Cisco IOS has statefull firewall features like reflexive access lists. By using this feature you can use your Cisco router as a second firewall (the choke point concept in Cisco firewall trainings) and increase your network security by layered approach.
You can use an access control list (ACL) for the filtering one way traffic but what about the responding packages. You have to add an incoming ACL but it should include only sessions started from internal. Reflexive ACLs helps us in this point.
To use reflexive ACLs
1. You MUST use named access lists
2. You MUST add “reflect samplename” to the end of permit line.
3. You MUST create a second named access list and add “evaluate samplename” line for responding traffic.
In our example we have a proxy server (e.g. Microsoft ISA Server) with 126.96.36.199 IP address. This server needs access to internet via http (tcp 80) for web browsing and via dns (udp 53) for name resolution.
description Internet connection
ip access-group INBOUND in
ip access-group OUTBOUND out
ip access-list extended OUTBOUND
permit tcp host 188.8.131.52 any reflect PROXYTCP
permit udp host 184.108.40.206 any eq domain reflect PROXYUDP
ip access-list extended INBOUND
We used reflect command to create reverse ACL and we added it to inbound ACL with evaluate command.
This feature is 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.