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Chap 5 – Access Control Lists Learning Objectives

Chap 5 – Access Control Lists Learning Objectives. Explain how ACLs are used to secure a medium-size Enterprise branch office network. Configure standard ACLs in a medium-size Enterprise branch office network. Configure extended ACLs in a medium-size Enterprise branch office network.

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Chap 5 – Access Control Lists Learning Objectives

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  1. Chap 5 – Access Control Lists Learning Objectives • Explain how ACLs are used to secure a medium-size Enterprise branch office network. • Configure standard ACLs in a medium-size Enterprise branch office network. • Configure extended ACLs in a medium-size Enterprise branch office network. • Describe complex ACLs in a medium-size Enterprise branch office network. • Implement, verify and troubleshoot ACLs in an enterprise network environment.

  2. Access Control Lists (ACL) ACLs are lists of instructions applied to a router's interface to tell the router which kind of packets to permit and which kind to deny.

  3. Access Control Lists (ACL) • An ACL is a sequential list of permit or deny statements that apply to IP addresses or upper-layer protocols. The ACL can extract the following information from the packet header, test it against its rules, and make "allow" or "deny" decisions based on: • Source IP address • Destination IP address • ICMP message type • The ACL can also extract upper layer information and test it against its rules. Upper layer information includes: • TCP/UDP source port • TCP/UDP destination port

  4. Access Control Lists (ACL) • Limit network traffic and increase network performance. Provide traffic flow control. • ACLs can restrict the delivery of routing updates. If updates are not required because of network conditions, bandwidth is preserved. • Provide a basic level of security for network access. ACLs can allow one host to access a part of the network and prevent another host from accessing the same area. • Decide which types of traffic are forwarded or blocked at the router interfaces. ACLs can permit e-mail traffic to be routed, but block all Telnet traffic. • Control which areas a client can access on a network. • ACLs can be used to permit or deny a user to access file types such as FTP or HTTP.

  5. Asking For port 80? Asking For port 80? Packet Filtering Frame Header Packet Header Segment Header Data From which network? 192.168.1.0 Yes Permit No Deny 192.168.2.0 Yes Deny No Permit

  6. How ACLs Work • ACLs must be defined on a per protocol, per direction, or per port basis. • To control traffic flow on an interface, an ACL must be defined for each protocol enabled on the interface. • ACLs control traffic in one direction at a time on an interface.

  7. Any matches? No Permit? Permit? How ACLs Work • ACL statements operate in sequential, logical order, from top to bottom. • If a condition is matched, the packet is permitted or denied and the rest of the ACL isn’t checked. • An implicitdeny any statement is at end of all lists by default. • This last line "deny any" is not visible but it will not allow any unmatched packets to be permitted. Route packet to outbound interface ACL on interface? Yes No Frame arrives at inbound interface Yes ACL on interface? No L2 address match? Default Deny Any matches? Yes No Yes No Default Deny Yes Yes Yes Forward packet No

  8. Standard ACLs • Standard ACLs allow you to permit or deny traffic from source IP addresses. • The destination of the packet and the ports involved do not matter. • The example allows all traffic from network 192.168.30.0/24 network. • Because of the implied deny any at the end, all other traffic is blocked with this ACL. • Standard ACLs are created in global configuration mode.

  9. Standard ACLs • Extended ACLs filter IP packets based on several attributes, for example, protocol type, source IP address, destination IP address, source TCP or UDP ports, destination TCP or UDP ports, and optional protocol type information for finer granularity of control. • In the example, ACL 103 permits traffic originating from any address on the 192.168.30.0/24 network to any destination host port 80 (HTTP). • Extended ACLs are created in global configuration mode.

  10. Numbering & Naming ACLs Numbered ACLs - Assign a number based on which protocol is to be filtered filtered: • (1 to 99) and (1300 to 1999): Standard IP ACL • (100 to 199) and (2000 to 2699): Extended IP ACL • Named ACLs - assign a name by providing the name of • the ACL: • Names can contain alphanumeric characters. • Recommended that the name be written in CAPITAL LETTERS. • Names cannot contain spaces or punctuation and mustbegin with a letter. • Possible to add or delete entries within the ACL.

  11. ACL Placement • Standard ACLs should be placed close to the destination. • Extended ACLs should be placed close to the source.

