1. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 1 Version 4.0 Filtering Traffic Using Access Control Lists Introducing Routing and Switching in the Enterprise – Chapter 8

2. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 2 Objectives  Describe traffic filtering and explain how Access Control Lists (ACLs) can filter traffic at router interfaces.  Analyze the use of wildcard masks.  Configure and implement ACLs.  Create and apply ACLs to control specific types of traffic.  Log ACL activity and integrate ACL best practices.

3. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 3 Describe Traffic Filtering  Analyze the contents of a packet  Allow or block the packet  Based on source IP, destination IP, MAC address, protocol, application type

4. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 4 Describe Traffic Filtering Devices providing traffic filtering:  Firewalls built into integrated routers  Dedicated security appliances  Servers

5. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 5 Describe Traffic Filtering Uses for ACLs:  Specify internal hosts for NAT  Classify traffic for QoS  Restrict routing updates, limit debug outputs, control virtual terminal access

6. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 6 Describe Traffic Filtering Possible issues with ACLs:  Increased load on router  Possible network disruption  Unintended consequences from incorrect placement

7. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 7 Describe Traffic Filtering  Standard ACLs filter based on source IP address  Extended ACLs filter on source and destination, as well as protocol and port number  Named ACLs can be either standard or extended

8. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 8 Describe Traffic Filtering  ACLs consist of statements  At least one statement must be a permit statement  Final statement is an implicit deny  ACL must be applied to an interface in order to work

9. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 9 Describe Traffic Filtering  ACL is applied inbound or outbound  Direction is from the router’s perspective  Each interface can have one ACL per direction for each network protocol

10. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 10 Analyze the Use of Wildcard Masks  Wildcard mask can block a range of addresses or a whole network with one statement  0s indicate which part of an IP address must match the ACL  1s indicate which part does not have to match specifically

11. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 11 Analyze the Use of Wildcard Masks  Use the host parameter in place of a 0.0.0.0 wildcard  Use the any parameter in place of a 255.255.255.255 wildcard

12. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 12 Configure and Implement Access Control Lists  Determine traffic filtering requirements  Decide which type of ACL to use  Determine the router and interface on which to apply the ACL  Determine in which direction to filter traffic

13. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 13 Configure and Implement Access Control Lists: Numbered Standard ACL  Use access-list command to enter statements  Use the same number for all statements  Number ranges: 1-99, 1300-1999  Apply as close to the destination as possible

14. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 14 Configure and Implement Access Control Lists: Numbered Extended ACL  Use access-list command to enter statements  Use the same number for all statements  Number ranges: 100-199, 2000-2699  Specify a protocol to permit or deny  Place as close to the source as possible

15. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 15 Configure and Implement Access Control Lists: Named ACLs  Descriptive name replaces number range  Use ip access-list command to enter initial statement  Start succeeding statements with either permit or deny  Apply in the same way as standard or extended ACL

16. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 16 Configure and Implement Access Control Lists: VTY access  Create the ACL in line configuration mode  Use the access-class command to initiate the ACL  Use a numbered ACL  Apply identical restrictions to all VTY lines

17. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 17 Create and Apply ACLs to Control Specific Types of Traffic  Use a specified condition when filtering on port numbers: eq, lt, gt  Deny all appropriate ports for multi-port applications like FTP  Use the range operator to filter a group of ports

18. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 18 Create and Apply ACLs to Control Specific Types of Traffic  Block harmful external traffic while allowing internal users free access  Ping: allow echo replies while denying echo requests from outside the network  Stateful Packet Inspection

19. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 19 Create and Apply ACLs to Control Specific Types of Traffic  Account for NAT when creating and applying ACLs to a NAT interface  Filter public addresses on a NAT outside interface  Filter private addresses on a NAT inside interface

20. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 20 Create and Apply ACLs to Control Specific Types of Traffic  Examine every ACL one line at a time to avoid unintended consequences

21. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 21 Create and Apply ACLs to Control Specific Types of Traffic  Apply ACLs to VLAN interfaces or subinterfaces just as with physical interfaces

22. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 22 Log ACL Activity and ACL Best Practices  Logging provides additional details on packets denied or permitted  Add the log option to the end of each ACL statement to be tracked

23. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 23 Log ACL Activity and ACL Best Practices Syslog messages:  Status of router interfaces  ACL messages  Bandwidth, protocols in use, configuration events

24. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 24 Log ACL Activity and ACL Best Practices  Always test basic connectivity before applying ACLs  Add deny ip any to the end of an ACL when logging  Use reload in 30 when testing ACLs on remote routers

25. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 25 Summary  ACLs enable traffic management and secure access to and from a network and its resources  Apply an ACL to filter inbound or outbound traffic  ACLs can be standard, extended, or named  Using a wildcard mask provides flexibility  There is an implicit deny statement at the end of an ACL  Account for NAT when creating and applying ACLs  Logging provides additional details on filtered traffic

26. © 2006 Cisco Systems, Inc. All rights reserved.Cisco Public 26