Reflexive Access List

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:

3 cisco routers

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?

You don’t have to add the “deny ip any any” to an access-list, it’s always there at the bottom but invisible. An advantage of adding it is that you will see the number of packets that are dropped this way.

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.

Configuration

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:

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Forum Replies

  1. Rene,

    I have been working on access-lists and NAT on my little lab. I have a Cisco router connected to a D Link router that is in turn connected to a vonage router which in turn connects to a Cable mode - my gateway to the internet. I am able to ping the D Link IP address from the cisco router and also the internet. The D link using 192.168.0.0/24 subnet. I configured other subnets behind the cisco router. I managed to use NAT to be able to ping the D Link router but could never be able to ping anythin on the internet. All my other subnets behind the cisco

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  2. Good question, there are quite some differences.

    The reflexive access-list can match on L2-L4 attributes, just like the normal extended access-list. It’s quite “dumb” since the only thing it does is track the outgoing traffic and creating an access-list entry automatically that reverses the source / destination IP and port numbers. This works for traffic like HTTP but not for applications with dynamic port numbers.

    CBAC is a lot smarter, it can match up to L7 attributes and supports a wide range of protocols. The reflexive access-list and CBAC are both configur

    ... Continue reading in our forum

  3. Hi Nabeel,

    The short answer…not very well. Reflexive ACLs just create a temporary permit statement that is the opposite of the outgoing traffic. If you want to use this you’ll need to use passive FTP.

    If you only can use active FTP, take a look at CBAC.

    René

  4. Do relexive access lists imply that by default, Cisco’s ACL’s are not stateful?

  5. Hi,
    What is the difference between a reflexive ACL and normal extended access-list with established attribute?
    When I should use the first one instead the other one?
    Thank you

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