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Advanced ACL Configuration

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When working with network devices, one of the simplest methods of controlling traffic is using access control lists (ACL). This article extends on the ‘Basic Access List Configuration’ (insert link) article that covered the basics and how to configure a basic access list. This article reviews the different advanced options that exist when implementing ACLs, including lists using both source and destination address, reflexive ACLs, and time-basic ACLs.
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When working with network devices, one of the simplest methods of controlling traffic is using access control lists (ACL). This article extends on the ‘Basic Access List Configuration’ article that covered the basics and how to configure a basic access list. This article reviews the different advanced options that exist when implementing ACLs, including lists using both source and destination address, reflexive ACLs and time-basic ACLs.

Overview

When trying to control network traffic, it is not often possible to do this with only the source address; this is where more advanced extended ACLs come in. With an extended ACL, it becomes possible to use the source and destination address or network information, as well as a number of other pieces of information, to control traffic. This provides an ACL to be developed that can more specifically target the specific traffic to be filtered. Not only can the list be created to block a specific address or network, but it can be created to block traffic from a specific address or network going to a specific address or network. It is even possible with an extended ACL to define what protocol that is being permitted or denied. As with standard ACLs, there is a specific number range that is used to specify an extended access list; this range is from 100-199 and 2000-2699.

Other advanced ACLs exist (and will also be covered in this article); these include a review of reflexive ACLs and time based ACLs. Reflexive ACLs implement a form of session filtering that can be used to permit traffic back in through an outside interface that was originated inside a network. This is one of many solutions that Cisco offers in their IOS to limit unauthorized traffic into a networking device. Time-based ACLs work the same as other extended ACLs but provide a mechanism that applies the ACL within a specific timeframe. This option is useful in situations where access to the network is only permitted at specific times; for example, during the working day or during a specific event.

Access List Configuration

An extended access list not only provides the ability to match traffic based on the source address but also on a number of other criteria. As the space for this article is limited, the basic options available will be covered; if interested in the full command syntax, visit cisco.com. The command syntax of an extended ACL is as follows:

router(config)#access-list access-list-number {deny | permit} protocol {source source-wildcard |
 host hostname} {destination destination-wildcard | host hostname}
 [time-range time-range-name] [established]

or

router(config)#ip access-list extended {access-list-name}
router(config-ext-nacl)#[sequence-number] {permit | deny} protocol {source source-wildcard |
 host hostname} {destination destination-wildcard | host hostname}
 [time-range time-range-name] [established] [reflect name]
  • When creating an extended ACL, the access-list-number will always be from 100-199 or 2000-2699 as stated above.
  • The choice of the permit or deny action dictates what action is being taken.
  • The protocol parameter is used to set the protocol that is being filtered; the common values include ip, tcp, and udp.
  • The source parameter is the source IP network that is being matched by the ACL.
  • The source-wildcard parameter is used to affect the source addresses being matched by the ACL by masking off the target addresses with an inverse mask.
  • The host hostname parameter is used to match a specific host instead of a network.
  • The destination parameter is the destination IP network that is being matched by the ACL.
  • The destination-wildcard parameter is used to affect the destination addresses being matched by the ACL by masking off the target addresses with an inverse mask; the host hostname operates the same.
  • The time-range time-range-name parameter is used to link a preconfigured time-range with the ACL making it a time based ACL.
  • The established parameter is used to configure a generic form of session filtering when using TCP.
  • The reflect name parameter is used to create a reflexive access list; this parameter is only available with using named ACL configuration mode. This parameter is used along with the evaluate command as shown later in this article.

The second shown method of configuration uses the named ACL configuration; this method allows some additional functionality that does not exist with the original method including the ability to edit individual lines in an ACL. When using a numbered ACL, the whole list must be deleted and reentered in order to make a change.

In order for an ACL to be functional, it must be applied to a specific interface. When being applied, the access list is configured in a specific direction from the perspective of the interface with the options of coming into the interface (in) or going out of the interface (out). The general rule when applying access lists is to apply standard IP access lists as close to the destination as possible and to apply extended access lists as close to the source as possible. The reasoning for this rule is that standard access lists lack granularity; it is better to implement them as close to the destination as possible. Extended access lists have more potential granularity, thus they are better implemented close to the source. The command syntax to apply an ACL is as follows:

router(config-if)#ip access-group {access-list-number | access-list-name} {in | out}

Reflexive access lists allow traffic to be permitted based on an entry in a temporary table. The entries in this table are created when traffic is sourced from an “inside” interface and is going through an “outside” interface. Information about this session is entered into this table and return traffic is monitored. If the returning traffic matches the outbound information recorded in the table it is allowed in through the interface (“outside”); if it does not, it will be denied. For those who are aware of security terminology, a reflexive ACL works similarly to a stateful firewall.

Examples

Examples of configurations of both an extended ACL and a reflective ACL are shown below.

Extended ACL example

Figure 1 shows a simple network topology with a couple different networks.

In this example, an ACL is required to permit the traffic coming from the 10.10.10.0/24 network to the web server at IP address 172.16.1.100/24 on TCP port 80 but deny all other traffic going to the web server. Standard practice says to place an extended ACL as close to the source as possible; however, in this example, traffic is being blocked from several other sources as well that are off different interfaces. If the ACL would be applied at the source interface, the traffic from the other interfaces would not be affected.

The configuration required to setup the ACL in this example is as follows:

router(config)#access-list 100 permit tcp 10.10.10.0 0.0.0.255 host 172.16.1.100 eq 80

Since in this case the access list is supposed to block traffic from all other sources, no other configuration is required as access lists have an implicit deny at the end of them.

The next step would be to apply the ACL onto the correct interface in the appropriate direction. In this case, the best interface to apply the ACL would be on the F0/1 interface outbound. This would restrict all traffic from entering the 172.16.1.0/24 network destined for the web server that did not come from the 10.10.10.0/24 network.

The configuration required to apply the ACL is as follows:

router(config)#interface f0/1
router(config-if)#ip access-group 100 out

Reflexive ACL example

Figure 2 shows a simple network topology that includes an internal network with hosts that access the Internet.

In this example, the goal is to set up a reflexive ACL so that the only traffic that is allowed out the F0/1 interface is TCP traffic, and the only traffic allowed back into the F0/1 interface is TCP traffic that was initiated from the internal network.

The configuration required to setup the ACL in this example is as follows:

router(config)#ip access-list extended out-going
router(config-ext-nacl)#permit tcp any any reflect sample-reflection
router(config)#ip access-list extended in-coming
router(config-ext-nacl)#evaluate sample-reflection

Notice that a named ACL is being used; reflexive ACLs require that a named ACL be used for configuration. These ACLs must be applied to a specific interface; in this case they are both applied to the F0/1 interface.

The configuration required to apply the ACL is as follows:

router(config)#interface f0/1
router(config-if)#ip access-group out-going out
router(config-if)#ip access-group in-coming in

Summary

The options available for extended ACLs are quite numerous, and the examples discussed in this article only cover the basics. Hopefully, the content in this article will allow a good base of understanding. If you are interested in the other options available for extended ACLs, please visit cisco.com.

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