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 network.

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.

Understanding IP

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 thereof.

Attacks Against IP

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.

IP Spoofing

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 )[2]. These are both good examples how trusting untrustable layers can provide security that is -- at best -- weak.

IP Session Hijackin.

This is a relatively sophisticated attack, first described by Steve Bellovin [3]. 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 doing things.
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 B 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 happened.
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 session.

TCP

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.

Guaranteed Packet Delivery

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

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.

Lower Overhead than 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.

Risk Management: The Game of Security

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.

Types And Sources Of Network Threats

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.

Denial-of-Service

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

Obviously forged packets would include those that claim to come from your own hosts, addresses reserved for private networks as defined in RFC 1918 [4], and the loopback network (127.0.0.0).

Unauthorized Access

``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 administrator.

Executing Commands Illicitly

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 host.

Confidentiality Breaches

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.)

Destructive Behavior

Among the destructive sorts of break-ins and attacks, there are two major categories.

Data Diddling

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?

Data Destruction

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 completely destroyed.

Where Do They Come From?

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.

Lessons Learned

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.

Hope you have backups

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 your data.

Don't put data where it doesn't need to be

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.

Avoid systems with single points of failure

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.

Stay current with relevant operating system patches

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.

Watch for relevant security advisories

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

Have someone on staff be familiar with security practices

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, etc.

Firewalls

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.
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.
Router
A special purpose computer for connecting networks together. Routers also handle certain functions, such as routing , or managing the traffic on the networks they connect.
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 given network.
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.
Proxy
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.

Types of Firewalls

Application Gateways

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.

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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

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 filtering gateway.
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 than that.

Hybrid Systems

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.

Figure 6: A sample packet filtering gateway

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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.

So, what's best for me?

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.

Some Words of Caution

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 network devices.
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.

Single Points of Failure

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.

Secure Network Devices

It's important to remember that the firewall 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.

Secure Modems; Dial-Back Systems

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 password 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.

Crypto-Capable Routers

A feature that is being built into some routers is the ability to 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 secure routes.
See the Snake Oil FAQ [6] for a description of cryptography, ideas for evaluating cryptographic products, and how to determine which will most likely meet your needs.

Virtual Private Networks

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 be achievable.
If your computer is not connected to any other computers and doesn't have a modem, the only way anyone can access your computer's information is by physically coming to the computer and sitting at it. So securing the room it's in will secure the computer [1]. As soon as your computer is connected to another computer you add the possibility that someone using the other computer can access your computer's information.
If your network (your connected computers) consists only of other computers in the same building you can still secure the network by securing the rooms the computers are in. An example of this would be two computers sharing the same files and printer, but not having a modem and not being connected to any other computers.
However, it's wise to learn about other ways to secure a network of connected computers, in case you add something later. Networks have a tendency to grow. If you have a network, an intruder who gains access to one computer has at least some access to all of them.

The Locked Front Door

As soon as your network connects to somewhere outside your building, you need the virtual equivalent of a locked front door. If you don't have that, all the information you have on your computers is vulnerable to anyone who wants to gain access.
Like real doors, virtual doors come in a wide variety of types, security levels, and expense.
The simplest, but not the safest, way to secure your network is to keep 'moving' - if you're connected to the internet through a modem and have a 'dynamic IP address' (ask your service provider), your address keeps changing. If your address keeps changing, and you're never on the internet for very long, it's very hard for someone to deliberately intrude on you. However, many computer intruders are like teenagers - they will go to great lengths for what they perceive as 'fun'. I recommend at least some security beyond this, even if all you ever do is read and write email.
As soon as you have a stable address and a permanent connection, you lose the 'obscurity' advantage that a dynamic IP and sporadic connection provides. You must install a real 'front door'.

Passwords

The most basic lock for your front door is a password. Ensure that every computer on your network requires a password before anyone from the network can read your information or write to your hard drive. If a password isn't required, there is no front door at all. If you're not sure how to ensure that passwords are necessary, I strongly recommend getting hold of a computer expert, or at least a very good manual.

Most computer systems will not password-lock someone sitting at the computer itself. There are ways to do it, but there's usually a way that someone at the computer itself (not on the network) can get in and change the passwords. This is to prevent the computer from becoming an expensive doorstop if the passwords are forgotten. This does, however, mean that you still need physical security.

