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This chapter describes reflexive access list commands, which are used to configure IP session filtering. IP session filtering provides the ability to filter IP packets based on upper-layer protocol "session" information.
Refer to the Command Reference Master Index or search online to find complete descriptions of other commands used when configuring reflexive access lists.
For reflexive access list configuration information, refer to the "Configuring IP Session Filtering (Reflexive Access Lists)" chapter in the Cisco IOS Security Configuration Guide.
To nest a reflexive access list within an access list, use the evaluate command in access-list configuration mode. Use the no form of this command to remove a nested reflexive access list from the access list.
evaluate name
Syntax Description
name The name of the reflexive access list that you want evaluated for IP traffic entering your internal network. This is the name defined in the permit (reflexive) command.
Defaults
Reflexive access lists are not evaluated.
Command Modes
Access-list configuration
Command History
11.3 This command was introduced.
Release
Modification
Usage Guidelines
This command is used to achieve reflexive filtering, a form of session filtering.
Before this command will work, you must define the reflexive access list using the permit (reflexive) command.
This command nests a reflexive access list within an extended named IP access list.
If you are configuring reflexive access lists for an external interface, the extended named IP access list should be one which is applied to inbound traffic. If you are configuring reflexive access lists for an internal interface, the extended named IP access list should be one which is applied to outbound traffic. (In other words, use the access list opposite of the one used to define the reflexive access list.)
This command allows IP traffic entering your internal network to be evaluated against the reflexive access list. Use this command as an entry (condition statement) in the IP access list; the entry "points" to the reflexive access list to be evaluated.
As with all access list entries, the order of entries is important. Normally, when a packet is evaluated against entries in an access list, the entries are evaluated in sequential order, and when a match occurs, no more entries are evaluated. With a reflexive access list nested in an extended access list, the extended access list entries are evaluated sequentially up to the nested entry, then the reflexive access list entries are evaluated sequentially, and then the remaining entries in the extended access list are evaluated sequentially. As usual, after a packet matches any of these entries, no more entries will be evaluated.
The following example shows reflexive filtering at an external interface. This example defines an extended named IP access list inboundfilters, and applies it to inbound traffic at the interface. The access list definition permits all BGP and Enhanced IGRP traffic, denies all ICMP traffic, and causes all TCP traffic to be evaluated against the reflexive access list tcptraffic.
If the reflexive access list tcptraffic has an entry that matches an inbound packet, the packet will be permitted into the network. tcptraffic only has entries that permit inbound traffic for existing TCP sessions.
interface Serial 1 description Access to the Internet via this interface ip access-group inboundfilters in ! ip access-list extended inboundfilters permit bgp any any permit eigrp any any deny icmp any any evaluate tcptraffic
Related Commands
ip access-list Defines an IP access list by name. Specifies the length of time that reflexive access list entries will continue to exist when no packets in the session are detected. Creates a reflexive access list and enables its temporary entries to be automatically generated.
Command
Description
To specify the length of time that reflexive access list entries will continue to exist when no packets in the session are detected, use the ip reflexive-list timeout command in global configuration mode. Use the no form to reset the timeout period to the default timeout.
ip reflexive-list timeout seconds
Syntax Description
seconds Specifies the number of seconds to wait (when no session traffic is being detected) before temporary access list entries expire. Use a positive integer from 0 to 232-1.
Defaults
The reflexive access list entry is removed after no packets in the session are detected for 300 seconds.
Command Modes
Global configuration
Command History
11.3 This command was introduced.
Release
Modification
Usage Guidelines
This command is used with reflexive filtering, a form of session filtering.
This command specifies when a reflexive access list entry will be removed after a period of no traffic for the session (the timeout period).
With reflexive filtering, when an IP upper-layer session begins from within your network, a temporary entry is created within the reflexive access list, and a timer is set. Whenever a packet belonging to this session is forwarded (inbound or outbound) the timer is reset. When this timer counts down to zero without being reset, the temporary reflexive access list entry is removed.
The timer is set to the timeout period. Individual timeout periods can be defined for specific reflexive access lists, but for reflexive access lists that do not have individually defined timeout periods, the global timeout period is used. The global timeout value is 300 seconds by default; however, you can change the global timeout to a different value at any time using this command.
This command does not take effect for reflexive access list entries that were already created when the command is entered; this command only changes the timeout period for entries created after the command is entered.
