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Year 2 - Chapter 6/Cisco 3 - Module 6 ACLs

Year 2 - Chapter 6/Cisco 3 - Module 6 ACLs. By Carl Marandola CCAI-CCNA. Objectives. Define and describe the purpose and operation of ACLs Explain the processes involved in testing packets with ACLs Describe ACL configuration commands, global statements, and interface commands

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Year 2 - Chapter 6/Cisco 3 - Module 6 ACLs

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  1. Year 2 - Chapter 6/Cisco 3 - Module 6ACLs By Carl Marandola CCAI-CCNA

  2. Objectives • Define and describe the purpose and operation of ACLs • Explain the processes involved in testing packets with ACLs • Describe ACL configuration commands, global statements, and interface commands • Define and explain the function and operation of wildcard masks bits and the wildcards any and host • Describe standard ACLs • Describe extended ACLs • Describe named ACLs • Monitor and verify selected ACL operations on the router

  3. ACL Overview • Reasons to Create ACLs • Creating an ACL in the Correct Order • Using ACLs • How ACLs Work

  4. What Are ACLs? • Lists of instructions you apply to a router's interface. • Tell the router what kinds of packets to accept and what kinds to deny based on criteria defined in the ACL. • Cisco ACLs support all routed network protocols (IP, IPX). • Each protocol must have its own ACLs.

  5. Reasons to Create ACLs • Limit network traffic and increase network performance • Provide traffic flow control • Provide a basic level of security for network access • Decide which types of traffic are forwarded or blocked at the router interfaces

  6. ACL Configuration Tasks • Grouping (Applying) ACLs to Interfaces • Assigning a Unique Number to Each ACL • Using Wildcard Mask Bits

  7. Basic ACL Creation Steps Router#config t Router(config)#access-list 50 permit 192.168.1.10 Router(config)#access-list 50 deny 192.168.1.0 0.0.0.255 Router(config)#access-list 50 permit any Router(config)#interface Ethernet0 Router(config-if)#ip address 192.168.5.1 255.255.255.0 Router(config-if)#ip access-group 50 out (applying the ACL) • Create a List of Permit or Deny Statements. • Created in global configuration mode. • Created in sequential order. • Each new item is added to the bottom of the ACL. • ACLs are processed in sequential order (top down). • Apply the ACL to a Task. • Although ACLs can be used in many ways, the CCNA exam covers only filtering interfaces.

  8. Applying ACLs to Interfaces Router#config t Router(config)#access-list 50 permit 192.168.1.10 Router(config)#access-list 50 deny 192.168.1.0 0.0.0.255 Router(config)#access-list 50 permit any Router(config)#interface Ethernet0 Router(config-if)#ip address 192.168.5.1 255.255.255.0 Router(config-if)#ip access-group 50 out • Keyword ip access-group applies ACL to an interface. • In the above example, the ACL is applied “outbound” meaning: • Only outbound traffic is checked by the ACL. • Out or in is always viewed from the center of the router. • The default direction is out (that is, if no designation is mad). • Each interface can have an in and out ACL for each protocol configured on the interface.

  9. ACLs Command-Line Basics Router#config t Router(config)#access-list 50 permit 192.168.1.10 Router(config)#access-list 50 deny 192.168.1.0 0.0.0.255 Router(config)#access-list 50 permit any Router(config)#interface Ethernet0 Router(config-if)#ip address 192.168.5.1 255.255.255.0 Router(config-if)#ip access-group 50 out • Keyword access-list required; identifies an ACL item. • ACL number (50 above) is the identifier for this particular list. • Things to know about ACL numbers: • Standard IP ACLs use numbers 1–99. • Extended IP ACLs use numbers 100–199. • Each item in the list will have the same number. • No two ACLs on a router can use the same number. • ACLs and ACL numbers are router-specific. • Keywords permit / deny allow or prevent passage, respectively. • Balance of the ACL is the criteria (rules for filtering).

  10. How an Outbound ACL Works

  11. Flowchart of the ACL Test Matching Process ACL lines are processed sequentially (top-down). When a criteria is matched, the packet is permitted or denied and no other testing occurs on that packet. If no ACL test matches, the packet it is denied by default.

