IP Access List Overview
IP Access List Overview
Last Updated: May 14, 2011
Access control lists (ACLs) perform packet filtering to control which packets move through the network and where. Such control provides security by helping to limit network traffic, restrict the access of users and devices to the network, and prevent traffic from leaving a network. IP access lists can reduce the chance of spoofing and denial-of-service attacks and allow dynamic, temporary user access through a firewall.
IP access lists can also be used for purposes other than security, such as bandwidth control, restricting the content of routing updates, redistributing routes, triggering dial-on-demand (DDR) calls, limiting debug output, and identifying or classifying traffic for quality of service (QoS) features. This module provides an overview of IP access lists.
Information About IP Access Lists
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Benefits of IP Access Lists
Access control lists (ACLs) perform packet filtering to control which packets move through the network and where. Such control can restrict the access of users and devices to the network, providing a measure of security. Access lists can save network resources by reducing traffic. Access lists provide diverse benefits, depending on how they are used. Many of the benefits fall into the following categories:
Block Unwanted Traffic or Users
Access lists can filter incoming or outgoing packets on an interface, thereby controlling access based on source addresses, destination addresses, or user authentication. You can also use access lists to determine which types of traffic are forwarded or blocked at the router interfaces. For example, you can permit e-mail traffic to be routed, but at the same time block all Telnet traffic.
Reduce the Chance of DOS Attacks
There are a number of ways to reduce the chance of denial-of-service attacks. For example, by specifying IP source addresses, you can control whether traffic from hosts, networks, or users access your network. You can filter on specific time-to-live (TTL) values in packets to control how many hops a packet can take before reaching a router in your network. By configuring the TCP Intercept feature, you can prevent servers from being flooded with requests for a connection.
Control Access to Virtual Terminal Lines
You can place an access list on inbound vty (Telnet) line access from certain nodes or networks. You can also place an access list on outbound vty access, blocking or permitting Telnet access to other devices.
Restrict the Content of Routing Updates
Access lists can control routing updates being sent, received, or redistributed.
Identify or Classify Traffic for QoS Features
Access lists can provide congestion avoidance by setting IP precedence for WRED or CAR. It can provide congestion management for class-based weighted fair queuing (WFQ), priority queuing, and custom queuing.
Provide NAT Control
Access lists can control which addresses are translated by Network Address Translation (NAT).
Authenticate Incoming RSH and RCP Requests
To enable the Cisco IOS software to receive incoming remote shell (rsh) protocol and remote copy (rcp) protocol requests, customers must configure an authentication database to control access to the router. Access lists can simplify the identification of local users, remote hosts, and remote users in the database authentication configuration.
Border Routers and Firewall Routers Should Use Access Lists
There are many reasons to configure access lists; for example, you can use access lists to restrict contents of routing updates or to provide traffic flow control. One of the most important reasons to configure access lists is to provide a basic level of security for your network by controlling access to it. If you do not configure access lists on your router, all packets passing through the router could be allowed onto all parts of your network.
An access list can allow one host to access a part of your network and prevent another host from accessing the same area. In the figure below, by applying an appropriate access list to the interfaces of the router, Host A is allowed to access the Human Resources network and Host B is prevented from accessing the Human Resources network.
Access lists should be used in firewall routers, which are often positioned between your internal network and an external network such as the Internet. You can also use access lists on a router positioned between two parts of your network, to control traffic entering or exiting a specific part of your internal network.
To provide some security benefits of access lists, you should at least configure access lists on border routers--routers located at the edges of your networks. Such an access list provides a basic buffer from the outside network or from a less controlled area of your own network into a more sensitive area of your network. On these border routers, you should configure access lists for each network protocol configured on the router interfaces. You can configure access lists so that inbound traffic or outbound traffic or both are filtered on an interface.
Access lists are defined on a per-protocol basis. In other words, you should define access lists for every protocol enabled on an interface if you want to control traffic flow for that protocol.
Definition of an Access List
An access list is a sequential list consisting of at least one permit statement and possibly one or more deny statements. In the case of IP access lists, the statements can apply to IP addresses, upper-layer IP protocols, or other fields in IP packets. The access list is identified and referenced by a name or a number. The access list acts as a packet filter, filtering packets based on the criteria defined in the access list.
