What command would you use to apply the access list you created in Question 1 to an Ethernet interface outbound?
Which command would you use to apply an access list to a router interface?A. ip access-list 101 out Show B. access-list ip 101 in C. ip access-group 101 in D. access-group ip 101 in Answer: Option C Solution(By Examveda Team)To apply an access list, the proper command is ip access-group 101 in.clear access-list ipv4To clear IPv4 access list counters, use the clear access-list ipv4 command in EXEC mode. clear access-list ipv4 access-list name [ sequence-number | hardware { ingress | egress}] [interface type interface-path-id] [ location node-id | sequence number ] Syntax Description
Traffic Filtering in the Cisco Internetwork Operating SystemEric Knipp, ... Edgar DanielyanTechnical Editor, in Managing Cisco Network Security (Second Edition), 2002 Extended IP Access ListsAlthough there are times when we only need to filter traffic based on the source address, more often than not we will need to match traffic with a higher level of detail. An option for more precise traffic-filtering control would be an extended IP access list. Here, both the source and destination address are checked. In addition, you also have the ability to specify the protocol and optional TCP or UDP port number to filter more precisely. In the following example, any field represented by {} is mandatory for the access list, while any field represented by [] is optional. The format of an extended IP access list is: access-list access-list-number {permit | deny} protocol source source-wildcard [operator source-port] destination destination-wildcard [operator destination-port] [precedence precedence-number] [tos tos] [established] [log | log-input] Bold items represent keywords that are part of the access list syntax. Table4.4 lists the configuration for a standard IP access list. Table4.4. Extended IP Access List Configuration
In the following access list, we get very specific about what host we want to access a particular network or host on a network. In the first three lines, we are permitting or allowing packets from individual hosts on subnet 172.16.130.0 to any host on network 10.0.0.0. In line 4, we are denying packets with the source address that belongs to subnet 172.16.130.0 to the destination of host 192.168.10.118. Line 5 tells us that we are permitting all IP packets with no concern of a source or destination address. The implicit deny all at the end of the list will never be matched against a packet because the previous permit statement will match all packets. In Figure4.3, we would apply this access list on the serial 0 interface in the outbound direction as follows: Figure4.3. An Example Network Router(conflg)# interface serial 0 Router(config-if)# ip access-group 141 out An example of an extended access list is as follows: access-list 141 permit icmp host 172.16.130.88 10.0.0.0 0.255.255.255 access-list 141 permit tcp host 172.16.130.89 eq 734 10.0.0.0 0. 255.255.255 range 10000 10010 access-list 141 permit udp host 172.16.130.90 10.0.0.0 0.255.255.255 eq tftp access-list 141 deny ip 172.16.130.0 0.0.0.255 host 192.168.10.118 access-list 141 permit ip any any Table4.5 describes the extended access list commands. Table4.5. A Description of Access List Commands
Just as in our standard access list, the extended access list will require a hyphen between the words access and list. Next is the list number. Since we are referencing an extended IP access list, the numbers would range from 100 to 199. The access list number serves the same dual purpose here as we looked at earlier with the standard access list. The router must have a way to distinguish between access lists. The number performs this purpose along with tying the lines of an access list together and designates which access list the filter is part of The number also tells the router the type of access list. Desinging & Planning…Placement of Access ListsOften you have a few options about how to apply your access lists and stili achieve the same affect on the traffic flowing through the router, in the case of the previous example, access list 141 was applied outbound on the serial 0 interface. Because access list 141 was designed to only filter traffic originating from the 172.16.130.0 network, and not traffic from 172.17.0.0, this list could have been applied in the inbound direction on Ethernet 0. Both approaches will have the same affect on the traffic flowing through the router. There is a minor difference between these two approaches, though. When the ACL is applied outbound on the Serial0 interface, the traffic enters the Ethernet0 interface and is processed against the routing table. The packet is then passed to the outbound interface, where it is checked against any outbound ACLs. If the outbound interface is Serial 0, it checks packets against access list 141 and will permit or deny the traffic based on the rules defined in that list. When the ACL is applied inbound on the Ethernet0 interface, the traffic is permitted or denied before it is processed against the routing table. On a router under heavy traffic loads, this could make a considerable difference in the delay that is introduced because the router does not have to process packets that will be dropped by the outbound interface. Although inbound filtering has the advantage with respect to route processing, that does not necessarily make it the better way to apply access lists. Under different circumstances, you may want to prevent access to an external subnet from both Ethernet interfaces. In this case, it may be easier to apply the access lists in the outbound direction of Serial0 because packets from both Ethernet interfaces will have to pass through Serial0 to get to the external subnet. In other words, you are applying the access list to the bottleneck in traffic. Otherwise, you will have to keep two separate access lists, one specific for Ethernet0 and the other specific for Ethernet1. If the router is under light traffic loads, it may be easier to maintain a single access list. There is disagreement among network and security professionals about which approach is better, but neither approach should be considered