1 Lecture #5 Access Control Lists (ACLs) Asst.Prof. Dr.Anan Phonphoem Department of Computer Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand

2 Overview ACL fundamentals ACL operations Types of ACLs (Standard / Extended) Implementing ACLs

3 Access Control Lists (ACLs) List of conditions to test the traffic Router can permit or deny( like a filter) Provides Security Bandwidth Management Come in two Types STANDARD and EXTENDED

4 What is ACL? A List of Criteria to which all Packets are compared. Is this Packet from Network Yes - Forward the Packet No - Check with Next Statement Is this a Telnet Protocol Packet from Yes - Forward the Packet No - Check Next Statement Deny All Other Traffic

5 ACL Operations Packets are compared to Each Statement in an Access-list SEQUENTIALLY- From the Top Down. The sooner a decision is made the better. Well written Access-lists take care of the most abundant type of traffic first. All Access-lists End with an Implicit Deny All statement

6 ACL operations

7 ACL numbers

8 Standard ACL Are given a # from 1-99 Filtering based only on Source Address Should be applied closest to the Destination

9 Extended ACL Are given a # from Much more flexible and complex Can filter based on: Source address Destination address Session Layer Protocol (ICMP, TCP, UDP..) Port Number (80 http, 23 telnet…) Should be applied closest to the Source

10 Implementing ACLs Step 1 - Create the Access-list Step 2 -Apply the Access-list to an Interface Must be in interface config mode (config-if)# IP access-group # in/out (routers point of view)

11 Standard ACL format # 1-99 permit/denyswitch the packet or drop it sourceIPsource IP address to which the packet should be compared. Can also use ANY wildcard (inverse mask) see next slides access-list # permit/deny sourceIP wildcard

12 Wildcard Mark 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

13 Wildcard Mark

14 Wildcard Example Network Wildcard Result: Match the first three octets exactly but ignore the last octet thru is a match since the last octet does not matter.

15 Implementing ACLs Remember the Implicit Deny All at the end of each access-list. Two Approaches: 1. List the traffic you know you want to permit Deny all other traffic 2. List the traffic you want to deny Permit all other traffic (permit any)

16 Standard ACL

17 Standard ACL example (I) A(config)#access-list 5 deny A(config)#access-list 5 deny A(config)#access-list 5 permit any So what does this access list do? Deny any host Deny any host All other traffic can go

18 Standard ACL example (II) A(config)#access-list 5 deny A(config)#access-list 5 deny A(config)#access-list 5 permit any A(config)#access-list 5 deny Why does the last line have no affect? How could you correct this situation?

19 Extended ACL

20 Placing ACLs Standard : Closed to source Extended: Closed to destination

21 Firewall DMZ External Internal

22 Restricted ACL access

23 Verifying ACLs show ip interface show access-lists Show running-config

24 Implementing ACLs Tips You cannot selectively add or remove statements from an Access-list Typically modifications are made in a text editor and then pasted to the router as a new access-list. The new access list is then applied and the old one removed Document your Access-list After each line indicate exactly what that line is supposed to do.

25 Implementing ACLs Tips Verifying Your Access-list Show Access-lists Show IP Interfaces Revisit your access-list after a few days Routers keep track of the number of packets that match each statement in an access-list Use this information to reorder your access-list and thus improve it efficiency Never remove an access-list that is applied to a port - this can crash a router.

26 Summary Are Created and then Applied to an interface Are Implemented Sequentially- Top Down End with an implicit Deny ALL statement #1-99 Standard and # Extended Standard - source address only Extended - source, destination, protocol, port

27 References C.Dodge slide in Cisco Website Cisco curriculum materials