An access list may be configured, but it does not take effect until the access list is either applied to an interface (with the ip access-group command), a virtual terminal line (vty) (with the access-classcommand), or referenced by some other command that accepts an access list. Access lists have many uses, and therefore many Cisco IOS software commands accept a reference to an access list in their command syntax. Multiple commands can reference the same access list.
In the following configuration excerpt, the first three lines are an example of an IP access list named branchoffices, which is applied to serial interface 0 on incoming packets. No sources other than those on the networks specified by each source address and mask pair can access this interface. The destinations for packets coming from sources on network 172.20.7.0 are unrestricted. The destination for packets coming from sources on network 172.29.2.0 must be 172.25.5.4.
ip access-list extended branchoffices 10 permit 172.20.7.0 0.0.0.3 any 20 permit 172.29.2.0 0.0.0.255 host 172.25.5.4 ! interface serial 0 ip access-group branchoffices in
Software Processing of an Access List
The following general steps describe how the Cisco IOS software processes an access list when it is applied to an interface, a vty, or referenced by some other Cisco IOS command. These steps apply to an access list that has 13 or fewer access list entries.
In later Cisco IOS releases such as Release 12.4, 12.2S, and 12.0S, by default, an access list that has more than 13 access list entries is processed differently from one that has 13 or fewer entries. In order to be more efficient, an access list with more than 13 entries is processed using a trie-based lookup algorithm. This process will happen automatically; it does not need to be configured.
Access List Rules
Keep the following rules and characteristics of access lists in mind when creating one:
Helpful Hints for Creating IP Access Lists
The following tips will help you avoid unintended consequences and help you create more efficient, useful access lists.
Named or Numbered Access Lists
All access lists must be identified by a name or a number. Named and numbered access lists have different command syntax. Named access lists are compatible with Cisco IOS Release 11.2 and later. Named access lists are more convenient than numbered access lists because you can specify a meaningful name that is easier to remember and associate with a purpose. You may reorder statements in or add statements to a named access list.
Named access list are newer than numbered access lists and support the following features that are not supported in numbered access lists:
Not all commands that accept a numbered access list will accept a named access list. For example, virtual terminal lines use only numbered access lists.
Standard or Extended Access Lists
All access lists are either standard or extended access lists. If you only intend to filter on a source address, the simpler standard access list is sufficient. For filtering on anything other than a source address, an extended access list is necessary.
Standard Access Lists
Standard IP access lists test only source addresses of packets (except for two exceptions). Because standard access lists test source addresses, they are very efficient at blocking traffic close to a destination. There are two exceptions when the address in a standard access list is not a source address:
Extended Access Lists
Extended access lists are good for blocking traffic anywhere. Extended access lists test source and destination addresses and other IP packet data, such as protocols, TCP or UDP port numbers, type of service (ToS), precedence, TCP flags, IP options, and TTL value. Extended access lists can also provide capabilities that standard access lists cannot, such as the following:
IP Packet Fields You Can Filter to Control Access
You can use an extended access list to filter on any of the following fields in an IP packet. Source address and destination address are the two most frequently specified fields on which to base an access list:
Wildcard Mask for Addresses in an Access List
Address filtering uses wildcard masking to indicate to the software whether to check or ignore corresponding IP address bits when comparing the address bits in an access list entry to a packet being submitted to the access list. By carefully setting wildcard masks, you can specify one or more IP addresses for permit or deny tests.
Wildcard masking for IP address bits uses the number 1 and the number 0 to specify how the software treats the corresponding IP address bits. A wildcard mask is sometimes referred to as an inverted mask because a 1 and 0 mean the opposite of what they mean in a subnet (network) mask.
If you do not supply a wildcard mask with a source or destination address in an access list statement, the software assumes an implicit wildcard mask of 0.0.0.0, meaning all values must match.
Unlike subnet masks, which require contiguous bits indicating network and subnet to be ones, wildcard masks allow noncontiguous bits in the mask.
The table below shows examples of IP addresses and masks from an access list, along with the corresponding addresses that are considered a match.
Access List Sequence Numbers
The ability to apply sequence numbers to IP access list entries simplifies access list changes. Prior to the IP Access List Entry Sequence Numbering feature, there was no way to specify the position of an entry within an access list. If you wanted to insert an entry in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.
This feature allows users to add sequence numbers to access list entries and resequence them. When you add a new entry, you specify the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry.
