it claims it did. A packet simply claims to originate from a given
address, and there isn't a way to be sure that the host that sent the
packet is telling the truth. This isn't necessarily a weakness,
per se , but it is an important point, because it means that the
facility of host authentication has to be provided at a higher layer
on the ISO/OSI Reference Model. Today, applications that require
strong host authentication (such as cryptographic applications) do
this at the application layer.
This is where one host claims to have the
IP address of another. Since many systems (such as router access
control lists) define which packets may and which packets may not pass
based on the sender's IP address, this is a useful technique to an
attacker: he can send packets to a host, perhaps causing it to take
some sort of action.
Additionally, some applications allow login based on the IP address of
the person making the request (such as the Berkeley
r-commands ). These are both good examples
how trusting untrustable layers can provide security that is -- at
best -- weak.
This is a relatively sophisticated attack, first described by Steve
Bellovin . This is very dangerous, however,
because there are now toolkits available in the underground community
that allow otherwise unskilled bad-guy-wannabes to perpetrate this
attack. IP Session Hijacking is an attack whereby a user's session is
taken over, being in the control of the attacker. If the user was in
the middle of email, the attacker is looking at the email, and then
can execute any commands he wishes as the attacked user. The attacked
user simply sees his session dropped, and may simply login again,
perhaps not even noticing that the attacker is still logged in and
For the description of the attack, let's return to our large network
of networks in Figure 4. In this attack, a user on
host A is carrying on a session with host G. Perhaps
this is a telnet session, where the user is reading his email,
or using a Unix shell account from home. Somewhere in the network
between A and G sits host H which is run by a
naughty person. The naughty person on host H watches the
traffic between A and G, and runs a tool which starts to
impersonate A to G, and at the same time tells A
to shut up, perhaps trying to convince it that G is no longer
on the net (which might happen in the event of a crash, or major
network outage). After a few seconds of this, if the attack is
successful, naughty person has ``hijacked'' the session of our user.
Anything that the user can do legitimately can now be done by the
attacker, illegitimately. As far as G knows, nothing has
This can be solved by replacing standard telnet-type
applications with encrypted versions of the same thing. In this case,
the attacker can still take over the session, but he'll see only
``gibberish'' because the session is encrypted. The attacker will not
have the needed cryptographic key(s) to decrypt the data stream from
G, and will, therefore, be unable to do anything with the
TCP is a transport-layer protocol. It needs to sit on top of a
network-layer protocol, and was designed to ride atop IP. (Just as IP
was designed to carry, among other things, TCP packets.) Because TCP
and IP were designed together and wherever you have one, you typically
have the other, the entire suite of Internet protocols are known
collectively as ``TCP/IP.'' TCP itself has a number of important
features that we'll cover briefly.
Probably the most important is guaranteed packet delivery. Host
A sending packets to host B expects to get
acknowledgments back for each packet. If B does not send an
acknowledgment within a specified amount of time, A will
resend the packet.
Applications on host B will expect a data stream from a TCP
session to be complete, and in order. As noted, if a packet is
missing, it will be resent by A, and if packets arrive out of
order, B will arrange them in proper order before passing the
data to the requesting application.
This is suited well toward a number of applications, such as a
telnet session. A user wants to be sure every keystroke is
received by the remote host, and that it gets every packet sent back,
even if this means occasional slight delays in responsiveness while a
lost packet is resent, or while out-of-order packets are rearranged.
It is not suited well toward other applications, such as streaming
audio or video, however. In these, it doesn't really matter if a
packet is lost (a lost packet in a stream of 100 won't be
distinguishable) but it does matter if they arrive late (i.e.,
because of a host resending a packet presumed lost), since the data
stream will be paused while the lost packet is being resent. Once the
lost packet is received, it will be put in the proper slot in the data
stream, and then passed up to the application.