  12. ACL Best Practice Using ACLs requires attention to detail and great care. Mistakes can be costly in terms of downtime, troubleshooting efforts, and poor network service. Before starting to configure an ACL, basic planning is required: • Base ACLs on the security policy of the organisation. • Prepare a description of what ACLs are required to do. • Use a text editor to create, edit and save ACLs. • Test ACLs on a development network before implementing them on a production network.

  13. R1 Configuring Standard ACLs 192.168.30.0/24 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Both ACLs have the same effect, due to the implicit deny at the end of all ACLs:

  14. Creating Standard ACLs access-list # permit/deny source IP wildcard To delete:

  15. Asking for 192.168.10.1? Asking for 192.168.10.0 0.0.0.255? Asking for 192.168.0.0 0.0.255.255? Asking for 192.0.0.0 0.255.255.255? Packet Header Segment Header Data Standard ACL access-list 2 deny 192.168.10.1 access-list 2 permit 192.168.10.0 0.0.0.255 access-list 2 deny 192.168.0.0 0.0.255.255 access-list 2 permit 192.0.0.0 0.255.255.255 Yes Deny Yes No Permit Yes No Deny Yes No Permit No Implicit Deny

  16. Verify Standard ACLs • The remark keyword is used for documentation and makes access lists a great deal easier to understand. Each remark is limited to 100 characters. • When reviewing the ACL in the configuration, the remark is also displayed.

  17. Wildcard Masks • ACLs statements include masks, also called wildcard masks. The mask determines how much of an IP source or destination address to apply to the address match. The numbers 1 and 0 in the mask identify how to treat the corresponding IP address bits. They are different to subnet masks, and follow different rules. • Wildcard masks and subnet masks are both 32 bits long and use binary 1s and 0s. Subnet masks use binary 1s and 0s to identify the network, subnet, and host portion of an IP address. • Wildcard masks use binary 1s and 0s to filter individual or groups of IP addresses to permit or deny access to resources based on an IP address. By carefully setting wildcard masks, you can permit or deny a single or several IP addresses

  18. Wildcards (Inverse Mask) • Allows you to indicate a host, subnet, network or range of IP addresses. • The two binary values in the wildcard have different meanings: 0 = Must Match Exactly 1 = Ignore

  19. Must match Must match Must match Must match Wildcard Masks Source IP Wildcard 172.16.10.10 0.0.0.0 10101100. 00010000. 00001010. 00001010 Source IP Wildcard 00000000. 00000000. 00000000. 00000000 Range of matching addresses: 172.16.10.10 only

  20. Must match Must match Must match Don’t Care Wildcard Masks Source IP Wildcard 172.16.10.0 0.0.0.255 10101100. 00010000. 00001010. 00000000 Source IP Wildcard 00000000. 00000000. 00000000. 11111111 Range of matching addresses: 172.16.10.0 to 172.16.10.255

  21. Must match Must match Don’t Care Don’t Care Wildcard Masks Source IP Wildcard 172.16.10.0 0.0.255.255 10101100. 00010000. 00001010. 00000000 Source IP Wildcard 00000000. 00000000. 11111111. 11111111 Range of matching addresses: 172.16.0.0 to 172.16.255.255

  22. ACL Example 1 Standard Access List Format: access-list # permit/deny source IP wildcard A(config)#access-list 5 deny 172.22.5.2 0.0.0.0 A(config)#access-list 5 deny 172.22.5.3 0.0.0.0 A(config)#access-list 5 permit any So what does this access list do?

  23. ACL Example 2 Sample: A(config)#access-list 5 deny 172.22.5.2 0.0.0.0 A(config)#access-list 5 deny 172.22.5.3 0.0.0.0 A(config)#access-list 5 permit any What happens if you now type in the following: A(config)#access-list 5 deny 172.22.5.4 0.0.0.0?

  24. ACL Example 3 Source IP Wildcard 172.16.10.0 0.0.31.255 128 64 32 16 8 4 2 1 31 in binary is 0 0 0 1 1 1 1 1 3rd octet = 10 = 00001010. All packets are compared to this value and the mask ’31’ So the first three bits must be 0’s and the last 5 bits do not matter. So acceptable values are 172.16.0.0 through 172.16.31.255 don’t care Must match

  25. ACL Example 4 The hosts on subnet 192.168.1.32/27 are to be split, with the lower half denied access to a router. Write the required access list. • 192.168.1.32/27 has an increment size of 32 • The first address in the 2nd half of the subnet = 32+16 = 48 • Compare last address from 1st half with 1st address in 2nd half in binary: 128 64 32 16 8 4 2 1 47 = 0 0 1 0 1 1 1 1 48 = 0 0 1 1 0 0 0 0 • All numbers between 32 and 47 have bit 16 = 0 • All numbers after 47 have bit 16 = 1