better than the other in all cases. It is up to you to decide which is best for your situation. Keywords permit or denyA keyword permit or deny specifies to the router the action to be performed. For example, the keyword permit would allow the packet to exit or enter the interface, depending on whether you specify the filtering to be performed in or out. Again, this option provides the same function as in our standard access list. The last line of our extended access list example could have read as follows: access-list 141 permit ip any any ProtocolYou have the option of filtering several different protocols using the extended access list. The protocol field in the IP header is an 8-bit number that defines what protocol is used inside the IP packet. TCP and UDP are only two of the possible protocols that can be filtered on, although they are most common. Other protocols, such as ICMP and EIGRP, have their own protocol numbers because they are not encapsulated inside TCP or UDP. If we use a question mark when defining an access list, we can see the protocol numbers that have been defined by name inside the router. Router (config)#access-list 191 permit? <0-255> An IP protocol number ahp Authentication Header Protocol eigrp Cisco's EIGRP routing protocol esp Encapsulation Security Payload gre Cisco's GRE tunneling icmp Internet Control Message Protocol igmp Internet Gateway Message Protocol igrp Cisco's IGRP routing protocol ip Any Internet Protocol ipinip IP in IP tunneling nos KA9Q NOS compatible IP over IP tunneling ospf OSPF routing protocol pcp Payload Compression Protocol pim Protocol Independent Multicast tcp Transmission Control Protocol udp User Datagram Protocol Protocols not on the preceding list may also be filtered with extended access lists, but they must be referenced by their protocol number. A full list of assigned IP protocol numbers can be found at www.iana.org/assignments/protocol-numbers. It is important to remember that the IP keyword in the protocol field matches all protocol numbers.You must use a systematic approach here when designing your access list. For example, if your first line in the access list permits IP for a specific address, and the second line denies UDP for the same address, the second statement would have no effect. The first line would permit IP, including all the above layers. An option here may be to reverse the order of the statements. With the statements reversed, UDP would be denied from that address and all other protocols would be permitted. Source Address and Wildcard-maskThe source address and source wildcard-mask perform the same function here as in a standard IP access list. So, in the preceding example we could have used the wildcard mask instead of the host and any keywords. The access list would then look as follows: access-list 141 permit ip 172.16.130.88 0.0.0.0 i0.0.0.0 0.255.255.255 access-list 141 permit ip 172.16.130.89 0.0.0.0 i0.0.0.0 0.255.255.255 access-list 141 permit ip 172.16.130.90 0.0.0.0 i0.0.0.0 0.255.255.255 access-list 141 permit ip 172.16.130.0 0.0.0.255 192.168.10.118 0.0.0.0 access-list 141 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 In the first three lines, we are permitting or allowing packets from individual hosts on subnet 172.16.130.0 to any host on network 10.0.0.0. In line 4, we are permitting packets with the source address that belongs to subnet 172.16.130.0 to the destination of host 192.168.10.118. Line 5 tells us that we are permitting all packets regardless of the source or destination address. Remember that standard IP access lists have a default mask of 0.0.0.0. This does not apply to extended access lists so we must specify one. Destination Address and Wildcard-maskThe destination address and wildcard-mask have the same effect and structure as the source address and wildcard-mask. So, here the keywords host and any are also available. You can utilize these keywords to specify any destination address as well as a specific destination without using the wildcard mask. Remember that extended access lists try a match on both source and destination. A common mistake here is trying to build an extended access list with the idea of only filtering the source address, and forgetting to specify the destination address. Source and Destination Port NumberMany times, we don’t want to deny all access to a particular server. When you put a Web server out on the Internet, you want everyone to be able to access it on port 80 (WWW), but you don’t want to allow access to any other ports, because it gives hackers the opportunity to exploit other services you may not be aware of (although you should know of them in the first place). Restricting access to this level of detail is another benefit of extended ACLs. We have the option of specifying a source and destination port number in the access list. Let’s look at a simple example: Router(config)# interface Serial 0 Router(config-if)# ip access-group 111 in Router(config)#access-list 111 permit tcp any host 172.17.11.19 eq 25 Router(config)#access-list 111 permit tcp any host 172.17.11.19 eq 23 These commands are explained in Table 4.6. Table4.6. Router Commands
In line 1, we are permitting TCP packets from any source to the destination of host 172.22.11.19 if the destination port is 25 (SMTP). In line 2, we are permitting TCP packets from any source to the destination of host 172.22.11.19 if the destination port is 23 (Telnet). The implicit deny statement at the end of this access list will prevent all other traffic from making it into our network. Let’s take a look at filtering with TCP and UDP. When using TCP, for example, the access list will examine the source and destination port numbers inside the TCP segment header. So, when using an extended access list, you have the capability to filter to and from a network address and also to and from a particular port number. You have several options when deciding which operator to use, such as: ■eq equal to ■neq not equal to ■gt greater than ■lt less than ■range specifies an inclusive range or ports (Here, two port numbers are specified.) View chapterPurchase book Read full chapter URL:https://www.sciencedirect.com/science/article/pii/B9781931836562500088 |