UDP (User Datagram Protocol) is a simple transport-layer
protocol. It does not provide the same features as TCP, and is thus
considered ``unreliable.'' Again, although this is unsuitable for
some applications, it does have much more applicability in other
applications than the more reliable and robust TCP.
One of the things that makes UDP nice is its simplicity. Because it
doesn't need to keep track of the sequence of packets, whether they
ever made it to their destination, etc., it has lower overhead than
TCP. This is another reason why it's more suited to streaming-data
applications: there's less screwing around that needs to be done with
making sure all the packets are there, in the right order, and that
sort of thing.
It's very important to understand that in security, one simply cannot
say ``what's the best firewall?'' There are two extremes: absolute
security and absolute access. The closest we can get to an absolutely
secure machine is one unplugged from the network, power supply, locked
in a safe, and thrown at the bottom of the ocean. Unfortunately, it
isn't terribly useful in this state. A machine with absolute access
is extremely convenient to use: it's simply there, and will do
whatever you tell it, without questions, authorization, passwords, or
any other mechanism. Unfortunately, this isn't terribly practical,
either: the Internet is a bad neighborhood now, and it isn't long
before some bonehead will tell the computer to do something like
self-destruct, after which, it isn't terribly useful to you.
This is no different from our daily lives. We constantly make
decisions about what risks we're willing to accept. When we get in a
car and drive to work, there's a certain risk that we're taking. It's
possible that something completely out of control will cause us to
become part of an accident on the highway. When we get on an
airplane, we're accepting the level of risk involved as the price of
convenience. However, most people have a mental picture of what an
acceptable risk is, and won't go beyond that in most circumstances.
If I happen to be upstairs at home, and want to leave for work, I'm
not going to jump out the window. Yes, it would be more convenient,
but the risk of injury outweighs the advantage of convenience.
Every organization needs to decide for itself where between the two
extremes of total security and total access they need to be. A policy
needs to articulate this, and then define how that will be
enforced with practices and such. Everything that is done in the name
of security, then, must enforce that policy uniformly.
Now, we've covered enough background information on networking that we
can actually get into the security aspects of all of this. First of
all, we'll get into the types of threats there are against networked
computers, and then some things that can be done to protect yourself
against various threats.
DoS (Denial-of-Service) attacks are probably the nastiest, and
most difficult to address. These are the nastiest, because they're
very easy to launch, difficult (sometimes impossible) to track, and it
isn't easy to refuse the requests of the attacker, without also
refusing legitimate requests for service.
The premise of a DoS attack is simple: send more requests to the
machine than it can handle. There are toolkits available in the
underground community that make this a simple matter of running a
program and telling it which host to blast with requests. The
attacker's program simply makes a connection on some service port,
perhaps forging the packet's header information that says where the
packet came from, and then dropping the connection. If the host is
able to answer 20 requests per second, and the attacker is sending 50
per second, obviously the host will be unable to service all of the
attacker's requests, much less any legitimate requests (hits on the
web site running there, for example).
Such attacks were fairly common in late 1996 and early 1997, but are
now becoming less popular.
Some things that can be done to reduce the risk of being stung by a
denial of service attack include
``Unauthorized access'' is a very high-level term that can refer to a
number of different sorts of attacks. The goal of these attacks is to
access some resource that your machine should not provide the
attacker. For example, a host might be a web server, and should
provide anyone with requested web pages. However, that host should
not provide command shell access without being sure that the person
making such a request is someone who should get it, such as a local
It's obviously undesirable for an unknown and untrusted person to be
able to execute commands on your server machines. There are two main
classifications of the severity of this problem: normal user access,
and administrator access. A normal user can do a number of things on
a system (such as read files, mail them to other people, etc.) that
an attacker should not be able to do. This might, then, be all the
access that an attacker needs. On the other hand, an attacker might
wish to make configuration changes to a host (perhaps changing its IP
address, putting a start-up script in place to cause the machine to
shut down every time it's started, or something similar). In this
case, the attacker will need to gain administrator privileges on the
We need to examine the threat model: what is it that you're trying to
protect yourself against? There is certain information that could be
quite damaging if it fell into the hands of a competitor, an enemy, or
the public. In these cases, it's possible that compromise of a normal
user's account on the machine can be enough to cause damage (perhaps
in the form of PR, or obtaining information that can be used against
the company, etc.)
