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Firewall

Firewalls

  1. What is a firewall?
  2. What does a firewall do?
  3. What can’t a firewall do?
  4. Who needs a firewall?
  5. How does a firewall work?
  6. What are the OSI and TCP/IP Network models?
  7. What different types of firewalls are there?

1. What is a firewall?

A firewall protects networked computers from intentional hostile intrusion that could compromise confidentiality or result in data corruption or denial of service. It may be a hardware device (see Figure 1) or a software program (see Figure 2) running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to.

A firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet. The earliest firewalls were simply routers. The term firewall comes from the fact that by segmenting a network into different physical subnetworks, they limited the damage that could spread from one subnet to another just like firedoors or firewalls.

Figure 1: Hardware Firewall.

Hardware firewall providing protection to a Local Network.

Figure 2: Computer with Firewall Software.
Computer running firewall software to provide protection

2. What does a firewall do?

A firewall examines all traffic routed between the two networks to see if it meets certain criteria. If it does, it is routed between the networks, otherwise it is stopped. A firewall filters both inbound and outbound traffic. It can also manage public access to private networked resources such as host applications. It can be used to log all attempts to enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter packets based on their source and destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state.

3. What can’t a firewall do?

A firewall cannot prevent individual users with modems from dialling into or out of the network, bypassing the firewall altogether. Employee misconduct or carelessness cannot be controlled by firewalls. Policies involving the use and misuse of passwords and user accounts must be strictly enforced. These are management issues that should be raised during the planning of any security policy but that cannot be solved with firewalls alone.

The arrest of the Phonemasters cracker ring brought these security issues to light. Although they were accused of breaking into information systems run by AT&T Corp., British Telecommunications Inc., GTE Corp., MCI WorldCom, Southwestern Bell, and Sprint Corp, the group did not use any high tech methods such as IP spoofing (see question 10). They used a combination of social engineering and dumpster diving. Social engineering involves skills not unlike those of a confidence trickster. People are tricked into revealing sensitive information. Dumpster diving or garbology, as the name suggests, is just plain old looking through company trash. Firewalls cannot be effective against either of these techniques.

4. Who needs a firewall?

Anyone who is responsible for a private network that is connected to a public network needs firewall protection. Furthermore, anyone who connects so much as a single computer to the Internet via modem should have personal firewall software. Many dial-up Internet users believe that anonymity will protect them. They feel that no malicious intruder would be motivated to break into their computer. Dial up users who have been victims of malicious attacks and who have lost entire days of work, perhaps having to reinstall their operating system, know that this is not true. Irresponsible pranksters can use automated robots to scan random IP addresses and attack whenever the opportunity presents itself.

5. How does a firewall work?

There are two access denial methodologies used by firewalls. A firewall may allow all traffic through unless it meets certain criteria, or it may deny all traffic unless it meets certain criteria  . The type of criteria used to determine whether traffic should be allowed through varies from one type of firewall to another. Firewalls may be concerned with the type of traffic, or with source or destination addresses and ports. They may also use complex rule bases that analyse the application data to determine if the traffic should be allowed through. How a firewall determines what traffic to let through depends on which network layer it operates at. A discussion on network layers and architecture follows.

6. What are the OSI and TCP/IP Network models?

To understand how firewalls work it helps to understand how the different layers of a network interact. Network architecture is designed around a seven layer model. Each layer has its own set of responsibilities, and handles them in a well-defined manner. This enables networks to mix and match network protocols and physical supports. In a given network, a single protocol can travel over more than one physical support (layer one) because the physical layer has been dissociated from the protocol layers (layers three to seven). Similarly, a single physical cable can carry more than one protocol. The TCP/IP model is older than the OSI industry standard model which is why it does not comply in every respect. The first four layers are so closely analogous to OSI layers however that interoperability is a day to day reality.

Firewalls operate at different layers to use different criteria to restrict traffic. The lowest layer at which a firewall can work is layer three. In the OSI model this is the network layer. In TCP/IP it is the Internet Protocol layer. This layer is concerned with routing packets to their destination. At this layer a firewall can determine whether a packet is from a trusted source, but cannot be concerned with what it contains or what other packets it is associated with. Firewalls that operate at the transport layer know a little more about a packet, and are able to grant or deny access depending on more sophisticated criteria. At the application level, firewalls know a great deal about what is going on and can be very selective in granting access.

7. What different types of firewalls are there?

Firewalls fall into four broad categories: packet filters, circuit level gateways, application level gateways and stateful multilayer inspection firewalls.

Packet filtering firewalls work at the network level of the OSI model, or the IP layer of TCP/IP. They are usually part of a router. A router is a device that receives packets from one network and forwards them to another network. In a packet filtering firewall each packet is compared to a set of criteria before it is forwarded. Depending on the packet and the criteria, the firewall can drop the packet, forward it or send a message to the originator. Rules can include source and destination IP address, source and destination port number and protocol used. The advantage of packet filtering firewalls is their low cost and low impact on network performance. Most routers support packet filtering. Even if other firewalls are used, implementing packet filtering at the router level affords an initial degree of security at a low network layer. This type of firewall only works at the network layer however and does not support sophisticated rule based models (see Figure 5). Network Address Translation (NAT) routers offer the advantages of packet filtering firewalls but can also hide the IP addresses of computers behind the firewall, and offer a level of circuit-based filtering.

 

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