  26. ACL Example 4 128 64 32 16 8 4 2 1 47 = 0 0 1 0 1 1 1 1 48 = 0 0 1 1 0 0 0 0 Wildcard Mask = 0 0 0 0 1 1 1 1 15 Access-list 20 deny 192.168.1.32 0.0.0.15

  27. ACL Example 5 The hosts on subnet 192.168.23.64/28 are to be split, with the upper half denied access to a router. Write the required access list. • 192.168.23.64/28 has an increment size of 16 • The first address in the 2nd half of the subnet = 64+8 = 72 • Compare last address from 1st half with 1st address in 2nd half in binary: 128 64 32 16 8 4 2 1 64 = 0 1 0 0 0 0 0 0 72 = 0 1 0 0 1 0 0 0 • All numbers between 64 and 71 have bit 8 = 0 • All numbers after 72 have bit 8 = 1

  28. ACL Example 5 128 64 32 16 8 4 2 1 64 = 0 1 0 0 0 0 0 0 72 = 0 1 0 0 1 0 0 0 Wildcard Mask = 0 0 0 0 0 1 1 1 7 Access-list 20 permit 192.168.23.64 0.0.0.7

  29. ACL Example 6 • Permit network access for the 14 users in the subnet 192.168.3.32 /28. • Subtract the subnet mask of the network from 255.255.255.255: 255.255.255.255 255.255.255.240 0 . 0 . 0 . 15 Access-list 20 permit 192.168.3.32 0.0.0.15

  30. Wildcard Mask Keywords • The keywords host and any help identify the most common uses of wildcard masking, eliminating the need to enter wildcard masks when identifying a specific host or network. • The host option substitutes for the 0.0.0.0 mask: • Instead of entering 192.168.10.10 0.0.0.0, use host 192.168.10.10. • The any option substitutes for the IP address and 255.255.255.255 mask: • instead of entering 0.0.0.0 255.255.255.255, can use the keyword any by itself.

  31. R1 Applying Standard ACLs 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • After a standard ACL is configured, it is linked to an interface using the ip access-group command: • Direction refers to the direction in which packets must be are flowing in order for the ACL to check them.

  32. R1 Standard ACLs to Control VTY Access 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Restricting VTY access allows the definition of which IP addresses are allowed Telnet access to the router EXEC process. This technique can be used with with SSH to further improve administrative access security.

  33. Editing Numbered ACLs • When configuring an ACL, the statements are added in the order that they are entered at the end of the ACL. However, there is no built-in editing feature that allows you to edit a change in an ACL - selectively inserting or deleting lines is not possible. • Therefore, any ACL is best constructed in a text editor such as MS Notepad, allowing the ACL to be edited and then pasted into the router as follows: • Display the ACL using the sh run command. • Highlight the ACL, copy it, and then paste it into MS Notepad. Edit the list as required. Once the ACL is correctly displayed in MS Notepad, highlight it and copy it. • In global configuration mode, remove the old access list using the no access-list command. Then paste the new ACL into the configuration of the router.

  34. R1 Creating Named ACLs 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Naming an ACL makes it easier to understand its function. Named ACLs have a different configuration mode and command syntax:

  35. Verifying ACLs There are many show commands that will verify the content and placement of ACLs on the router: • show ip interface • show access-lists • show access-list <ACL-number> • show running-config

  36. R1 Editing Named ACLs 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Use of sequence • numbers allows lines to be added and removed from named ACLs

  37. Extended ACLs • Extended ACLs are used more often than standard ACLs because they provide a greater range of control and, therefore, add additional security. • Like standard ACLs, extended ACLs check the source packet addresses, but they also check the destination address, protocols and port numbers (or services).

  38. Extended ACLs At the end of the extended ACL statement, an administrator can specify a TCP or UDP port number. Using Port Numbers: Using Keywords:

  39. Extended ACLs • Use the ‘?’ to display a list of layer-4 protocols and their associated port numbers

  40. R2 R3 R1 ISP Creating Extended ACLs S0/1/0 209.165.200.255/27 S0/0/1 10.2.2.1/30 S0/0/0 10.1.1.1/30 S0/0/1 10.2.2. 2/30 192.168.11.0/24 192.168.10.0/24 Fa0/0 Fa0/0 • Administrator needs to restrict Internet access to allow only website browsing. ACL 103 applies to traffic leaving the 192.168.10.0 network, and ACL 104 to traffic coming into the network.

