|Securing and Optimizing Linux: RedHat Edition -A Hands on Guide|
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What is a Network Firewall Security Policy? Network firewall security policy defines those services that will be explicitly allowed or denied, how these services will be used and the exceptions to these rules. An organization's overall security policy must be determined according to security and business-need analysis. Since a firewall relates to network security alone, a firewall has little value unless the overall security policy is properly defined. Every rule in the network firewall security policy should be implemented on a firewall. Generally, a firewall uses one of the following methods.
Everything not specifically permitted is denied. This approach blocks all traffic between two networks except for those services and applications that are permitted. Therefore, each desired service and application should be implemented one by one. No service or application that might be a potential hole on the firewall should be permitted. This is the most secure method, denying services and applications unless explicitly allowed by the administrator. On the other hand, from the point of users, it might be more restrictive and less convenient. This is the method we will use in our Firewall configuration files in this book.
Everything not specifically denied is permitted This approach allows all traffic between two networks except for those services and applications that are denied. Therefore, each untrusted or potentially harmful service or application should be denied one by one. Although this is a flexible and convenient method for the users, it could potentially cause some serious security problems.
What is Packet Filtering? Packet Filtering is the type of firewall built into the Linux kernel. A filtering firewall works at the network level. Data is only allowed to leave the system if the firewall rules allow it. As packets arrive they are filtered by their type, source address, destination address, and port information contained in each packet. Most of the time, packet filtering is accomplished by using a router that can forward packets according to filtering rules. When a packet arrives at the packet-filtering router, the router extracts certain information from the packet header and makes decisions according to the filter rules as to whether the packet will pass through or be discarded.
The following information can be extracted from the packet header:
Source IP address Destination IP address TCP/UDP source port TCP/UDP destination port ICMP message type Encapsulated protocol information (TCP, UDP, ICMP or IP tunnel)