IP Access List Overview

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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:

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.

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

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.

• The software receives an IP packet and tests parts of each packet being filtered against the conditions in the access list, one condition (permit or deny statement) at a time. For example, the software tests the source and destination addresses of the packet against the source and destination addresses in a permit or denystatement.
• If a packet does not match an access list statement, the packet is then tested against the next statement in the list.
• If a packet and an access list statement match, the rest of the statements in the list are skipped and the packet is permitted or denied as specified in the matched statement. The first entry that the packet matches determines whether the software permits or denies the packet. That is, after the first match, no subsequent entries are considered.
• If the access list denies a packet, the software discards the packet and returns an ICMP Host Unreachable message.
• If no conditions match, the software drops the packet. This is because each access list ends with an unwritten, implicit deny statement. That is, if the packet has not been permitted by the time it was tested against each statement, it is denied.

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.

Keep the following rules and characteristics of access lists in mind when creating one:

• Only one access list per interface, per protocol, per direction is allowed.
• The access list must contain at least one permit statement or else all packets are denied.
• Because the software stops testing conditions after the first match, the order of the conditions is critical. The same permit or deny statements specified in a different order could result in a packet being passed under one circumstance and denied in another circumstance.
• If an access list is referenced by name in a command, but the access list does not exist, all packets pass. That is, an interface or command with an empty access list applied to it permits all traffic.
• Standard access lists and extended access lists cannot have the same name.
• Inbound access lists process packets arriving at the router. Incoming packets are processed before being routed to an outbound interface. An inbound access list is efficient because it saves the overhead of routing lookups if the packet is to be discarded because it is denied by the filtering tests. If the packet is permitted by the tests, it is then processed for routing. For inbound lists, permit means continue to process the packet after receiving it on an inbound interface; deny means discard the packet.
• Outbound access lists process packets before they leave the router. Incoming packets are routed to the outbound interface and then processed through the outbound access list. For outbound lists, permit means send it to the output buffer; deny means discard the packet.
• An access list can control traffic arriving at the router or leaving the router, but not traffic originating at the router.

The following tips will help you avoid unintended consequences and help you create more efficient, useful access lists.

• Create the access list before applying it to an interface (or elsewhere), because if you apply a nonexistent access list to an interface and then proceed to configure the access list, the first statement is put into effect, and the implicit deny statement that follows could cause you immediate access problems.
• Another reason to configure an access list before applying it is because an interface with an empty access list applied to it permits all traffic.
• All access lists need at least one permit statement; otherwise, all packets are denied and no traffic passes.
• Because the software stops testing conditions after it encounters the first match (to either a permit or deny statement), you will reduce processing time and resources if you put the statements that packets are most likely to match at the beginning of the access list. Place more frequently occurring conditions before less frequent conditions.
• Organize your access list so that more specific references in a network or subnet appear before more general ones.
• Use the statement permit any any if you want to allow all other packets not already denied. Using the statement permit any any in effect avoids denying all other packets with the implicit deny statement at the end of an access list. Do not make your first access list entry permit any any because all traffic will get through; no packets will reach the subsequent testing. In fact, once you specify permit any any, all traffic not already denied will get through.
• Although all access lists end with an implicit deny statement, we recommend use of an explicit deny statement (for example, deny ip any any). On most platforms, you can display the count of packets denied by issuing the show access-listcommand, thus finding out more information about who your access list is disallowing. Only packets denied by explicit deny statements are counted, which is why the explicit deny statement will yield more complete data for you.
• While you are creating an access list or after it is created, you might want to delete an entry.
  • You cannot delete an entry from a numbered access list; trying to do so will delete the entire access list. If you need to delete an entry, you need to delete the entire access list and start over.
  • You can delete an entry from a named access list. Use the no permitor no deny command to delete the appropriate entry.
• In order to make the purpose of individual statements more scannable and easily understood at a glance, you can write a helpful remark before or after any statement by using the remark command.
• If you want to deny access to a particular host or network and find out if someone from that network or host is attempting to gain access, include the log keyword with the corresponding deny statement so that the packets denied from that source are logged for you.
• This hint applies to the placement of your access list. When trying to save resources, remember that an inbound access list applies the filter conditions before the routing table lookup. An outbound access list applies the filter conditions after the routing table lookup.

