Firewalls are vital guardians of network security, establishing a barrier between trusted internal networks and potentially malicious external networks like the internet. Two essential policies govern how firewall rules are processed: “First Rule Win” and “Last Rule Win”. Let’s delve into these policies, examples of firewall software that use them, and the implications for network security.

First Rule Win

In a “First Rule Win” policy, firewall rules are evaluated sequentially from top to bottom. Once a rule matches the incoming or outgoing traffic, the actions specified in that rule are applied, and further rule processing stops. If no rule matches, the firewall either denies or allows the traffic based on a default policy.

Examples:

iptables (Linux)

iptables is a widely-used firewall tool for Linux systems that follows the “First Rule Win” policy by default.

Example Configuration:

# Allow SSH traffic from a specific IP address
iptables -A INPUT -s 192.168.1.10 -p tcp --dport 22 -j ACCEPT

# Deny all other incoming traffic
iptables -A INPUT -j DROP

In this example, traffic from IP address 192.168.1.10 destined for port 22 matches the first rule and is allowed. All other traffic is denied by the second rule.

Last Rule Win

Conversely, in a “Last Rule Win” policy, firewall rules are processed in reverse order, from bottom to top. The actions of the last rule that matches the traffic are applied, and further rule processing stops.

Examples:

Windows Firewall (Windows)

Windows Firewall on Windows operating systems uses the “Last Rule Win” policy.

Example Configuration using Windows Firewall with Advanced Security:

1. Deny all incoming traffic by default.
2. Allow incoming traffic on port 80 from a specific IP address.

In this configuration, even though the default “Deny” rule is processed first, it is overridden by the “Allow” rule for port 80 due to the “Last Rule Win” policy.

pfSense (Open Source)

pfSense is an open-source firewall based on FreeBSD that also follows the “Last Rule Win” policy by default.

Example Configuration:

1. Deny traffic from a specific IP address.
2. Allow traffic on port 443 for all.

In this example, the “Allow” rule for port 443 overrides the “Deny” rule for the specific IP address due to the “Last Rule Win” policy.

Conclusion

Understanding the “First Rule Win” and “Last Rule Win” policies is crucial for effective firewall management and network security. Whether you’re using iptables on Linux, Windows Firewall on Windows, or pfSense on FreeBSD, knowing the policy that governs your firewall rules is essential. It enables you to design rules that align with your network’s security requirements and ensure that your firewall operates as intended, providing a robust defense against potential threats.