Changing forgotten passwords isn't easy, however. It's better not to forget them in the first place. If your system has a 'master password' that has access to everything, make sure two people in your company or household know that password. If there's only one, what happens when that person is on vacation on that tropical island with no phones?
Passwords are only as secure as they are difficult to guess - if your password is your name, for instance, or the word 'password', it's like putting a lock on the front door and never bothering to actually lock it.
There are a lot of suggestions for how to make passwords difficult to guess - here're a few of them:

no less than eight characters long
include both upper and lower case letters, numbers and punctuation marks
don't use anything which can be guessed by someone who knows you or has your information - no names of family members or pets, no licence numbers or passport numbers or phone numbers or similar, not a street address (current or past!), not any words which are visible from your desk (like the brand of monitor)
no legitimate words in any language, brand names or logos
no swear words
not a simple substitution (ABC as 123, to as 2, Ziggy as 2166Y)
not the same password as on as another computer, or the same one you had last year. ANY password can be figured out in time, and if someone guesses one of your passwords they might try the same thing for another computer
not a common misspelling of a word

Suggestions for good passwords include

take something you'll recognise - a line from a book or a line of poetry - and use the third letter of each word. Include punctuation (but not spaces)
a really, REALLY bad misspelling of a word
two words from different languages stuck together with punctuation marks
a short phrase

Think up other suggestions. For passwords, weird and idiosyncratic is good.

Permissions

Passwords usually come with usernames as well. A good username-and-password system will enable you to set up several roles for your computers. Each role will need different types of access, will use different programs and different data.
If an intruder guesses or finds out one person's username and password, they will have access to any programs or data that that person usually has access to. For this reason, you might like to limit what each person is allowed to access.
Most computer systems have something in place which does this. Under most systems, it is called 'permissions'. Your computer manual or local expert can help you set it up on your computers.
Give each person what they need to do their jobs, plus a little personal space of their own. That personal space is often used to 'to-do' lists and other minor things which make their job easier or more comfortable.

Firewalls

If passwords provide a 'door' to cover the 'doorway' into your 'house', then firewalls provide 'shutters' to cover the 'windows'. Bear with me, we're extending the metaphor further than we probably should.
Your network has a lot of windows. These aren't just casual windows that let you see out, the metaphor is closer if you think of them as service windows, like at a drive-through of them have people (programs) at them to provide service, some of them are empty.
A firewall provides shutters to close the empty service windows.
A firewall does absolutely nothing to protect the windows you leave open - that's the job of the programs which provide the services at those windows. But if you don't have a firewall, there's all those empty windows that an intruder can use to break in through.
The firewall is ideally a separate computer which is between your network and the internet. It can be a purpose-built device - there are some available which are small black boxes which look like network hubs. Or it can be your brother's old 486, with a highly secure operating system that provides an inbuilt firewall. Whatever you choose, ensure that your local computer expert approves of it, and do your best to ensure that he knows how to make sure it really is secure.
None of your computers should be able to access the internet or be accessed from the internet without going through the firewall.

Other security measures

6.1. Unused programs

At each 'service window' that your firewall leaves open (technical term: 'open port'), you should have a computer program. This program should be providing some sort of service to your users.
Any program which isn't being used, but which has a connection outside your network, should be shut down and the 'service window' (port) closed at the firewall. Every port which isn't specifically in use should be shut down. Admittedly, this is a 'paranoia' position - the rationale for shutting them down being that a closed port is safer than an open one, regardless of how good the program is.

6.2. Bugs & patches

Programs which you are using need to stay operational, and their ports 'open'. However, occasionally programs are vulnerable to clever attackers.
Vulnerabilities are reported to organisations on the Internet which make a point of informing the companies or groups who write those programs, and distributing the modifications that these companies or groups produce to patch the vulnerabilities.
Every so often someone in your company should go to those sites, read their reports for your programs, and install the patches. Once a month is common, but you need to determine your own balance between security and convenience.

6.3. Monitoring

How do you know if someone has broken into your system? The only way to know for sure is to monitor it.
Some common types of monitoring tools are:

The tripwire: On a read-only medium (like a write-protected floppy), store a program and a small database. The program checks every file in the database to find out when it was last changed, and sends the user the list of all the files which have changed since it first ran. To prevent false reporting, the database should only include files which should never be changed.

If any of the files have been changed, you may have been broken into. (Or your system administrator installed a new version of the operating system and forgot to warn whoever does the monitoring!)

