access control lists

Access Control Lists

Types Standard Extended

Standard ACLs Use only the packet’s source

address for comparison 1-99

Extended ACLs Provide more precise (finer tuned)

packet selection based on: Source and destination addresses Protocols Port numbers

100-199

Steps to Create an ACL Create ACL in global config Assign to interface Decide the direction

In Out

How do ACLs work? Processing occurs line by line from

top to bottom. New lines are added at the end of

the current list. Last line of an ACL is an implicit

“deny any.”

How does a Standard ACL work? If source IP address is matched:

Permit or deny statement is processed

Permit – action in ACL is performed Deny – packet is dropped Implicit Deny – If a packet’s address does

not match an earlier statement an implicit deny any occurs at the end of every ACL and the packet is dropped.

Wildcard Masks Are used to specify (by bits) the

traffic you are trying to filter by address.

Use 1s to ignore, 0s to match. In the example below, only the 1st

2 octets will be examined: 172.16.0.0 0.0.255.255

Global Standard ACL command access-list access-list-number

{permit |deny} source-ip-address wildcard-mask [log]

Log – causes each packet that matches this statement to generate a log entry that is recorded by the router.

Examples of Standard ACLs To permit all packets for the

network number 172.16.0.0 Access-list 20 permit 172.16.0.0

0.0.255.255

Examples Cont’d To permit traffic from the host

172.16.1.1 only Access-list 20 permit 172.16.1.1

0.0.0.0

Examples Cont’d To permit traffic from any source

address. Access-list 20 permit 0.0.0.0

255.255.255.255OR

Access-list 20 permit any

Examples Cont’d To permit traffic from the subnet

12.16.0.0 through 12.31.0.0 Access-list 20 permit 12.16.0.0

0.15.255.255

Identical Statements Access-list 22 permit 0.0.0.0

255.255.255.255 Access-list 22 permit any

Identical Statements Access-list 23 permit 172.16.1.1

0.0.0.0 Access-list 23 permit host

172.16.1.1

How does an Extended ACL work? All conditions must match Test sequence in this order

Source Address Destination Address Protocol Port No. or Protocol Options Permit or Deny decision

Extended ACL command access-list number {permit|

deny} protocol source-ip-address source-wildcard-mask destination-ip-address destination-wildcard-mask eq port-number [log]

Some Protocols with Port Numbers FTP – 21 Telnet – 23 SMTP – 25 DNS – 53 TFTP – 69 WWW, HTML – 80 POP3 - 110 SNMP - 161

Major differences Standard ACL

Use only source address and requires fewer CPU cycles.

Place as close to destination as possible. Extended ACL

More flexible and requires more CPU cycles. Place as close to source as possible. (This

keeps undesired traffic and ICMP messages away from the network backbone.)

Do I place an ACL in? In

Requires less CPU processing because every packet bypasses processing before it is routed.

Filtering decision is made prior to the routing table.

Do I place an ACL out? Out

Routing decision has been made and the packet is switched to the proper outbound interface before it is tested against the access list.

ACLs are outbound unless otherwise specified.