  12. Wildcard Mask Bits #1 Router#config t Router(config)#access-list 50 deny 192.168.1.0 0.0.0.255 Router(config)#access-list 50 permit any Router(config)#interface Ethernet0 Router(config-if)#ip address 192.168.5.1 255.255.255.0 Router(config-if)#ip access-group 50 out Although maybe not a perfect explanation, it is often easiest to see wildcard masks as being just the opposite of subnet masks. The subnet mask 255.255.255.0 identifies the network, the wildcard mask 0.0.0.255 identifies the hosts. A subnet mask is a 32-bit value made up of contiguous 1s and then 0s. A wildcard is a 32-bit value made up of contiguous 0s indicating a “must match” and then 1s. In the above example, the 0s mean the first 3 octets must match exactly. The 255 means any value from 0 to 255 can be here. Both actually occur in binary and can often be understood only in binary.

  13. Wildcard Mask Bits #2

  14. It is possible to identify a range of subnets. 172.30.16.0 0.0.15.255 The 255 means the last octet can be 0 to 255. 3rd Octet = 16 This means any value that can be made using 4 bits (0 to 15), so the range is 16+0 to 16+15. Wildcard Mask Bits #3

  15. Keywords Any and Host • Any • Same as 0.0.0.0 255.255.255.255 • Example: access-list 12 permit any • Host • Same as IP-address 0.0.0.0 • Example: 192.168.1.10 0.0.0.0 • Example: access-list 12 deny host 192.168.1.10 • Can be omitted in standard IP ACLs Router(config)#access-list 75 deny host 192.168.1.10 Router(config)#access-list 75 deny 192.168.17.123 Router(config)#access-list 75 permit any

  16. Implicit Deny Any Statement • By default, an interface will allow all traffic in both directions. • Once an ACL is applied, the default state switches to deny all traffic in the direction of the ACL (in or out). • Many ACLs end with access-list # permit any to avoid the implied deny any. Router(config)#access-list 75 deny host 192.168.1.10 Router(config)#access-list 75 deny 192.168.17.123 Router(config)#access-list 75 permit any

  17. Permit and Deny Statements • Except in special circumstances, every ACL must have at least one permit statement. • An ACL made up of only deny statements combined with the implicit deny any statement will block all traffic in the direction of the ACL. • No similar requirement for deny statements.

  18. Standard ACLs • Criteria is limited to source network address only. • Can identify a host, subnet, or network • Denies or permits all traffic from the target address for the protocol • Necessary to place close to the destination

  19. Standard ACLs Example #1 This one-line ACL will allow into each LAN only packets from the other LAN, not from the cloud. The implicit deny any blocks any other traffic.

  20. A Network X Standard ACLs Example #2 This two-line ACL will deny traffic from a single host A from getting to the X network. The second line could and should have been access-list 1 permit any. The implicit deny any is negated by the previous line, which allowed every thing through.

  21. Standard ACLs Example #3 This two-line ACL will deny traffic from the entire 172.16.4.0 network. The second line allows any traffic from the cloud. The implicit deny any is negated by the permit any line, which allowed everything else through.

  22. Extended ACLs Can Filter on One or Many Data Fields

  23. Extended ACLs – Creating an Extended Access List As with standard lists, the access-list command is used to create each condition of the list – using one condition per line. The syntax for each line in the list is: access-list access-list-number {permit | deny} {protocol | protocol keyword} {source | any} [source-wildcard] [source port] {destination | any} [destination-wildcard] [destination port] [options] Example: Lab-X#config t Lab-X(config)#Access-list 101 deny tcp 192.168.1.0 0.0.0.255 any eq www Lab-X(config)#Access-list 101 deny tcp any eq ftp 192.168.1.25 Lab-X(config)#Access-list 101 permit ip any any Lab-X(config)#interface Fastethernet 0/0 Lab-X(config-if)#ip access-group 101 out The access list-number range for IP extended access lists is 100 to 199. The protocol entry defines the protocol to be filtered, such as IP, TCP, UDP, or ICMP for example. Because IP headers transport TCP, UDP, and ICMP, it is important to specify the protocol or you could end up inadvertently filtering more than you want to.

