Figure 8 2: A sample network configuration in Software

Maker QR Code ISO/IEC18004 in Software Figure 8 2: A sample network configuration

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The use of reflexive access lists is best illustrated with an example In Figure 8 2, we show a sample network diagram We also show the use of reflexive access lists to monitor all outbound TCP, UDP, and ICMP traffic in the code below Only relevant portions of the configuration are shown
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Figure 8 2: A sample network configuration
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interface serial 0 ip access-group infilter in ip access-group outfilter out !
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ip reflexive-list timeout 120 ! ip access-list extended outfilter permit tcp any any reflect my_packets permit udp any any reflect my_packets permit icmp any any reflect my_packets ! ip access-list extended infilter deny ip 10000 0255255255 any deny ip 1721600 031255255 any deny ip 19216800 00255255 any evaluate my_packets Examine the configuration above The first step is to create an extended named access list, which will include a permit statement for all the protocols we wish to enable to create "reflected" entries The use of named access lists is required In the earlier example, we have created a named access list called "outfilter" Within the body of this named access list, we have created entries for TCP, UDP, and ICMP Notice that the entries are all permit entries Also notice the use of the keyword "reflect" This keyword signals the use of reflexive access lists Also notice the use of the name "my_packets" for each of the entries This name is used later to reference the reflexive access list entries As shown in the code above, the reflexive access list entries are "nested" beneath a traditional named access list called "infilter" Although it isn't shown, the named access list "outfilter" could contain other non-reflexive access list entries If the access list entries contain both reflexive and non-reflexive entries, the order placement is crucial Only packets that reach the reflexive access list entries will be enabled to create "reflected" entries If a packet matches an entry higher in the named access list, the packet will not be evaluated by the reflexive entries As an example, here's a modified version of our "outfilter" access list: ip access-list extended outfilter permit ip any any permit tcp any any reflect my_packets permit udp any any reflect my_packets permit icmp any any reflect my_packets Notice the addition of an entry permitting all IP packets Notice, too, that this entry does not use the keyword "reflect," indicating that the entry is not a reflexive access list entry Because all IP packets would be matched by this entry, none of our remaining reflexive access list entries would ever be matched so no "reflected" entries would ever be created in our inbound access list "infilter" An additional point worth noting is that non-reflexive access list entries could be interwoven with reflexive access list entries within the same named access list No requirement exists that all of the reflexive entries appear contiguously This characteristic provides a great deal of flexibility when
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creating reflexive access lists Certain protocols and IP address ranges can be included as reflexive access list entries, and others might not be included Here's a simple example to illustrate this point: ip access-list extended outfilter permit ip 160100101 any permit tcp any any reflect my_packets permit udp any any reflect my_packets deny icmp 160100100 000255 any echo-reply permit icmp any any reflect my_packets Let's return now to our original example showing the use of reflexive access lists to monitor all outbound TCP, UDP, and ICMP traffic Notice that the reflexive access list "my_packets" is referenced by the named access list "infilter" The command "evaluate my_packets" signals the placement of the dynamic entries that will be created by the reflexive access list "my_packets" Notice that there are other access list entries preceding the "evaluate" command These entries will be considered prior to any entries created by our reflexive access list "my_packets" We could also create additional access list entries after the "evaluate" command These entries would be examined after any entries created by our reflexive access list "my_packets" As with all other access lists, the order is critical Once a matching access list entry is found, no other entries are considered In the code sample below, we show the output of "show access list" command before and after an inside client initiates an outbound Telnet session The Telnet session has the following characteristics: Source IP address: 160101100 Source TCP port: 1045 Destination IP address: 175100101 Destination TCP port: 23 (Telnet) Before: router#sh access-list Extended IP access list infilter deny ip 10000 0255255255 any deny ip 1721600 031255255 any deny ip 19216800 00255255 any evaluate my_packets extended IP access list outfilter permit tcp any any reflect my_packets permit udp any any reflect my_packets
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permit icmp any any reflect my_packets After router#sh access-list Extended IP access list infilter deny ip 10000 0255255255 any deny ip 1721600 031255255 any deny ip 19216800 00255255 any evaluate my_packets extended IP access list outfilter permit tcp any any reflect my_packets permit udp any any reflect my_packets permit icmp any any reflect my_packets reflexive IP access list my_packets permit tcp host 175100101 eq telnet host 160101100 eq 1045 (10 matches) (time left 110 seconds) Notice that the reflexive access list "my_traffic" now appears with a new entry One final point regarding reflexive access lists is the use of a timeout variable We mentioned earlier that only TCP traffic is actively closed A TCP reflexive access list entry will be closed immediately after receipt of a packet, with the RST bit set and within five seconds of the detection of two FIN bits Because UDP and ICMP do not have similar options in their headers, an idle timeout is necessary to determine when entries for these kinds of protocols should be deleted The default idle timeout is 300 seconds In the configuration sample of reflexive access lists to monitor all outbound TCP, UDP, and ICMP traffic, we changed the global timeout to 120 seconds with the global command ip reflexive-list timeout 120 The timeout value can also be set independently for each reflexive access list entry by using the keyword "timeout" ip access list extended outfilter permit tcp any any reflect my_packets permit udp any any reflect my_packets timeout 60 permit icmp any any reflect my_packets The configuration change described earlier sets the timeout for UDP entries to 60 seconds While reflexive access lists are powerful and provide a significant enhancement for traditional access lists, they are still limited by their lack of knowledge about the behavior of multi-channel applications In the next section, we discuss CBAC, which is capable of understanding the behavior of many multi-channel applications and is much more flexible than reflexive access lists
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Context Based Access Control (CBAC)
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CBAC was originally introduced in IOS Version 112 in a special release called the Firewall Feature Set, or simply FFS The original release was available only on the 1600- and 2500-series platforms In release 120, Cisco included CBAC support for the 1700-, 2600- and 3600-series router platforms CBAC is similar in concept to reflexive access lists, because CBAC provides the capacity to dynamically create openings through a filtering router when a connection is initiated from within the protected network CBAC includes additional intelligence, however, that enables it to filter based on application-layer protocol information to learn about the state of a UDP or TCP session This feature enables support of applications that involve multiple channels created through client/server negotiation, such as FTP As noted previously, reflexive access lists are not capable of handling these kinds of applications CBAC is much more than an access list enhancement; rather, it is a comprehensive set of security tools In addition to providing application-layer filtering capabilities, CBAC provides the following items: Java blocking Denial-of-service prevention and detection Real-time alerts and audit trails We will examine each of these features in detail in the following sections
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