The following example sets the global timeout period for reflexive access list entries to 120 seconds:
ip reflexive-list timeout 120
The following example returns the global timeout period to the default of 300 seconds:
no ip reflexive-list timeout
Related Commands
Nests a reflexive access list within an access list. ip access-list Defines an IP access list by name. Creates a reflexive access list and enables its temporary entries to be automatically generated.
Command
Description
To create a reflexive access list and to enable its temporary entries to be automatically generated, use the permit command in access-list configuration mode. Use the no form of this command to delete the reflexive access list (if only one protocol was defined) or to delete protocol entries from the reflexive access list (if multiple protocols are defined).
permit protocol source source-wildcard destination destination-wildcard reflect name [timeout seconds]
Syntax Description
protocol Name or number of an IP protocol. It can be one of the keywords gre, icmp, ip, ipinip, nos, tcp, or udp, or an integer in the range 0 to 255 representing an IP protocol number. To match any Internet protocol (including ICMP, TCP, and UDP), use the keyword ip. source Number of the network or host from which the packet is being sent. There are three other ways to specify the source: source-wildcard Wildcard bits (mask) to be applied to source. There are three other ways to specify the source wildcard: destination Number of the network or host to which the packet is being sent. There are three other ways to specify the destination: destination- wildcard Wildcard bits to be applied to the destination. There are three other ways to specify the destination wildcard: reflect Identifies this access list as a reflexive access list. name Specifies the name of the reflexive access list. Names cannot contain a space or quotation mark, and must begin with an alphabetic character to prevent ambiguity with numbered access lists. The name can be up to 64 characters long. timeout seconds (Optional) Specifies the number of seconds to wait (when no session traffic is being detected) before entries expire in this reflexive access list. Use a positive integer from 0 to 232-1. If not specified, the number of seconds defaults to the global timeout value.
Defaults
If this command is not configured, no reflexive access lists will exist, and no session filtering will occur.
If this command is configured without specifying a timeout value, entries in this reflexive access list will expire after the global timeout period.
Command Modes
Access-list configuration
Command History
11.3 This command was introduced.
Release
Modification
Usage Guidelines
This command is used to achieve reflexive filtering, a form of session filtering.
For this command to work, you must also nest the reflexive access list using the evaluate command.
This command creates a reflexive access list and triggers the creation of entries in the same reflexive access list. This command must be an entry (condition statement) in an extended named IP access list.
If you are configuring reflexive access lists for an external interface, the extended named IP access list should be one which is applied to outbound traffic.
If you are configuring reflexive access lists for an internal interface, the extended named IP access list should be one which is applied to inbound traffic.
IP sessions that originate from within your network are initiated with a packet exiting your network. When such a packet is evaluated against the statements in the extended named IP access list, the packet is also evaluated against this reflexive permit entry.
As with all access list entries, the order of entries is important, because they are evaluated in sequential order. When an IP packet reaches the interface, it will be evaluated sequentially by each entry in the access list until a match occurs.
If the packet matches an entry prior to the reflexive permit entry, the packet will not be evaluated by the reflexive permit entry, and no temporary entry will be created for the reflexive access list (session filtering will not be triggered).
The packet will be evaluated by the reflexive permit entry if no other match occurs first. Then, if the packet matches the protocol specified in the reflexive permit entry, the packet is forwarded and a corresponding temporary entry is created in the reflexive access list (unless the corresponding entry already exists, indicating the packet belongs to a session in progress). The temporary entry specifies criteria that permits traffic into your network only for the same session.
Characteristics of Reflexive Access List Entries
This command enables the creation of temporary entries in the same reflexive access list that was defined by this command. The temporary entries are created when a packet exiting your network matches the protocol specified in this command. (The packet "triggers" the creation of a temporary entry.) These entries have the following characteristics:
The following example defines a reflexive access list tcptraffic, in an outbound access list that permits all BGP and Enhanced IGRP traffic and denies all ICMP traffic. This example is for an external interface (an interface connecting to an external network).
First, the interface is defined and the access list is applied to the interface for outbound traffic.
interface Serial 1 description Access to the Internet via this interface ip access-group outboundfilters out
Next, the outbound access list is defined and the reflexive access list tcptraffic is created with a reflexive permit entry.
ip access-list extended outboundfilters permit tcp any any reflect tcptraffic
Related Commands
Nests a reflexive access list within an access list. ip access-list Defines an IP access list by name. Specifies the length of time that reflexive access list entries will continue to exist when no packets in the session are detected.
Command
Description
Posted: Tue Apr 4 17:37:34 PDT 2000
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