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:

• TCP flag filtering
• IP option filtering
• noncontiguous ports
• reflexive access lists
• ability to delete entries with the no permit or no deny command

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.

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.

• Named access lists are specified as standard or extended based on the keyword standard or extended in the ip access-list command syntax.
• Numbered access lists are specified as standard or extended based on their number in the access-list command syntax. Standard IP access lists are numbered 1 to 99 or 1300 to 1999; extended IP access lists are numbered 100 to 199 or 2000 to 2699. The range of standard IP access lists was initially only 1 to 99, and was subsequently expanded with the range 1300 to 1999 (the intervening numbers were assigned to other protocols). The extended access list range was similarly expanded.

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:

• Source address--Specifies a source address to control packets coming from certain networking devices or hosts.
• Destination address--Specifies a destination address to control packets being sent to certain networking devices or hosts.
• Protocol--Specifies an IP protocol indicated by the keyword eigrp, gre, icmp, igmp, ip, ipinip, nos, ospf, tcp, or udp, or indicated by an integer in the range from 0 to 255 (representing an Internet protocol). If you specify a transport layer protocol (icmp, igmp, tcp, or udp), the command has a specific syntax.
  • Ports and non-contiguous ports--Specifies TCP or UDP ports by a port name or port number. The port numbers can be noncontiguous port numbers. Port numbers can be useful to filter Telnet traffic or HTTP traffic, for example.
  • TCP flags--Specifies that packets match any flag or all flags set in TCP packets. Filtering on specific TCP flags can help prevent false synchronization packets.
• IP options--Specifies IP options; one reason to filter on IP options is to prevent routers from being saturated with spurious packets containing them.

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.

• A wildcard mask bit 0 means check the corresponding bit value; they must match.
• A wildcard mask bit 1 means ignore that corresponding bit value; they need not match.

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.

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.

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.

Caution

If you set the number-of-matches argument to 1, a log message is sent right away, rather than caching it; every packet that matches an access list causes a log message. A setting of 1 is not recommended because the volume of log messages could overwhelm the system.

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.

Note

The logging facility might drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.

• Alternative to Access List Logging

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 "Refining an Access List."

• You can impose dates and times when permit or deny statements in an extended access list are in effect, making your access list more granular and specific to an absolute or periodic time period.
• After you create a named access list, you might want to add entries or change the order of the entries, known as resequencing an access list.
• You can achieve finer granularity when filtering packets by filtering on noninitial fragments of packets.

Time-based access lists implement access list entries based on particular times of the day or week. This is an advantage when you don't 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.

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.

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 router's interface or process level (in the case of filtering on TTL values).

• When an inbound access list is applied to an interface, after the software receives a packet, the software checks the packet against the access list statements. If the access list permits the packet, the software continues to process the packet. Therefore, filtering on incoming packets can save router resources because filtered packets will not go through the router.
• Access lists that apply to outbound packets are filtering packets that have already gone through the router. Packets that pass the access list are transmitted (sent) out the interface.
• The TCP ACL splitting feature of Rate-Based Satellite Control Protocol (RBSCP) is an example of a feature that can be used on an outgoing interface. The access list controls which packets are subject to TCP ACK splitting.

Access lists can be used in ways other than applying them to interfaces. The following are additional places to apply an access list.

• To restrict incoming and outgoing connections between a particular vty (into a Cisco device) and the network devices at addresses in an access list, apply an access list to a line. See the "Controlling Access to a Virtual Terminal Line" module.
• Referencing an access list from a debug command limits the amount of information displayed to only the information permitted by the access list, such as sources, destinations, or protocols, for example.
• Access lists can be used to control routing updates, to control dial-on-demand routing (DDR), and to control quality of service (QoS) features, for example. See the appropriate configuration chapters for using access lists with these features.