Access Control Lists (ACLs)

Chapter 11 (Access Control Lists (ACLs))

Overview
Network administrators must be able to deny unwanted access to a network and allow authorized users to access necessary services. Security tools such as passwords, callback equipment, and physical security devices are helpful. However, they often lack the flexibility of basic traffic filters and the specific controls that most administrators prefer. For example, a network administrator may want to allow users access to the Internet, but not permit external users Telnet access into the LAN.
Routers provide the capability to filter traffic, such as blocking Internet traffic, with access control lists (ACLs). An ACL is a sequential list of permit or deny statements that apply to addresses or upper-layer protocols. This module will introduce standard and extended ACLs as a way to control network traffic and explain how they are used as part of a security solution.
This module includes tips, considerations, recommendations, and general guidelines on how to use ACLs. It also includes the commands and configurations needed to create ACLs. Finally, this module provides examples of standard and extended ACLs and describes ACL placement on router interfaces.
An ACL can be as simple as a single line that permits packets from a specific host or it can be a complex set of rules and conditions that defines network traffic and determines the router processes. While many of the advanced uses of ACLs are beyond the scope of this course, this module provides details about standard and extended ACLs, the proper placement of ACLs, and some special applications of ACLs.


This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.


Students who complete this module should be able to perform the following tasks:
• Describe the differences between standard and extended ACLs
• Explain the rules for placement of ACLs
• Create and apply named ACLs
• Describe the function of firewalls
• Use ACLs to restrict virtual terminal access



11.1 Access Control List Fundamentals
11.1.1 Introduction to ACLs
This page will explain what ACLs are and how they are used.


ACLs are lists of conditions used to test network traffic that tries to travel across a router interface. These lists tell the router what types of packets to accept or deny. Acceptance and denial can be based on specified conditions. ACLs enable management of traffic and secure access to and from a network.
ACLs can be created for all routed network protocols such as IP and Internetwork Packet Exchange (IPX). ACLs can be configured at the router to control access to a network or subnet.
To filter network traffic, ACLs determine if routed packets are forwarded or blocked at the router interfaces. The router examines each packet and will forward or discard it based on the conditions specified in the ACL. An ACL makes routing decisions based on source address, destination address, protocols, and upper-layer port numbers.
ACLs must be defined on a per protocol, per direction, or per port basis. To control traffic flow on an interface, an ACL must be defined for each protocol enabled on the interface. ACLs control traffic in one direction at a time on an interface. Two separate ACLs must be created to control inbound and outbound traffic. Every interface can have multiple protocols and directions defined. If the router has two interfaces configured for IP, AppleTalk, and IPX, 12 separate ACLs would be needed. There would be one ACL for each protocol, times two for each direction, times two for the number of ports.






ACLs can be used to perform the following tasks:
• Limit network traffic and increase network performance. For example, ACLs that restrict video traffic could greatly reduce the network load and increase network performance.
• Provide traffic flow control. ACLs can restrict the delivery of routing updates. If updates are not required because of network conditions, bandwidth is preserved.
• Provide a basic level of security for network access. ACLs can allow one host to access a part of the network and prevent another host from accessing the same area. For example, Host A is allowed to access the Human Resources network and Host B is prevented from accessing it.
• Decide which types of traffic are forwarded or blocked at the router interfaces. ACLs can permit e-mail traffic to be routed, but block all Telnet traffic.
• Control which areas a client can access on a network.
• Screen hosts to permit or deny access to a network segment. ACLs can be used to permit or deny a user to access file types such as FTP or HTTP.
If ACLs are not configured on the router, all packets that pass through the router will be permitted to access the entire network.
The next page will describe how ACLs work.
11.1.2 How ACLs work
An ACL is made up of statements that define whether packets are accepted or rejected at inbound and outbound interfaces. This page will explain how these statements are edited and added to an ACL. These decisions are made by matching a condition statement in an access list and then performing the accept or reject action defined in the statement.






The order in which ACL statements are placed is important. The Cisco IOS software tests the packet against each condition statement in order from the top of the list to the bottom. Once a match is found in the list, the accept or reject action is performed and no other ACL statements are checked. If a condition statement that permits all traffic is located at the top of the list, no statements added below that will ever be checked.
If additional condition statements are needed in an access list, the entire ACL must be deleted and recreated with the new condition statements. To make the process of revising an ACL simpler it is a good idea to use a text editor such as Notepad and paste the ACL into the router configuration.