The sniffer: This tool checks all the traffic which goes through the network, looking for suspicious activity. It's usually installed on the firewall, or on a special box just to one side or the other of the firewall - though it would be more useful on the outside.

It doesn't attempt to block any activity, only to report it when it finds it.

The honeypot: One for special circumstances - this system has most of the useful programs (like directory listers or file removers or editors) removed and replaced with special programs that shut the computer down as soon as they're run. The shutdown prevents the intruder from further intrusion, and also from modifying the honeypot's logs.

These aren't very useful as working computers - they're simply traps.

Log analysis: This is difficult - most intruders will be careful to wipe traces of their activity out of the logs. I don't recommend its use by laymen, and include it here only because it is an important tool for more experienced administrators.

Most operating systems keep a set of logs of their network activity. This usually consists of things like 'opened this port', 'sent mail to this person', 'closed the port'. The content of the mail is not kept, but the fact of its being sent is. This sort of information is a useful tool for intrusion analysis (and for checking whether the system is running correctly).
Log analysis involves whoever does the monitoring going through the logs and looking for strange occurrences. Logs look something like this:

                   May 13 09:57:03 gondwanah dhclient-2.2.x: DHCPDISCOVER on lo to 255.255.255.255 port 67 interval 2

                   May 13 09:57:05 gondwanah dhclient-2.2.x: No DHCPOFFERS received.

                   May 13 09:57:05 gondwanah dhclient-2.2.x: No working leases in persistent database - sleeping.

                   May 13 09:57:05 gondwanah dhclient-2.2.x: No DHCPOFFERS received.

                   May 13 09:57:05 gondwanah dhclient-2.2.x: No working leases in persistent database - sleeping.

                   May 13 10:00:21 gondwanah dhclient-2.2.x: DHCPREQUEST on eth0 to 10.0.3.1 port 67

                   May 13 10:00:21 gondwanah dhclient-2.2.x: DHCPACK from 10.0.3.1

                   May 13 10:00:21 gondwanah dhclient-2.2.x: bound to 10.0.1.1 -- renewal in 3500 seconds.

You're not expected to understand what this is! It's an attempt by my computer to get an IP address (a number address) from the master computer on our home network. Log analysis involves reading a lot of stuff like this, knowing what's normal and what isn't, and dealing with the abnormalities.
Which is why I don't recommend it for laymen.

6.4. What do I do if I think I've been broken into?

If it was a physical break-in, call the police.
If it was a network break-in, either call the police or:

Shut your computer down.
Call your trusted computer-expert friend, or hire specialists in computer security.
Consider calling the police. Consider preserving the evidence.
Let the experts take your computer off the network, reboot it, and take a look at the logs. They will hopefully be able to figure out what type of attack it was.
If you chose to preserve the evidence, make sure your computer experts know this before they change anything.
Let the experts check your files for damage. They may recommend reinstalling the operating system, they may recommend restoring your data from your latest backup. Ask them for the pros and cons of each option they offer, and each recommendation they make. It's your data, but you hired them for their knowledge. So lean towards their advice, but you make the decision.
Get their advice on further securing your system. Listen to it.

Your security system is only as strong as its weakest part. A determined intruder will keep looking until they find a vulnerability.
Security through obscurity is weak. A hidden thing is more secure than a highly visible one, but don't trust hiding on its own to protect your data. A hidden safe is more secure than a sock under the floorboards.