Access List Logging
The Cisco IOS software can provide logging messages about packets permitted or denied by a single standard or extended IP access list entry. That is, any packet that matches the entry will cause an informational logging message about the packet to be sent to the console. The level of messages logged to the console is controlled by the logging console global configuration command.
The first packet that triggers the access list entry causes an immediate logging message, and subsequent packets are collected over 5-minute intervals before they are displayed or logged. The logging message includes the access list number, whether the packet was permitted or denied, the source IP address of the packet, and the number of packets from that source permitted or denied in the prior 5-minute interval.
However, you can use the ip access-list log-update command to set the number of packets that, when match an access list (and are permitted or denied), cause the system to generate a log message. You might want to do this to receive log messages more frequently than at 5-minute intervals.
Even if you use the ip access-list log-update command, the 5-minute timer remains in effect, so each cache is emptied at the end of 5 minutes, regardless of the count of messages in each cache. Regardless of when the log message is sent, the cache is flushed and the count reset to 0 for that message the same way it is when a threshold is not specified.
Alternative to Access List Logging
Packets matching an entry in an ACL with a log option are process switched. It is not recommended to use the log option on ACLs, but rather use NetFlow export and match on a destination interface of Null0. This is done in the CEF path. The destination interface of Null0 is set for any packet that is dropped by the ACL.
Additional IP Access List Features
Beyond the basic steps to create a standard or extended access list, you can enhance your access lists as mentioned below. Each of these methods is described completely in the module entitled âRefining an Access List.â
Time-Based and Distributed Time-Based Access Lists
Time-based access lists implement access list entries based on particular times of the day or week. This is an advantage when you donât want access list entries always in effect or in effect as soon as they are applied. Use time-based access lists to make the enforcement of permit or deny conditions granular, based on time and date.
Distributed time-based access lists are those that are supported on line cards for the Cisco 7500 series routers. Packets destined for an interface configured with time-based access lists are distributed switched through the line card.
Types of IP Access Lists
There are several types of access lists that are distinct because of how they are triggered, their temporary nature, or how their behavior differs from an ordinary access list.
Authentication proxy provides dynamic, per-user authentication and authorization, authenticating users against industry standard TACACS+ and RADIUS authentication protocols. Authenticating and authorizing connections by users provides more robust protection against network attacks.
Context-Based Access Control
Context-based access control (CBAC) examines not only network layer and transport layer information, but also the application-layer protocol information (such as FTP information) to learn about the state of TCP and UDP connections. CBAC maintains connection state information for individual connections. This state information is used to make intelligent decisions about whether packets should be permitted or denied, and dynamically creates and deletes temporary openings in the firewall.
Dynamic Access Lists with the Lock-and-Key Feature
Dynamic access lists provide temporary access to designated users who are using Telnet to reach designated hosts through a firewall. Dynamic access lists involve user authentication and authorization.
Reflexive Access Lists
Reflexive access lists provide filtering on upper-layer IP protocol sessions. They contain temporary entries that are automatically created when a new IP session begins. They are nested within extended, named IP access lists that are applied to an interface. Reflexive access lists are typically configured on border routers, which pass traffic between an internal and external network. These are often firewall routers. Reflexive access lists do not end with an implicit deny statement because they are nested within an access list and the subsequent statements need to be examined.
Where to Apply an Access List
If you are applying an access list to an interface, carefully consider whether to specify it as in (inbound) or out (outbound). Applying an access list to an incoming or outgoing interface controls the traffic that will enter or leave the routerâs interface or process level (in the case of filtering on TTL values).
Access lists can be used in ways other than applying them to interfaces. The following are additional places to apply an access list.
Where to Go Next
You must first decide what you want to restrict, and then select the type of access list that achieves your goal. Next, you will create an access list that permits or denies packets based on values in the fields you specify, and finally, you will apply the access list (which determines its placement).
Assuming you have decided what you want to restrict and what type of access list you need, your next step is to create an access list. Creating an access list based on source address, destination address, or protocol is described in the âCreating an IP Access List and Applying It to an Interfaceâ module. You could create an access list that filters on other fields, as described in âCreating an IP Access List to Filter IP Options, TCP Flags, Noncontiguous Ports, or TTL Values.â If you want to control access to a virtual line, see âControlling Access to a Virtual Terminal Line.â If the purpose of your access list is to control routing updates or QoS features, for example, see the appropriate technology chapter.
Feature Information for IP Access List Overview
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.