Comprehensive Guide to Internet Firewalls: Everything You Need to Know

 

Firewalls are an essential component of network security, designed to control the flow of traffic between different networks or devices. Whether you're managing a corporate network or securing your personal system at home, understanding firewalls can help you implement better security practices. This guide, based on the extensive Internet Firewalls FAQ compiled by experts, will provide you with a detailed overview of firewalls and address frequently asked questions.

1. What is a Network Firewall?

A firewall is a system that enforces an access control policy between two or more networks. It controls what traffic is allowed to pass between these networks and what is denied, acting as a barrier between a trusted internal network and untrusted external networks, like the internet. Firewalls can be configured to block certain types of traffic (like malicious requests) while allowing others.

2. Why Use a Firewall?

In today’s digital age, malicious entities frequently attempt to exploit vulnerabilities in networks to gain unauthorized access. A firewall helps prevent unwanted traffic from entering a network, making it a crucial security measure for protecting sensitive data and ensuring network safety.

3. What Can Firewalls Protect Against?

• Unauthorized access: Firewalls can block unauthorized attempts to access a network from external sources.
• Network-borne attacks: Some firewalls restrict access to only certain kinds of data (like emails) to limit exposure to vulnerabilities.
• Monitoring and auditing: Firewalls provide logging data, which can help identify suspicious activities on the network.

4. What Firewalls Cannot Protect Against

While firewalls are highly effective at protecting networks, they aren’t a catch-all solution:

• Insider threats: Firewalls won’t prevent employees or trusted users from accidentally or maliciously leaking sensitive information.
• Non-network-based attacks: Firewalls can’t protect against data exported on USB drives, CDs, or other media.
• Tunneling threats: If attackers hide malicious content in legitimate protocols (like HTTP or email), firewalls might not detect it.

5. Types of Firewalls

There are three primary types of firewalls, each with its advantages:

1. Network layer firewalls: These firewalls operate at the network layer and filter packets based on source, destination, and port.
2. Application layer firewalls: These firewalls filter traffic based on specific applications, offering more detailed logging and control but potentially reducing network speed.
3. Hybrid firewalls: These combine both network and application layer features, providing faster packet screening while offering detailed inspection when necessary.

6. Design and Implementation Considerations

When designing a firewall solution, organizations must consider:

• Access control policies: Clearly define what traffic is allowed or denied.
• Firewall location: Decide whether the firewall should sit at the perimeter of the network or between different internal segments.
• Cost and maintenance: While free firewalls exist, higher-end solutions may require dedicated staff and maintenance to ensure continued effectiveness.

7. Common Firewall Attacks and How to Protect Against Them

• Denial of Service (DoS) attacks: Firewalls can be configured to limit the impact of these attacks by blocking suspicious IP addresses.
• Source routing attacks: Source routing allows a packet to specify its route through the network, making it easier for attackers to spoof legitimate traffic. Blocking source-routed packets is a common firewall rule.
• ICMP redirect attacks: These attacks manipulate network traffic, but firewalls can be configured to block unnecessary ICMP messages to prevent these types of exploits.

8. How to Make Common Services Work with Firewalls

1. Web/HTTP: You can configure a proxy server to handle HTTP requests, ensuring security without sacrificing usability.
2. FTP: Passive FTP works better with firewalls, as it doesn’t require the firewall to manage dynamic incoming connections.
3. DNS: You can set up a dual DNS server configuration—one for internal use and one for the public—while using your firewall to filter DNS traffic.
4. SSL: Ensure that SSL traffic is allowed through your firewall, particularly for secure web connections.

9. Ports and Firewalls

• What is a port? A port is a virtual slot in your network stack that allows different applications to connect over a network.
• How do firewalls handle ports? Firewalls filter traffic based on port numbers, allowing only specific, trusted services to communicate over known ports (e.g., HTTP traffic on port 80).
• Are high-numbered ports safe? Just because a port is numbered above 1024 doesn’t mean it’s safe. Security depends on the application using that port, not the port number itself.

10. Glossary of Key Firewall Terms

• DMZ (Demilitarized Zone): A network segment that is isolated from both the internal network and the public internet, typically used to host services that need to be publicly accessible.
• Bastion Host: A highly secured system that is exposed to external threats and acts as a key point of defense.
• Proxy Server: A server that intermediates between users and the internet, providing control and logging of traffic.
• IP Spoofing: An attack method where the attacker manipulates the source IP address of a packet to make it appear as though it comes from a trusted source.

Resources for Further Reading

• Building Internet Firewalls by Elizabeth D. Zwicky et al. (2000) - A comprehensive guide to firewall design and implementation.
• Firewalls and Internet Security: Repelling the Wily Hacker by Cheswick, Bellovin, and Rubin (2003) - A foundational text for understanding network security through firewalls.

Conclusion

Firewalls play a crucial role in securing networks by filtering out malicious traffic and blocking unauthorized access. However, they are not a silver bullet—comprehensive security also involves antivirus solutions, employee training, and strong policies. By understanding the limitations and capabilities of firewalls, administrators can make informed decisions to protect their systems.

For more detailed information, the full Internet Firewalls FAQ can be accessed at: Internet Firewalls FAQ