What are Reflexive ACLs?
Reflexive ACLS also known as IP-Session-Filtering ACLs is used to allow traffic sessions from a source network while denying IP traffic for sessions coming from outside the network. It allows a network administrator to dynamically enable a network filtering router to manage session traffic.
The router examines the outbound traffic and when it sees a new connection, it adds an entry to a temporary ACL to allow replies back in. These entries are automatically created when a new IP session begins and are removed when the session ends.
Reflexive access lists are not applied directly to an interface but are “nested” within an extended named IP access list that is applied to the interface.
Reflective ACLs add a dynamic component to this traditional model. Unlike standard ACLs, which rely on predefined rules, reflective ACLs can modify these rules in real-time based on observed traffic patterns. This is particularly useful for initiating and managing stateful sessions, where the ACL dynamically adjusts to allow return traffic from an outside source that was previously requested by an internal host.
Here’s a more detailed breakdown:
Dynamic Nature: Reflective ACLs monitor outbound traffic from a protected network and dynamically permit return traffic from the same session. This contrasts with traditional ACLs, which are static and require explicit rules for both inbound and outbound traffic.
Session-Based Filtering: This type of ACLs are session-aware, meaning they can track the state of a network connection (e.g., a TCP session) and allow traffic related to established connections, even if such traffic would be blocked under normal circumstances.
Enhanced Security: By only allowing inbound traffic that corresponds to outbound requests, reflective ACLs reduce the risk of unsolicited or malicious inbound traffic. This is particularly effective against certain types of network attacks, such as unauthorized access attempts.
Configuration and Maintenance: Reflective ACLs require an initial configuration, but they can reduce the ongoing maintenance required with static ACLs, as they adapt to changing traffic patterns.
Limitations and Considerations: Despite their advantages, reflective ACLs are not a one-size-fits-all solution. They might not be as effective against complex, multi-stage cyber attacks, and they require careful initial configuration to avoid inadvertently blocking legitimate traffic. Also, they are more resource-intensive than static ACLs, as they need to maintain state information for each session.
Cisco recommends that reflexive access lists are to be configured on border routers—routers that pass traffic between an internal and external network. Often, these are firewall routers.
Reflexive ACLs are used to provide a firmer form of session filtering than an extended ACL that uses the established parameter.
Reflexive ACLs also work for UDP and ICMP, which have no ACK or RST bits. The established option also does not work with applications that dynamically alter the source port for the session traffic. The permit established statement only checks ACK and RST bits, not source and destination addresses.
Network Administrators use reflexive ACLs to secure against network hackers, and can be included in a firewall defence.
* It is Simple to use and, compared to standard ACLs, provides greater control over which packets enter your network. It provides a level of security against spoofing and certain DoS attacks.
In summary, reflective ACLs provide a more dynamic and context-aware approach to network traffic management. They offer enhanced security by adapting to real-time traffic flows, making them a valuable tool for modern network security. However, they should be deployed with an understanding of their limitations and in conjunction with other security measures for comprehensive network protection.
Related:
Complex ACLs Switchport ACLs IPv6 ACLs Extended ACLs
Troubleshooting ACLs Errors How to Control Password attack with ACLs