  41. WAN Extended ACLs - Established • Allow access to traffic that originated in the network only • Allow external network testing A(config)#access-list 101 Permit TCP Any Any Established A(config)#access-list 101 Permit ICMP Any Any Echo-Reply A(config)#access-list 101 Permit ICMP Any Any Unreachable In Out

  42. R2 R3 R1 ISP Applying Extended ACLs S0/1/0 209.165.200.255/27 S0/0/1 10.2.2.1/30 S0/0/0 10.1.1.1/30 S0/0/1 10.2.2. 2/30 192.168.11.0/24 192.168.10.0/24 Fa0/0 Fa0/0 • ACL 103 is allowing internal users to access the Internet – it is applied to the S0/0/0 outbound. • ACL 104 is allowing established Internet traffic to enter network 192.168.10.0 – it is applied to S0/0/0 inbound.

  43. R1 Applying Extended ACLs 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Deny FTP traffic from subnet 192.168.11.0 going to subnet 192.168.10.0, but permitting all other traffic. FTP requires ports 20 and 21, therefore y both eq 20 and eq 21 must be specified to deny FTP

  44. R1 Applying Extended ACLs 10.1.1.1/30 S0/0/0 192.168.10.1/24 192.168.11.1/24 Fa0/0 Fa0/1 PC1 192.168.10.10/24 PC2 192.168.11.10/24 Fa0/1 Fa0/1 Fa0/2 Fa0/2 • Deny Telnet traffic from 192.168.11.0 leaving interface Fa0/0, but allow all other IP traffic from any other source to any destination out Fa0/0. Note the use of the any keywords, meaning from anywhere going to anywhere.

  45. R2 R3 R1 ISP Named Extended ACLs S0/1/0 209.165.200.255/27 S0/0/1 10.2.2.1/30 S0/0/0 10.1.1.1/30 S0/0/1 10.2.2. 2/30 192.168.11.0/24 192.168.10.0/24 Fa0/0 Fa0/0 • Named extended ACLs are created in essentially the same way as named standard ACLs:

  46. Complex ACLs • Dynamic ACLs (lock-and-key) - Users that want to traverse the router are blocked until they use Telnet to connect to the router and are authenticated. • Reflexive ACLs - Allows outbound traffic and limits inbound traffic in response to sessions that originate inside the router. • Time-based ACLs - Allows for access control based on the time of day and week

  47. Dynamic ACLs Dynamic ACLs have the following security benefits over standard and static extended ACLs: • Use of a challenge mechanism to authenticate individual users • Simplified management in large internetworks • In many cases, reduction of the amount of router processing that is required for ACLs • Reduction of the opportunity for network break-ins by network hackers • Creation of dynamic user access through a firewall, without compromising other configured security restrictions

  48. R2 R3 R1 ISP Dynamic ACLs S0/1/0 209.165.200.255/27 S0/0/1 10.2.2.1/30 S0/0/0 10.1.1.1/30 S0/0/1 10.2.2. 2/30 PC1 192.168.10.10/24 PC2 192.168.30.10/24 Fa0/0 Fa0/0 • PC1 is an administrator that requires a back door access to the 192.168.30.0 /24 network located on router R3. • A dynamic ACL has been configured to allow FTP and HTTP on router R3 access but only for a limited time.

  49. Reflexive ACLs • Reflexive ACLs force the reply traffic from the destination of a known recent outbound packet to go to the source of that outbound packet. • Network administrators use reflexive ACLs to allow IP traffic for sessions originating from their network while denying IP traffic for sessions originating outside the network. These ACLs allow the router to manage session traffic dynamically. • Reflexive ACLs provide a truer form of session filtering than an extended ACL that uses the established parameter introduced earlier. Although similar in concept to the established parameter, reflexive ACLs also work for UDP and ICMP, which have no ACK or RST bits.

  50. Reflexive ACLs Reflexive ACLs have the following benefits: • Help secure networks against network hackers and can be included in a firewall defense. • Provide a level of security against spoofing and certain DoS attacks. Reflexive ACLs are much harder to spoof because more filter criteria must match before a packet is permitted through. For example, source and destination addresses and port numbers, not just ACK and RST bits, are checked. • Simple to use and, compared to basic ACLs, provide greatercontrol over which packets entering a network.

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