How does a stateless firewall differ from a stateful firewall?

A stateless firewall is designed to filter traffic based on a set of predefined rules without maintaining any context or tracking the state of active connections. This is in contrast to a stateful firewall, which monitors the full state of active connections and makes decisions based on both the rules and the state of traffic flows.

Given this, the requirement for explicit rules for return traffic is key to understanding how a stateless firewall operates. Unlike stateful firewalls that can automatically determine whether return traffic is part of an established connection, stateless firewalls do not have the capability to track this context. Therefore, each packet is evaluated independently based solely on defined rules, necessitating that administrators create specific rules to allow responses to outgoing traffic.

This inherent characteristic means that any traffic returning from an allowed outgoing connection must be explicitly permitted by the firewall rules since the firewall does not remember the initial request that led to the return traffic. Thus, return traffic from requests must be handled by separate rules, which is a fundamental difference from how stateful firewalls manage this aspect seamlessly.