The beginning of the router process is the same, whether ACLs are used or not. As a frame enters an interface, the router checks to see whether the Layer 2 address matches or if it is a broadcast frame. If the frame address is accepted, the frame information is stripped off and the router checks for an ACL on the inbound interface. If an ACL exists, the packet is now tested against the statements in the list. If the packet matches a statement, the packet is either accepted or rejected. If the packet is accepted in the interface, it will then be checked against routing table entries to determine the destination interface and switched to that interface. Next, the router checks whether the destination interface has an ACL. If an ACL exists, the packet is tested against the statements in the list. If the packet matches a statement, it is either accepted or rejected. If there is no ACL or the packet is accepted, the packet is encapsulated in the new Layer 2 protocol and forwarded out the interface to the next device.
As a review, ACL statements operate in sequential, logical order. If a condition match is true, the packet is permitted or denied and the rest of the ACL statements are not checked. If all the ACL statements are unmatched, an implicit deny any statement is placed at the end of the list by default. The invisible deny any statement at the end of the ACL will not allow unmatched packets to be accepted. When first learning how to create ACLs, it is a good idea to add the deny any at the end of ACLs to reinforce the dynamic presence of the implicit deny.
The next page will describe how ACLs are created.
11.1.3 Creating ACLs
This page will explain how ACLs are created in global configuration mode. There are many types of ACLs. This lesson explains standard ACLs, extended ACLs, and named ACLs. When ACLs are configured on a router, each ACL must have a unique identification number assigned to it. This number identifies the type of access list created and must fall within the specific range of numbers that is valid for that type of list.





After the proper command mode is entered and the list type number is decided upon, the user enters the access list statements using the keyword access-list, followed by the proper parameters. After the proper command mode is entered and the list type number is set, the user enters the access list statements with the access-list command followed by the proper parameters. This is the first of the two-step process. The second step of the process is assigning the ACL to the proper interface.
In TCP/IP, ACLs are assigned to one or more interfaces and can filter inbound traffic or outbound traffic by using the ip access-group command in interface configuration mode. The access-group command is issued in the interface configuration mode. When an ACL is assigned to an interface, inbound or outbound placement should be specified. The filter direction can be set to check packets that travel into or out of an interface. To determine if an ACL controls inbound or outbound traffic, the network administrator must view the interfaces as if looking at them from inside the router. This is a very important concept. Traffic that travels into an interface is filtered by the inbound access list. Traffic going out of an interface is filtered by the outbound access list. After a numbered ACL is created, it must be assigned to an interface. An ACL containing numbered ACL statements cannot be altered. It must be deleted by using the no access-list list-number command and then recreated.






Use the following rules to create and apply access lists:
• There should be one access list per protocol per direction.
• Standard access lists should be applied closest to the destination.
• Extended access lists should be applied closest to the source.
• The inbound or outbound interface should be referenced as if looking at the port from inside the router.
• Statements are processed sequentially from the top of the list to the bottom until a match is found. If no match is found then the packet is denied, and discarded.
• There is an implicit deny any at the end of all access lists. This will not appear in the configuration listing.
• Access list entries should filter in the order from specific to general. Specific hosts should be denied first, and groups or general filters should come last.
• The match condition is examined first. The permit or deny is examined only if the match is true.
• Never work with an access list that is actively applied.
• A text editor should be used to create comments that outline the logic. Then fill in the statements that perform the logic.
• New lines are always added to the end of the access list. A no access-list x command will remove the whole list. It is not possible to selectively add and remove lines with numbered ACLs
• An IP access list will send an ICMP host unreachable message to the sender of the rejected packet and will discard the packet in the bit bucket.
• An access list should be removed carefully. If an access list that is applied to a production interface is removed, some versions of IOS will apply a default deny any to the interface and all traffic will be halted.
• Outbound filters do not affect traffic that originates from the local router.


The Lab Activity will help students become more familiar with the syntax that is used to create an ACL.
The next page will discuss wildcard masks.




11.1.4 The function of a wildcard mask
This page will explain what a wildcard mask is and how it is used.













A wildcard mask is a 32-bit quantity that is divided into four octets. A wildcard mask is paired with an IP address. The numbers one and zero in the mask are used to identify how to treat the corresponding IP address bits. The term wildcard mask represents the ACL mask-bit matching process and comes from an analogy of a wildcard that matches any other card in the game of poker. Wildcard masks have no functional relationship with subnet masks. They are used for different purposes and follow different rules.
The subnet mask and the wildcard mask represent two different things when they are compared to an IP address. Subnet masks use binary ones and zeros to identify the network, subnet, and host portion of an IP address. Wildcard masks use binary ones and zeros to filter individual or groups of IP addresses to permit or deny access to resources based on an IP address. The only similarity between a wildcard mask and a subnet mask is that they are both thirty-two bits long and use binary ones and zeros.














