Filtering Traffic Using Access Control Lists

1. Filtering Traffic Using Access Control Lists Introducing Routing and Switching in the Enterprise– Chapter 8

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. 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. Describe Traffic Filtering Devices providing traffic filtering: • Firewalls built into integrated routers • Dedicated security appliances • Servers

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. Describe Traffic Filtering Possible issues with ACLs: • Increased load on router • Possible network disruption • Unintended consequences from incorrect placement

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Create and Apply ACLs to Control Specific Types of Traffic • Examine every ACL one line at a time to avoid unintended consequences

21. Create and Apply ACLs to Control Specific Types of Traffic • Apply ACLs to VLAN interfaces or subinterfaces just as with physical interfaces

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. Log ACL Activity and ACL Best Practices Syslog messages: • Status of router interfaces • ACL messages • Bandwidth, protocols in use, configuration events

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. 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