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Lecture #5 Access Control Lists (ACLs)

Asst.Prof. Dr.Anan Phonphoem

Department of Computer Engineering,

Faculty of Engineering, Kasetsart University,

Bangkok, Thailand

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Overview

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

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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

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What is ACL?

A List of Criteria to which all Packets are compared. Is this Packet from Network 10.5.2.0

Yes - Forward the Packet No - Check with Next Statement

Is this a Telnet Protocol Packet from 25.25.0.0 Yes - Forward the Packet No - Check Next Statement

Deny All Other Traffic

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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

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ACL operations

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ACL numbers

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Standard ACL

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

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Extended ACL

Are given a # from 100-199 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

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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)

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Standard ACL format

# 1-99

permit/deny switch the packet or drop it

sourceIP source 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

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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

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Wildcard Mark

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Wildcard Example

Network Wildcard 172.16.10.0 0.0.0.255 Result: Match the first three octets exactly but

ignore the last octet. 172.16.10.0 thru 172.16.10.255 is a match

since the last octet does not matter.

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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)

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Standard ACL

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Standard ACL example (I)

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?

•Deny any host 172.22.5.2•Deny any host 172.22.5.3•All other traffic can go

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Standard ACL example (II)

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

A(config)#access-list 5 deny 172.22.5.4 0.0.0.0

Why does the last line have no affect? How could you correct this situation?

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Extended ACL

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Placing ACLs

Standard : Closed to sourceExtended: Closed to destination

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Firewall

DMZ

External Internal

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Restricted ACL access

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Verifying ACLs

show ip interface show access-listsShow running-config

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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.

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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.

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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 # 100-199 Extended Standard - source address only Extended - source, destination, protocol, port

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References

C.Dodge slide in Cisco Website Cisco curriculum materials