While many of the perpetrators of these sorts of break-ins are merely
thrill-seekers interested in nothing more than to see a shell prompt
for your computer on their screen, there are those who are more
malicious, as we'll consider next. (Additionally, keep in mind that
it's possible that someone who is normally interested in nothing more
than the thrill could be persuaded to do more: perhaps an unscrupulous
competitor is willing to hire such a person to hurt you.)
Among the destructive sorts of break-ins and attacks, there are two
The data diddler is likely the worst sort, since the fact of a
break-in might not be immediately obvious. Perhaps he's toying with
the numbers in your spreadsheets, or changing the dates in your
projections and plans. Maybe he's changing the account numbers for
the auto-deposit of certain paychecks. In any case, rare is the case
when you'll come in to work one day, and simply know that something is
wrong. An accounting procedure might turn up a discrepancy in the
books three or four months after the fact. Trying to track the
problem down will certainly be difficult, and once that problem
is discovered, how can any of your numbers from that time period be
trusted? How far back do you have to go before you think that your
data is safe?
Some of those perpetrate attacks are simply twisted jerks who like to
delete things. In these cases, the impact on your computing
capability -- and consequently your business -- can be nothing less
than if a fire or other disaster caused your computing equipment to be
How, though, does an attacker gain access to your equipment?
Through any connection that you have to the outside world.
This includes Internet connections, dial-up modems, and even physical
access. (How do you know that one of the temps that you've brought in
to help with the data entry isn't really a system cracker looking for
passwords, data phone numbers, vulnerabilities and anything else that
can get him access to your equipment?)
In order to be able to adequately address security, all possible
avenues of entry must be identified and evaluated. The security of
that entry point must be consistent with your stated policy on
acceptable risk levels.
From looking at the sorts of attacks that are common, we can divine a
relatively short list of high-level practices that can help prevent
security disasters, and to help control the damage in the event that
preventative measures were unsuccessful in warding off an attack.
This isn't just a good idea from a security point of view.
Operational requirements should dictate the backup policy, and this
should be closely coordinated with a disaster recovery plan, such that
if an airplane crashes into your building one night, you'll be able to
carry on your business from another location. Similarly, these can be
useful in recovering your data in the event of an electronic disaster:
a hardware failure, or a breakin that changes or otherwise damages
Although this should go without saying, this doesn't occur to
lots of folks. As a result, information that doesn't need to be
accessible from the outside world sometimes is, and this can
needlessly increase the severity of a break-in dramatically.
Any security system that can be broken by breaking through any one
component isn't really very strong. In security, a degree of
redundancy is good, and can help you protect your organization from a
minor security breach becoming a catastrophe.
Be sure that someone who knows what you've got is watching the
vendors' security advisories. Exploiting old bugs is still one of the
most common (and most effective!) means of breaking into systems.
In addition to watching what the vendors are saying, keep a close
watch on groups like CERT and CIAC.
Make sure that at least one person (preferably more) is subscribed to
these mailing lists
Having at least one person who is charged with keeping abreast of
security developments is a good idea. This need not be a technical
wizard, but could be someone who is simply able to read advisories
issued by various incident response teams, and keep track of various
problems that arise. Such a person would then be a wise one to
consult with on security related issues, as he'll be the one who knows
if web server software version such-and-such has any known problems,
This person should also know the ``dos'' and ``don'ts'' of security,
from reading such things as the ``Site Security
As we've seen in our discussion of the Internet and similar networks,
connecting an organization to the Internet provides a two-way flow of
traffic. This is clearly undesirable in many organizations, as
proprietary information is often displayed freely within a corporate
intranet (that is, a TCP/IP network, modeled after the Internet
that only works within the organization).