  24. Extended ACLs – TCP Relational Operators The access list TCP protocol option supports both source and destination ports. You can access each by using either the port number or a mnemonic or acronym. Keyword relational operators such as those shown in the following code output precede these: Lab-X(config)#access-list 101 deny tcp any ? A.B.C.D Destination address. any Any destination host. eq Match only packets on a given port number. gt Match only packets with a greater port number. host A single destination host. lt Match only packets with a lower port number. neq Match only packets not on a given port number. range Match only packets in the range of port numbers.

  25. Extended ACLs – Protocol Identifiers After choosing a relational operator, specify a mnemonic (or acronym) or port number, such as the following code output for the TCP port names: Lab-X(config)#access-list 101 deny tcp any eq ? <0-65535> Port number bgp Border Gateway Protocol (179) chargen Character generator (19) cmd Remote commands (rcmd, 514) daytime Daytime (13) discard Discard (9) domain Domain Name Service (53) echo Echo (7) exec Exec (rsh, 512) finger Finger (79) ftp File Transfer Protocol (21) --Some output omitted— uucp Unix-to-Unix Copy Program (540) whois Nicname (43) www World Wide Web (HTTP, 80)

  26. UDP and TCP Port Numbers

  27. Extended ACLs – TCP Example The first statement could have used the mnemonic “telnet” in place of 23 with exactly the same result. The one advantage to using the mnemonic is that it is more intuitive to anyone having to support the device. Lab-X#config t Lab-X(config)#access-list 101 deny tcp 192.168.5.0 0.0.0.255 any eq 23 Lab-X(config)#access-list 101 permit ip any any Lab-X(config)#interface fastethernet 0/1 Lab-X(config-if)#ip access-group 101 in If you want to block network 192.168.5.0 from being able to surf the Web while still allowing other services such as FTP, use this code: Lab-X#config t Lab-X(config)#access-list 106 deny tcp 192.168.5.0 0.0.0.255 any eq www Lab-X(config)#access-list 106 permit ip any any Lab-X(config)#interface ethernet 0 Lab-X(config-if)#ip access-group 106 in

  28. Extended ACLs – Blocking RIP Routing Updates One example of using ACLs with UDP packets would be to block RIP routing updates from passing out a particular interface using an ACL such as the following: Lab-X#config t Lab-X(config)#access-list 150 deny udp any any eq rip Lab-X(config)#access-list 150 permit ip any any Lab-X(config)#int e0 Lab-X(config-if)#ip access-group 150 out

  29. Important Concept for Extended ACLs Both ACLs below implicitly deny all other IP traffic, not just all other TCP traffic. A common mistake many people make is assuming that because they specified only TCP in the main statement(s), they need only to refer to TCP in the final permit any statement (102 below). The unplanned result would be that all ICMP and UDP traffic matching the address/protocol criteria would be blocked (102 below). ACL 101’s permit IP any any statement will allow all other traffic. Lab-X#config t Lab-X(config)#Access-list 101 deny tcp any 192.168.1.25 eq ftp Lab-X(config)#Access-list 101 permit IP any any Lab-X(config)#Access-list 102 deny tcp 10.0.0.0 0.255.255.255 192.168.1.1 eq ftp Lab-X(config)#Access-list 102 permit TCP any any Lab-X(config)#interface Fastethernet 0/0 Lab-X(config-if)#ip access-group 101 out Lab-X(config-if)#ip access-group 102 in

  30. Extended Access List Processing

  31. Named ACLs – Basics The Cisco IOS release supports using named access lists rather than the traditional number designations. This ability to name a list makes them easier to recognize and can make them easier to debug. Another advantage is that it is possible to delete individual entries from a specific ACL instead of erasing the entire list. A couple things to consider when implementing named ACLs: 1. Names, like numbers, must be unique on each router. 2. Named ACLs do not work with IOS releases prior to 11.2. The first step is to create the ACL using the following syntax: Router(config)#ip access-list {standard | extended} name Example: Lab-X(config)#ip access-list extended BlockInternet

