The reflexive access-list is the poor man’s stateful firewall. By default an access-list on a Cisco router doesn’t keep track of any connections. The only thing it cares about is whether an incoming packet matches a certain statement or not. When it matches a statement it will perform an action (permit or deny) and if it doesn’t match…it’ll check the next statement. If none of the statements match it will hit the implicit deny any and the packet will be dropped.
When using the reflexive access-list, your Cisco IOS router will keep track of the outgoing connection(s) and it will automatically allow the return traffic. It’s best to explain this with an example, so let’s take a look at the following topology:
Above we have 3 routers…nothing fancy. Let’s say I want to protect R1 and R2 from whatever traffic R3 might send. I could do this with a very simple but effective access-list:
R2(config)#ip access-list extended 100 R2(config-ext-nacl)#deny ip any any R2(config)#interface fastEthernet 0/1 R2(config-if)#ip access-group 100 in
The access-list above will drop all traffic from R3. Problem solved right?
Now what if there’s a HTTP server behind R3 that I want to reach from R1?
R1#telnet 192.168.23.3 80 Trying 192.168.23.3, 80 ... % Connection timed out; remote host not responding
Perhaps our network is a bit too secure….The packets from R1 will make it to R3 but the return traffic will be dropped. If I want to allow this return traffic, I have to punch a hole in that access-list that I just created. There’s a better method, and that’s the reflexive access-list…let’s take a look.
Forget about the access-list that I just created, we start with fresh routers that don’t have any access-lists applied to them.
I’m going to create an access-list that will track all outgoing connections, this is how we do it:
R2(config)#ip access-list extended OUTBOUND R2(config-ext-nacl)#permit ip any any reflect EVALUATE R2(config)#interface fastEthernet 0/1 R2(config-if)#ip access-group OUTBOUND out
Above you seen an access-list called OUTBOUND that will permit everything but I’ve added the reflect keyword. This means that the router keeps track of this outgoing connection and it will automatically create a statement for the return traffic. It will save this statement in a temporary access-list called EVALUATE. We are halfway done, there’s one more access-list to create: