Access Control Lists (ACL)

Access-List Overview 4 A Filter through which all traffic must pass 4 Used to Permit or Deny Access to Network 4 Provides Security 4 Bandwidth Management 4 Come in two flavors –STANDARD AND EXTENDED

What is an Access-List 4 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

How an Access-List Works 4 Packets are compared to Each Statement in an Access-list SEQUENTIALLY - From the Top Down. 4 The sooner a decision is made the better. 4 Well written Access-lists take care of the most abundant type of traffic first. 4 All Access-lists End with an Implicit Deny All statement

Standard Access Lists 4 Are given a # from Filtering based only on Source Address 4 Should be applied closest to the Destination

Extended Access-lists 4 Are given a # from Much more flexible and complex 4 Can filter based on: –Source address –Destination address –Session Layer Protocol (ICMP, TCP, UDP..) –Port Number (80 http, 23 telnet…) 4 Should be applied closest to the Source

Two Steps - Create and Apply 4 Step 1 - Create the Access-list –access-list # permit/deny source IP wildcard # permit/deny - switch the packet or drop it source IP - source IP address to which the packet should be compared. Can also use ANY wildcard - see next page 4 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)

Wildcards 4 Allows you to indicate a Range of IP addresses 4 Two Values are Used: –0 = Must Match Exactly –1 = Does Not Matter

Wildcard Examples Network Wildcard Result: Match all four octets 4 Only is a match 4 Could also use host in place of the wildcard. Host indicates an exact match is needed.

Wildcard Examples 4 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.

Implementing Access-lists 4 Remember the Implicit Deny All at the end of each access-list. 4 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)

Implementing Access-lists 4 You cannot selectively add or remove statements from an Access-list 4 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 4 Document your Access-list –After each line indicate exactly what that line is supposed to do.

Implementing Access-lists 4 Verifying Your Access-list –Show Access-lists –Show IP Interfaces 4 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 4 Never remove an access-list that is applied to a port - this can crash a router.

Summary: Access-Lists 4 Are Created and then Applied to an interface 4 Are Implemented Sequentially- Top Down 4 End with an implicit Deny ALL statement 4 #1-99 Standard and # Extended 4 Standard - source address only 4 Extended - source, destination, protocol, port