In order to provide some level of separation between an organization's
intranet and the Internet, firewalls have been employed. A
firewall is simply a group of components that collectively form a
barrier between two networks.
A number of terms specific to firewalls and networking are going to be
used throughout this section, so let's introduce them all together.
There are three basic types of firewalls, and we'll consider each of
The first firewalls were application gateways, and are sometimes known
as proxy gateways. These are made up of bastion hosts that run
special software to act as a proxy server. This software runs at the
Application Layer of our old friend the ISO/OSI Reference
Model, hence the name. Clients behind the firewall must be
proxitized (that is, must know how to use the proxy, and be
configured to do so) in order to use Internet services.
Traditionally, these have been the most secure, because they don't
allow anything to pass by default, but need to have the programs
written and turned on in order to begin passing traffic.
- Bastion host.
- A general-purpose computer used to control
access between the internal (private) network (intranet) and
the Internet (or any other untrusted network). Typically,
these are hosts running a flavor of the Unix operating system
that has been customized in order to reduce its functionality
to only what is necessary in order to support its functions.
Many of the general-purpose features have been turned off, and
in many cases, completely removed, in order to improve the
security of the machine.
- A special purpose computer for connecting networks
together. Routers also handle certain functions, such as
routing , or managing the traffic on the networks they
- Access Control List (ACL).
- Many routers now have the ability
to selectively perform their duties, based on a number of
facts about a packet that comes to it. This includes things
like origination address, destination address, destination
service port, and so on. These can be employed to limit the
sorts of packets that are allowed to come in and go out of a
- Demilitarized Zone (DMZ).
- The DMZ is a critical part of a
firewall: it is a network that is neither part of the
untrusted network, nor part of the trusted network. But, this
is a network that connects the untrusted to the trusted. The
importance of a DMZ is tremendous: someone who breaks into
your network from the Internet should have to get through
several layers in order to successfully do so. Those layers
are provided by various components within the DMZ.
- This is the process of having one host act in behalf of
another. A host that has the ability to fetch documents from
the Internet might be configured as a proxy server , and
host on the intranet might be configured to be proxy
clients . In this situation, when a host on the intranet
wishes to fetch the <http://www.interhack.net/> web
page, for example, the browser will make a connection to the
proxy server, and request the given URL. The proxy server
will fetch the document, and return the result to the client.
In this way, all hosts on the intranet are able to access
resources on the Internet without having the ability to direct
talk to the Internet.
A sample application gateway
These are also typically the slowest, because more processes need to
be started in order to have a request serviced. Figure 5
shows a application gateway.
Packet filtering is a technique whereby routers have ACLs
(Access Control Lists) turned on. By default, a router will pass all
traffic sent it, and will do so without any sort of restrictions.
Employing ACLs is a method for enforcing your security policy with
regard to what sorts of access you allow the outside world to have to
your internal network, and vice versa.
There is less overhead in packet filtering than with an application
gateway, because the feature of access control is performed at a lower
ISO/OSI layer (typically, the transport or session layer). Due to the
lower overhead and the fact that packet filtering is done with
routers, which are specialized computers optimized for tasks related
to networking, a packet filtering gateway is often much faster than
its application layer cousins. Figure 6 shows a packet
Because we're working at a lower level, supporting new applications
either comes automatically, or is a simple matter of allowing a
specific packet type to pass through the gateway. (Not that the
possibility of something automatically makes it a good idea;
opening things up this way might very well compromise your level of
security below what your policy allows.)
There are problems with this method, though. Remember, TCP/IP has
absolutely no means of guaranteeing that the source address is really
what it claims to be. As a result, we have to use layers of packet
filters in order to localize the traffic. We can't get all the way
down to the actual host, but with two layers of packet filters, we can
differentiate between a packet that came from the Internet and one
that came from our internal network. We can identify which network
the packet came from with certainty, but we can't get more specific
In an attempt to marry the security of the application layer gateways
with the flexibility and speed of packet filtering, some vendors have
created systems that use the principles of both.
