A router filters network traffic by inspecting individual packets and applying a set of predefined rules to decide whether to allow or deny them passage.
Routers act as gateways between different networks, and a key function is controlling which data packets are allowed to travel from one network to another. This control is achieved through a process called packet filtering.
The Core Mechanism: Filtering Rules
According to the reference, a packet-filtering router either blocks or passes packets presented to it according to a set of filtering rules. This means that each packet arriving at the router is examined against a list of criteria.
These filtering rules are configured by network administrators and dictate the router's behavior. They essentially form a policy for traffic flow.
What Are Filtering Rules Based On?
The rules used for filtering are based on various features of the service or protocols involved. A primary basis for these rules is the packet header information, which contains vital details about the packet's origin and destination.
Key information often used for filtering includes:
- IP Source Address: The IP address of the computer or device that sent the packet.
- IP Destination Address: The IP address of the intended recipient of the packet.
- Port Numbers: Used to identify specific applications or services (e.g., port 80 for HTTP web traffic, port 443 for HTTPS).
- Protocol Type: Specifies the protocol being used (e.g., TCP, UDP, ICMP).
How Filtering Works in Practice
When a packet arrives at a router configured for filtering, the router performs the following steps:
- Packet Inspection: The router examines the packet's header information.
- Rule Comparison: The header information is compared against the predefined set of filtering rules. Rules are typically processed in order.
- Action Determination: Based on the match (or lack thereof) against the rules, the router determines the appropriate action.
- Execute Action:
- If a rule matches and the action is to pass, the router forwards the packet towards its destination network.
- If a rule matches and the action is to block, the router drops the packet (denies it entry or exit) and may send a notification back to the source.
Figure 14 (mentioned in the reference, but not provided) likely illustrates this flow, showing packets arriving at the router, being processed by filtering rules, and then either passing through or being blocked.
Benefits of Router Filtering
Implementing packet filtering on a router provides several benefits:
- Security: Blocks unauthorized access attempts and prevents certain types of malicious traffic from entering a network.
- Network Performance: Reduces unwanted traffic, saving bandwidth and processing resources.
- Access Control: Restricts specific users or networks from accessing certain services or resources.
By defining precise rules based on packet information like IP addresses, routers play a crucial role in network security and traffic management.