The mask in Figure would be written as 0.0.255.255. A zero indicates a value that will be checked. The Xs, or ones, are used to block values.
In the wildcard mask process, the IP address in the access-list statement has the wildcard mask applied to it. This creates the match value, which is used to compare and see if a packet should be processed by this ACL statement, or sent to the next statement to be checked. The second part of the ACL process is that any IP address that is checked by a particular ACL statement will have the wildcard mask of that statement applied to it. The result of the IP address and the wildcard mask must equal the match value of the ACL. This process is illustrated in the animation in Figure .






























































There are two special keywords that are used in ACLs, the any and host options. The any option substitutes 0.0.0.0 for the IP address and 255.255.255.255 for the wildcard mask. This option will match any address that it is compared against. The host option substitutes 0.0.0.0 for the mask. This mask requires that all bits of the ACL address and the packet address match. This option will match just one address.



The next page will teach students how to verify ACLs.
11.1.5 Verifying ACLs
This page will explain how show commands can be used to verify the content and placement of ACLs on a router.




The show ip interface command displays IP interface information and indicates whether any ACLs are assigned to the interface. The show access-lists command displays the contents of all ACLs on the router. To see a specific list, add the ACL name or number as an option for this command. The show running-config command will also reveal the access lists on a router and the interface assignment information.





















These show commands will verify the list contents and placement. It is also a good practice to test the access lists with sample traffic to ensure that the access list logic is correct.
In the Lab Activity, students will use show commands to verify ACLs on a router.
This page concludes this lesson. The next lesson will provide more information about ACLs. The first page will describe standard ACLs.
11.2 Access Control Lists (ACLs)
11.2.1 Standard ACLs
This page will explain the function of standard ACLs. Students will also learn the syntax used for a standard ACL.















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Standard ACLs check the source address of IP packets that are routed. The ACL will either permit or deny access for an entire protocol suite, based on the network, subnet, and host addresses. For example, packets that come in Fa0/0 are checked for their source addresses and protocols. If they are permitted, the packets are routed through the router to an output interface. If they are not permitted, they are dropped at the incoming interface.


The standard version of the access-list global configuration command is used to define a standard ACL with a number in the range of 1 to 99 (also from 1300 to 1999 in recent IOS). In Cisco IOS Software Release 12.0.1, standard ACLs began using additional numbers (1300 to 1999) to provide a maximum of 798 possible standard ACLs. These additional numbers are referred to as expanded IP ACLs. In the first ACL statement, notice that there is no wildcard mask. Since no list is shown, the default mask of 0.0.0.0 is used. The entire address must match or the router must check for a match in the next line in the ACL.
The full syntax of the standard ACL command is as follows:
Router(config)#access-listaccess-list-number deny permit remarksource [source-wildcard ] [log]
The no form of this command is used to remove a standard ACL. The remark keyword makes the access list easier to understand. Each remark is limited to 100 characters. For example, it is not immediately clear what the purpose of the following entry is:
Router(config)#access-list 1 permit 171.69.2.88
It is much easier to read a remark about the entry to understand its effect, as follows:
Router(config)#access-list 1 remark Permit only Jones workstation through access-list 1 permit 171.69.2.88
To remove a standard ACL, use the no form of the command. This is the syntax:
Router(config)#no access-listaccess-list-number
The no form of this command is used to remove a standard ACL. The syntax is as follows:
Router(config)#no access-list access-list-number
The ip access-group command links an existing standard ACL to an interface:
Router(config)#ip access-group {access-list-number | access-list-name } {in | out }
The table shows descriptions of the parameters used in this syntax.

















The Lab Activities will teach students how to plan, configure, and apply standard ACLs to permit or deny traffic.
The next page will describe extended ACLs.
11.2.2 Extended ACLs
This page will provide an overview of extended ACLs.
Extended ACLs are used more often than standard ACLs because they provide a greater range of control. Extended ACLs check the source and destination packet addresses and can also check for protocols and port numbers. This gives greater flexibility to describe what the ACL will check. Access can be permitted or denied based on where a packet originates, its destination, protocol type, and port addresses. An extended ACL can simultaneously allow e-mail traffic from Fa0/0 to specific S0/0 destinations and deny file transfers and Web browsing. When packets are discarded, some protocols send an echo packet to the sender, stating that the destination was unreachable.