This chapter is a general introduction to network security issues and solutions in the Internet; emphasis is placed on route filters and firewalls. It is not intended as a guide to setting up a secure network; its purpose is merely as an overview. Some knowledge of IP networking is assumed, although not crucial.
In the last decade, the number of computers in use has exploded. For quite some time now, computers have been a crucial element in how we entertain and educate ourselves, and most importantly, how we do business. It seems obvious in retrospect that a natural result of the explosive growth in computer use would be an even more explosive (although delayed) growth in the desire and need for computers to talk with each other. The growth of this industry has been driven by two separate forces which until recently have had different goals and end products. The first factor has been research interests and laboratories; these groups have always needed to share files, email and other information across wide areas. The research labs developed several protocols and methods for this data transfer, most notably TCP/IP. Business interests are the second factor in network growth. For quite some time, businesses were primarily interested in sharing data within an office or campus environment, this led to the development of various protocols suited specifically to this task.
Within the last five years, businesses have begun to need to share data across wide areas. This has prompted efforts to convert principally LAN-based protocols into WAN-friendly protocols. The result has spawned an entire industry of consultants who know how to manipulate routers, gateways and networks to force principally broadcast protocols across point-to-point links (two very different methods of transmitting packets across networks). Recently (within the last 2 or 3 years) more and more companies have realized that they need to settle on a common networking protocol. Frequently the protocol of choice has been TCP/IP, which is also the primary protocol run on the Internet. The emerging ubiquitousness of TCP/IP allows companies to interconnect with each other via private networks as well as through public networks.
This is a very rosy picture: businesses, governments and individuals communicating with each other across the world. While reality is rapidly approaching this utopian picture, several relatively minor issues have changed status from low priority to extreme importance. Security is probably the most well known of these problems. When businesses send private information across the net, they place a high value on it getting to its destination intact and without being intercepted by someone other than the intended recipient. Individuals sending private communications obviously desire secure communications. Finally, connecting a system to a network can open the system itself up to attacks. If a system is compromised, the risk of data loss is high.
It can be useful to break network security into two general classes:

methods used to secure data as it transits a network
methods which regulate what packets may transit the network

While both significantly effect the traffic going to and from a site, their objectives are quite different.

Transit Security

Currently, there are no systems in wide use that will keep data secure as it transits a public network. Several methods are available to encrypt traffic between a few coordinated sites. Unfortunately, none of the current solutions scale particularly well. Two general approaches dominate this area:
Virtual Private Networks: This is the concept of creating a private network by using TCP/IP to provide the lower levels of a second TCP/IP stack. This can be a confusing concept, and is best understood by comparing it to the way TCP/IP is normally implemented. In a nutshell, IP traffic is sent across various forms of physical networks. Each system that connects to the physical network implements a standard for sending IP messages across that link. Standards for IP transmission across various types of links exist, the most common are for Ethernet and Point to Point links (PPP and SLIP). Once an IP packet is received, it is passed up to higher layers of the TCP/IP stack as appropriate (UDP, TCP and eventually the application). When a virtual private network is implemented, the lowest levels of the TCP/IP protocol are implemented using an existing TCP/IP connection. There are a number of ways to accomplish this which tradeoff between abstraction and efficiency. The advantage this gives you in terms of secure data transfer is only a single step further away. Because a VPN gives you complete control over the physical layer, it is entirely within the network designers power to encrypt the connection at the physical (virtual) layer. By doing this, all traffic of any sort over the VPN will be encrypted, whether it be at the application layer (such as Mail or News) or at the lowest layers of the stack (IP, ICMP). The primary advantages of VPNs are: they allow private address space (you can have more machines on a network), and they allow the packet encryption/translation overhead to be done on dedicated systems, decreasing the load placed on production machines.
Packet Level Encryption: Another approach is to encrypt traffic at a higher layer in the TCP/IP stack. Several methods exist for the secure authentication and encryption of telnet and rlogin sessions (Kerberos, S/Key and DESlogin) which are examples of encryption at the highest level of the stack (the application layer). The advantages to encrypting traffic at the higher layer are that the processor overhead of dealing with a VPN is eliminated, inter-operability with current applications is not affected, and it is much easier to compile a client program that supports application layer encryption than to build a VPN. It is possible to encrypt traffic at essentially any of the layers in the IP stack. Particularly promising is encryption that is done at the TCP level which provides fairly transparent encryption to most network applications.
It is important to note that both of these methods can have performance impacts on the hosts that implement the protocols, and on the networks which connect those hosts. The relatively simple act of encapsulating or converting a packet into a new form requires CPU-time and uses additional network capacity. Encryption can be a very CPU-intensive process and encrypted packets may need to be padded to uniform length to guarantee the robustness of some algorithms. Further, both methods have impacts on other areas (security related and otherwise- such as address allocation, fault tolerance and load balancing) that need to be considered before any choice is made as to which is best for a particular case.

Traffic Regulation

The most common form of network security on the Internet today is to closely regulate which types of packets can move between networks. If a packet which may do something malicious to a remote host never gets there, the remote host will be unaffected. Traffic regulation provides this screen between hosts and remote sites. This typically happens at three basic areas of the network: routers, firewalls and hosts. Each provides similar service at different points in the network. In fact the line between them is somewhat ill-defined and arbitrary. In this article, I will use the following definitions:
Router traffic regulation: Any traffic regulation that occurs on a router or terminal server (hosts whose primary purpose is to forward the packets of other hosts) and is based on packet characteristics. This does not include application gateways but does include address translation.
Firewall traffic regulation: Traffic regulation or filtering that is performed via application gateways or proxies.
Host traffic regulation: Traffic regulation that is performed at the destination of a packet. Hosts are playing a smaller and smaller role in traffic regulation with the advent of filtering routers and firewalls.

