Reflexive Access Lists

Cisco IOS has stateful firewall features called reflexive access lists. By using this feature, you can use your Cisco router as a second firewall (the choke point concept in network security terminology) and increase your network security with a layered approach.

You can use an access control list (ACL) to filter one-way traffic, but what about the responding packages. You have to add an incoming ACL, which will only contain sessions started internally. Reflexive ACLs help us at this point.

Requirements
To use reflexive ACLs
1. You MUST use named access lists
2. You MUST add “reflect samplename” to the end of the permit line.
3. You MUST create a second named access list and add the “evaluate samplename” line for responding traffic.

Sample Scenario
In our example, we have a proxy server (e.g., Microsoft ISA Server) with the 122.22.22.1 IP address. This server needs access to the internet via HTTP (TCP 80) for web browsing and via DNS (UDP 53) for name resolution.

interface Serial0/0/0
description Internet connection
ip access-group INBOUND in
ip access-group OUTBOUND out
!
ip access-list extended OUTBOUND
permit tcp host 122.22.22.1 any reflect PROXYTCP
permit udp host 122.22.22.1 any eq domain reflect PROXYUDP
!
ip access-list extended INBOUND
evaluate PROXYTCP
evaluate PROXYUDP
!

We used reflect command to create reverse ACL, and we added it to the inbound ACL with evaluate command.

Last Words
This feature is a really powerful tool to increase network security, but you should not use it instead of a real firewall for Internet access. It should be used as another security layer.

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