A sample packet filtering gateway
In some of these systems, new connections must be authenticated and
approved at the application layer. Once this has been done, the
remainder of the connection is passed down to the session layer, where
packet filters watch the connection to ensure that only packets that
are part of an ongoing (already authenticated and approved)
conversation are being passed.
Other possibilities include using both packet filtering and
application layer proxies. The benefits here include providing a
measure of protection against your machines that provide services to
the Internet (such as a public web server), as well as provide the
security of an application layer gateway to the internal network.
Additionally, using this method, an attacker, in order to get to
services on the internal network, will have to break through the
access router, the bastion host, and the choke router.
Lots of options are available, and it makes sense to spend some time
with an expert, either in-house, or an experienced consultant who can
take the time to understand your organization's security policy, and
can design and build a firewall architecture that best implements
that policy. Other issues like services required, convenience, and
scalability might factor in to the final design.
The business of building firewalls is in the process of becoming a
commodity market. Along with commodity markets come lots of folks who
are looking for a way to make a buck without necessarily knowing what
they're doing. Additionally, vendors compete with each other to try
and claim the greatest security, the easiest to administer, and the
least visible to end users. In order to try to quantify the potential
security of firewalls, some organizations have taken to firewall
certifications. The certification of a firewall means nothing more
than the fact that it can be configured in such a way that it
can pass a series of tests. Similarly, claims about meeting or
exceeding U.S. Department of Defense ``Orange Book'' standards,
C-2, B-1, and such all simply mean that an organization
was able to configure a machine to pass a series of tests. This
doesn't mean that it was loaded with the vendor's software at the
time, or that the machine was even usable. In fact, one vendor has
been claiming their operating system is ``C-2 Certified''
didn't make mention of the fact that their operating system only
passed the C-2 tests without being connected to any sort of
Such gauges as market share, certification, and the like are no
guarantees of security or quality. Taking a little bit of time to
talk to some knowledgeable folks can go a long way in providing you a
comfortable level of security between your private network and the
big, bad Internet.
Additionally, it's important to note that many consultants these days
have become much less the advocate of their clients, and more of an
extension of the vendor. Ask any consultants you talk to about their
vendor affiliations, certifications, and whatnot. Ask what difference
it makes to them whether you choose one product over another, and vice
versa. And then ask yourself if a consultant who is certified in
technology XYZ is going to provide you with competing technology ABC,
even if ABC best fits your needs.
Many ``firewalls'' are sold as a single component: a bastion host, or
some other black box that you plug your networks into and get a
warm-fuzzy, feeling safe and secure. The term ``firewall''
refers to a number of components that collectively provide the
security of the system. Any time there is only one component paying
attention to what's going on between the internal and external
networks, an attacker has only one thing to break (or fool!) in order
to gain complete access to your internal networks.
See the Internet Firewalls FAQ for more
details on building and maintaining firewalls.
It's important to remember that the firewall is only one entry point to
your network. Modems, if you allow them to answer incoming calls, can
provide an easy means for an attacker to sneak around (rather
than through ) your front door (or, firewall). Just as castles
weren't built with moats only in the front, your network needs to be
protected at all of its entry points.
If modem access is to be provided, this should be guarded carefully.
The terminal server , or network device that provides dial-up
access to your network needs to be actively administered, and its logs
need to be examined for strange behavior. Its passwords need to be
strong -- not ones that can be guessed. Accounts that aren't
actively used should be disabled. In short, it's the easiest way to
get into your network from remote: guard it carefully.
There are some remote access systems that have the feature of a
two-part procedure to establish a connection. The first part is the
remote user dialing into the system, and providing the correct userid
and password. The system will then drop the connection, and call the
authenticated user back at a known telephone number. Once the remote
user's system answers that call, the connection is established, and
the user is on the network. This works well for folks working at
home, but can be problematic for users wishing to dial in from hotel
rooms and such when on business trips.