For a single ACL, multiple statements may be configured. Each statement should have the same access list number, to relate the statements to the same ACL. There can be as many condition statements as needed, limited only by the available router memory. Of course, the more statements there are, the more difficult it will be to comprehend and manage the ACL.



The syntax for the extended ACL statement can get very long and often will wrap in the terminal window. The wildcards also have the option of using the host or any keywords in the command.
At the end of the extended ACL statement, an administrator can specify a TCP or UDP port number. The well-known port numbers for TCP/IP are shown in Figure . Logical operations may be specified such as, equal (eq), not equal (neq), greater than (gt), and less than (lt). The extended ACL will perform these operations on specific protocols. Extended ACLs use an access-list-number in the range 100 to 199 (also from 2000 to 2699 in recent IOS). In Cisco IOS Software Release 12.0.1, extended ACLs began using additional numbers (2000 to 2699) to provide a maximum of 799 possible extended ACLs. These additional numbers are referred to as expanded IP ACLs.






















































































































































The ip access-group command links an existing extended ACL to an interface. Remember that only one ACL per interface, per direction, per protocol is allowed. The format of the command is as follows:
Router(config-if)#ip access-group access-list-number {in | out }
The Lab Activities on this page will help students plan, configure, and apply extended ACLs to filter network traffic.




The next page will describe named ACLs.



11.2.3 Named ACLs
This page will explain the benefits and restrictions of named ACLs.


IP named ACLs were introduced in Cisco IOS Software Release 11.2. Named ACLs allow standard and extended ACLs to be given names instead of numbers. The following are advantages that are provided by a named access list:
• Alphanumeric names can be used to identify ACLs.
• The IOS does not limit the number of named ACLs that can be configured.
• Named ACLs provide the ability to modify ACLs without deletion and reconfiguration. However, a named access list will only allow for statements to be inserted at the end of a list. It is a good idea to use a text editor to create named ACLs.
Consider the following before implementing named ACLs.
Named ACLs are not compatible with Cisco IOS releases prior to Release 11.2.
The same name may not be used for multiple ACLs. For example, it is not permissible to specify both a standard and extended ACL named George.
It is important to be aware of named access lists because of the advantages just discussed. Advanced access list operations such as named ACLs will be presented in the CCNP curriculum.















































A named ACL is created with the ip access-list command. This places the user in the ACL configuration mode. In ACL configuration mode, specify one or more conditions to be permitted or denied. This determines whether the packet is passed or dropped when the ACL statement matches.






The configuration in Figure creates a standard ACL named Internetfilter and an extended ACL named marketing_group. The figure also shows how the named access lists are applied to an interface.
The Lab Activities on this page will show students how to create named ACLs to control network traffic.
The next page will discuss the placement of ACLs.






11.2.4 Placing ACLs
This page will explain where an ACL should be placed. The placement of ACLs is an important consideration.
Proper ACL placement will filter traffic and make the network more efficient. The ACL should be placed where it has the greatest impact on efficiency.


In Figure the administrator wants to deny Telnet or FTP traffic from the Router A Ethernet LAN segment to the switched Ethernet LAN Fa0/1 on Router D. At the same time, other traffic must be permitted. There are several ways to do this. The recommended solution is an extended ACL that specifies both source and destination addresses. Place this extended ACL in Router A. Then, packets do not cross the Router A Ethernet segment or the serial interfaces of Routers B and C, and do not enter Router D. Traffic with different source and destination addresses will still be permitted.
The general rule is to put the extended ACLs as close as possible to the source of the traffic denied. Standard ACLs do not specify destination addresses, so they should be placed as close to the destination as possible. For example, a standard ACL should be placed on Fa0/0 of Router D to prevent traffic from Router A.
Administrators can only place access lists on devices that they control. Therefore access list placement must be determined in the context of where the network administrator's control extends.
The Interactive Media Activity will teach students where to place ACLs.
The next page will discuss firewalls.
11.2.5 Firewalls
This page will explain how ACLs are used in firewall routers.
A firewall is an architectural structure that exists between the user and the outside world to protect the internal network from intruders. In most circumstances, intruders come from the global Internet and the thousands of remote networks that it interconnects. Typically, a network firewall consists of several different machines that work together to prevent unwanted and illegal access.