Filters and access lists

Regulating which packets can go between two sites is a fairly simple concept on the surface- it shouldn't be and isn't difficult for any router or firewall to decide simply not to forward all packets from a particular site. Unfortunately, the reason most people connect to the Internet is so that they may exchange packets with remote sites. Developing a plan that allows the right packets through at the right time and denies the malicious packets is a thorny task which is far beyond this article's scope. A few basic techniques are worth discussing, however.

Restricting access in, but not out: Almost all packets (besides those at the lowest levels which deal with network reachability) are sent to destination sockets of either UDP or TCP. Typically, packets from remote hosts will attempt to reach one of what are known as the well known ports. These ports are monitored by applications which provide services such as Mail Transfer and Delivery, Usenet News, the time, Domain Name Service, and various login protocols. It is trivial for modern routers or firewalls only to allow these types of packets through to the specific machine that provides a given service. Attempts to send any other type of packet will not be forwarded. This protects the internal hosts, but still allows all packets to get out. Unfortunately this isn't the panacea that it might seem.
The problem of returning packets: Let's pretend that you don't want to let remote users log into your systems unless they use a secure, encrypting application such as S/Key. However, you are willing to allow your users to attempt to connect to remote sites with telnet or ftp. At first glance, this looks simple: you merely restrict remote connections to one type of packet and allow any type of outgoing connection. Unfortunately, due to the nature of interactive protocols, they must negotiate a unique port number to use once a connection is established. If they didn't, at any given time, there could only be one of each type of interactive session between any given two machines. This results in a dilemma: all of a sudden, a remote site is going to try to send packets destined for a seemingly random port. Normally, these packets would be dropped. However, modern routers and firewalls now support the ability to dynamically open a small window for these packets to pass through if packets have been recently transmitted from an internal host to the external host on the same port. This allows connections that are initiated internally to connect, yet still denies external connection attempts unless they are desired.
Dynamic route filters: A relatively recent technique is the ability to dynamically add entire sets of route filters for a remote site when a particular set of circumstances occur. With these techniques, it is possible to have a router automatically detect suspicious activity (such as ISS or SATAN) and deny a machine or entire site access for a short time. In many cases this will thwart any sort of automated attack on a site.

Filters and access lists are typically placed on all three types of systems, although they are most common on routers.
Address Translation: Another advancement has been to have a router modify outgoing packets to contain their own IP number. This prevents an external site from knowing any information about the internal network, it also allows for certain tricks to be played which provide for a tremendous number of additional internal hosts with a small allocated address space. The router maintains a table which maps an external IP number and socket with an internal number and socket. Whenever an internal packet is destined for the outside, it is simply forwarded with the routers IP number in the source field of the IP header. When an external packet arrives, it is analyzed for its destination port and re-mapped before it is sent on to the internal host. The procedure does have its pitfalls; checksums have to be recalculated because they are based in part on IP numbers, and some upper layer protocols encode/depend on the IP number. These protocols will not work through simple address translation routers.
Application gateways and proxies: The primary difference between firewalls and routers is that firewalls actually run applications. These applications frequently include mail daemons, ftp servers and web servers. Firewalls also usually run what are known as application gateways or proxies. These are best described as programs which understand a protocol's syntax, but do not implement any of the functionality of the protocol. Rather, after verifying that a message from an external site is appropriate, they send the message on to the real daemon which processes the data. This provides security for those applications that are particularly susceptible to interactive attacks. One advantage of using a firewall for these services is that it makes it very easy to monitor all activity, and very easy to quickly control what gets in and out of a network.
There are two basic types of network security, transit security and traffic regulation, which when combined can help guarantee that the right information is securely delivered to the right place. It should be apparent that there is also a need for ensuring that the hosts that receive the information will properly process it, this raises the entire specter of host security: a wide area which varies tremendously for each type of system. With the growth in business use of the Internet, network security is rapidly becoming crucial to the development of the Internet. Soon, security will be an integral part of our day to day use of the Internet and other networks.