Other possibilities include one-time password schemes, where the user
enters his userid, and is presented with a ``challenge,'' a string of
between six and eight numbers. He types this challenge into a small
device that he carries with him that looks like a calculator. He then
presses enter, and a ``response'' is displayed on the LCD screen. The
user types the response, and if all is correct, he login will
proceed. These are useful devices for solving the problem of good
passwords, without requiring dial-back access. However, these have
their own problems, as they require the user to carry them, and they
must be tracked, much like building and office keys.
No doubt many other schemes exist. Take a look at your options, and
find out how what the vendors have to offer will help you
enforce your security policy effectively.
A feature that is being built into some routers is the ability to
use session encryption between specified routers. Because traffic
traveling across the Internet can be seen by people in the middle who
have the resources (and time) to snoop around, these are advantageous
for providing connectivity between two sites, such that there can be
See the Snake Oil FAQ  for a description
of cryptography, ideas for evaluating cryptographic products, and how
to determine which will most likely meet your needs.
Given the ubiquity of the Internet, and the considerable expense in
private leased lines, many organizations have been building
VPNs (Virtual Private Networks). Traditionally, for an
organization to provide connectivity between a main office and a
satellite one, an expensive data line had to be leased in order to
provide direct connectivity between the two offices. Now, a solution
that is often more economical is to provide both offices connectivity
to the Internet. Then, using the Internet as the medium, the two
offices can communicate.
The danger in doing this, of course, is that there is no privacy on
this channel, and it's difficult to provide the other office access to
``internal'' resources without providing those resources to everyone
on the Internet.
VPNs provide the ability for two offices to communicate with each
other in such a way that it looks like they're directly connected over
a private leased line. The session between them, although going over
the Internet, is private (because the link is encrypted), and the link
is convenient, because each can see each others' internal resources
without showing them off to the entire world.
A number of firewall vendors are including the ability to build VPNs
in their offerings, either directly with their base product, or as an
add-on. If you have need to connect several offices together, this
might very well be the best way to do it.
Security is a very difficult topic. Everyone has a different idea of
what ``security'' is, and what levels of risk are acceptable. The key
for building a secure network is to define what security means
to your organization . Once that has been defined, everything that
goes on with the network can be evaluated with respect to that
policy. Projects and systems can then be broken down into their
components, and it becomes much simpler to decide whether what is
proposed will conflict with your security policies and practices.
Many people pay great amounts of lip service to security, but do not
want to be bothered with it when it gets in their way. It's important
to build systems and networks in such a way that the user is not
constantly reminded of the security system around him. Users who find
security policies and systems too restrictive will find ways around
them. It's important to get their feedback to understand what can be
improved, and it's important to let them know why what's been
done has been, the sorts of risks that are deemed unacceptable, and
what has been done to minimize the organization's exposure to them.
Security is everybody's business, and only with everyone's
cooperation, an intelligent policy, and consistent practices, will it
Introduction to Network Security
- The New Lexicon Webster's
Encyclopedic Dictionary of the English Language. New York:
- R.T. Morris, 1985. A Weakness in the
4.2BSD Unix TCP/IP Software. Computing Science Technical
Report No. 117, AT&T Bell Laboratories, Murray Hill, New
- S.M. Bellovin. Security
Problems in the TCP/IP Protocol Suite. Computer
Communication Review, Vol. 19, No. 2, pp. 32-48, April 1989.
- Y. Rekhter, R. Moskowitz, D. Karrenberg,
G. de Groot, E. Lear, ``Address Allocation for Private Internets.'' RFC 1918.
- J.P. Holbrook, J.K. Reynolds. ``Site Security Handbook.'' RFC 1244.
- M. Curtin, ``Snake Oil Warning Signs: Encryption Software to Avoid.'' USENET <sci.crypt>
Frequently Asked Questions File.
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