  32. Named ACLs – Example Lab-X#conf t Lab-X(config)#ip access-list ?(to see the options) extended Extended Access List log-update Control access list log updates logging Control access list logging standard Standard Access List Lab-X(config)#ip access-list standard ProtectLAN Lab-X(config-std-nacl)#deny 192.168.20.0 0.0.0.255 Lab-X(config-std-nacl)#deny 192.168.30.0 0.0.0.255 Lab-X(config-std-nacl)#permit any Lab-X(config-std-nacl)#exit Lab-X(config)#ip access-list extended FilterOutside Lab-X(config-ext-nacl)#permit tcp any 192.168.5.0 0.0.0.255 established Lab-X(config-ext-nacl)#deny icmp any 192.168.5.0 0.0.0.255 Lab-X(config-ext-nacl)#permit ip any any Lab-X(config-ext-nacl)#^Z Lab-X# Named ACLs are applied to interfaces the same as other ACLs.

  33. ACL Placement Standard ACLs must be placed close to the destination. Extended ACLs should be placed close to the source.

  34. ACLs and Firewall Routers Border routers are charged with providing network security from potential attacks from outside the network. The figure shows the simplest situation where the border router becomes the clear demarcation between the network and the rest of the world. Typically intruders come from the global Internet and the thousands of networks connected to it.

  35. ACLs and Firewall Routers Another type of firewall implementation uses two or more routers to provide the security. The border router, A, is ultimately responsible for securing the LAN(s). The firewall router, F, is responsible for protecting both the shared servers and the internal networks from attack from outside. In an optimal solution, this would be a specialty device like Cisco’s PIX box, which combines hardware and software to secure the connection. Whether a specialty device or a router with the firewall feature set, ACLs will be a part of the solution.

  36. Verifying ACLs Lab-X#show ip interface Ethernet0 is up, line protocol is up Internet address is 192.168.5.1/24 Broadcast address is 255.255.255.255 Address determined by non-volatile memory MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Multicast reserved groups joined: 224.0.0.9 Outgoing access list is 50 Inbound access list is 70 Proxy ARP is enabled The show ip interface command will tell whether an inbound or outbound access list has been applied to an interface. Rows 9 and 10 above contain the information. The rest of the lines do not pertain to ACLs, so they have been omitted.

  37. The show access-lists Command Lab-X#show access-lists Standard IP access list 50 deny 192.168.1.10 log permit any Standard IP access list 75 deny 192.168.17.123 deny 192.168.1.10 permit any Lab-X# The show access-lists command will display all access lists on the router but does not show whether or where they are applied. Another command, show ip access-lists, would include only IP access lists. Both commands enable you to specify an ACL number or name after the command to display just that ACL.

  38. The show run Command Lab-X#show run hostname Lab-X ! ! (output abbreviated) ! access-list 50 deny 192.168.1.10 log access-list 50 permit any access-list 75 deny 192.168.17.123 access-list 75 deny 192.168.1.10 access-list 75 permit any ! One way to see your access lists and how they are applied is to use the show run command to see the active configuration. The above lines show the output of a show run command with some of the unrelated lines removed.

  39. Verifying Named ACLs Lab-X#show run ! (output edited and omitted) ip access-list standard ProtectLAN deny 192.168.20.0 0.0.0.255 deny 192.168.30.0 0.0.0.255 permit any ! ip access-list extended FilterOutside permit tcp any 192.168.5.0 0.0.0.255 established deny icmp any 192.168.5.0 0.0.0.255 permit ip any any ! Lab-X#show access-list Standard IP access list ProtectLAN deny 192.168.20.0, wildcard bits 0.0.0.255 deny 192.168.30.0, wildcard bits 0.0.0.255 permit any Extended IP access list FilterOutside permit tcp any 192.168.5.0 0.0.0.255 established deny icmp any 192.168.5.0 0.0.0.255 permit ip any any Lab-X#

  40. ACL Labs and exercises • Complete the e-labs below: (6/09 and 6/11) • 6.3.2 6.3.3 6.3.6.1 • 6.3.6.2 6.4.2.1 6.4.2.2 • 6.4.2.3 6.5.1 6.8.1.1 • 6.8.1.2 • Regular lab—Handouts

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