In this architecture, the router that is connected to the Internet, referred to as the exterior router, forces all incoming traffic to go to the application gateway. The router that is connected to the internal network, the interior router, accepts packets only from the application gateway. The gateway controls the delivery of network-based services both into and from the internal network. For example, only certain users might be allowed to communicate with the Internet, or only certain applications might be permitted to establish connections between an interior and exterior host. If the only application that is permitted is e-mail, then only e-mail packets should be allowed through the router. This protects the application gateway and avoids overwhelming it with packets that it would otherwise discard.
ACLs should be used in firewall routers, which are often positioned between the internal network and an external network, such as the Internet. This allows control of traffic entering or exiting a specific part of the internal network. The firewall router provides a point of isolation so that the rest of the internal network structure is not affected.
A configuration of ACLs on border routers, which are routers situated on the boundaries of the network, is necessary to provide security benefits. This provides basic security from the outside network, or from a less controlled area of the network, into a more private area of the network. On these border routers, ACLs can be created for each network protocol configured on the router interfaces.
The next page will describe virtual ports.
11.2.6 Restricting virtual terminal access
This page will explain how ACLs are created for virtual ports.
Standard and extended access lists apply to packets that travel through a router. They are not designed to block packets that originate within the router. An outbound Telnet extended access list does not prevent router initiated Telnet sessions, by default.


Just as there are physical ports or interfaces, such as Fa0/0 and S0/0 on the router, there are also virtual ports. These virtual ports are called vty lines. There are five vty lines, which are numbered 0 through 4, as shown in Figure . For security purposes, users can be denied or permitted virtual terminal access to the router but denied access to destinations from that router.
The purpose of restricted vty access is increased network security. The Telnet protocol can also be used to create a nonphysical vty connection to the router. There is only one type of vty access list. Identical restrictions should be placed on all vty lines since it is not possible to control the line on which a user will connect.
The process to create the vty access list is the same as described for an interface. However, applying the ACL to a terminal line requires the access-class command instead of the access-group command.
The following should be considered when configuring access lists on vty lines:
• A name or number can be used to control access to an interface.
• Only numbered access lists can be applied to virtual lines.
• Identical restrictions should be set on all the virtual terminal lines, because a user can attempt to connect to any of them.


In the second Lab Activity, students will use ACLs to control IP traffic.
This page concludes this lesson. The next page will summarize the main points from this module.
Summary
This page summarizes the topics discussed in this module.


ACLs are lists of conditions that are applied to traffic that travels across a router interface. They can be created for all routed network protocols such as IP and IPX. Packets are accepted or denied based on these lists.
Network administrators create ACLs to control network access. ACLs provide the ability to limit network traffic, increase performance, and manage security issues. ACL statements operate in sequential, logical order. When a condition is matched as true, the packet is permitted or denied and the rest of the ACL statements are not checked. If all the ACL statements are unmatched, an implicit deny any statement is placed at the end of the list by default. The invisible deny any statement at the end of the ACL will not allow unmatched packets to be accepted. When first learning how to create ACLs, it is a good idea to add the deny any at the end of ACLs to reinforce the dynamic presence implicit deny.
ACLs are created in the global configuration mode and the basic rules should be applied. Each ACL on a router must be configured with a unique number or a name. When a numbered ACL is used, the number identifies the type of access list. Numbered ACLs may be either standard or extended, and must fall within the specific range of numbers that is valid for that type of list . Standard IP ACLs use the numbers from 1 to 99. Extended IP ACLs use the numbers from 100 to 199. ACLs are created by entering the command access-list. Once created, the list is then assigned to the proper interface.
The placement of an ACL has a great impact on network efficiency. The general rule is to put the extended ACLs as close as possible to the source of the traffic denied. Standard ACLs do not specify destination addresses, so they should be placed as close to the destination as possible.
A wildcard mask is a 32-bit quantity that is divided into four octets. The numbers one and zero in the mask are used to determine the treatment of the corresponding IP address bits. In the wildcard mask process, the IP address in the access-list statement has the wildcard mask applied to it. This creates the match value, which compares the two and determines whether the packet should be processed by this ACL statement, or sent to the next statement to be checked.
The show ip interface command displays IP interface information and indicates whether any ACLs are set. The show access-lists command displays the contents of all ACLs on the router. To see a specific list, add the ACL name or number as an option for this command. The show running-config command will also display the access lists on a router and the interface assignment information.
Standard ACLs check the source IP address of packets that are routed. The ACL will permit or deny access based on the network, subnet, and host address. Extended ACLs are used more often than standard ACLs because they provide a greater range of control. Extended ACLs check the source and destination packet addresses and can also check for protocols and port numbers. A named ACL may be either an extended or standard ACL. Named ACLs provide the ability to modify ACLs without deleting and then reconfiguring them. A named access list will allow the deletion of statements but will only allow for statements to be inserted at the end of a list.