Chap 5 – Access Control Lists
Learning Objectives
•
•
•
Explain how ACLs are used to secure a medium-size
Enterprise branch office network.
Configure standard ACLs in a medium-size Enterprise
branch office network.
Configure extended ACLs in a medium-size Enterprise
branch office network.
•
•
•
Not NAMED ACLs – Different configuration method.
Describe complex ACLs in a medium-size Enterprise branch
office network.
Implement, verify and troubleshoot ACLs in an enterprise
network environment.
1
Chapter 5
How it fits in
•
•
Chapter 5 of CCNA4 online material
Assessed in skills test
•
•
Assessed in end test
•
•
You’ll need to create an ACL
There’ll be some questions about ACLS
Recommended labs –
•
•
5.2.4 and 5.2.8 (at end of lecture)
5.5.1, 5.5.2, 5.5.3 (on askgeoff)
2
Chapter 5
Access Control Lists (ACL)
ACLs are lists of instructions applied to a router's interface
to tell the router which kind of packets to permit and which
kind to deny.
3
Chapter 5
Access Control Lists (ACL)
•
An ACL is a sequential list of permit or deny statements
that apply to IP addresses or upper-layer protocols.
The ACL can extract the following information from the
packet header, test it against its rules, and make "allow"
or "deny" decisions based on:
1.
2.
3.
•
Source IP address
Destination IP address
ICMP message type
The ACL can also extract upper layer information and
test it against its rules. Upper layer information
includes:
1.
2.
TCP/UDP source port
TCP/UDP destination port
4
Chapter 5
Access Control Lists (ACL)
•Limit network traffic and increase network performance. Provide
traffic flow control.
•ACLs can restrict the delivery of routing updates. If updates are not
required because of network conditions, bandwidth is preserved.
•Provide a basic level of security for network access. ACLs can allow one
host to access a part of the network and prevent another host from
accessing the same area.
•Decide which types of traffic are forwarded or blocked at the router
interfaces. ACLs can permit e-mail traffic to be routed, but block all
Telnet traffic.
•Control which areas a client can access on a network.
•ACLs can be used to permit or deny a user to access file types such as
FTP or HTTP.
5
Chapter 5
How ACLs Work
•ACLs must be defined on a per protocol, per direction, or per
port basis.
•To control traffic flow on an interface, an ACL must be
defined for each protocol enabled on the interface.
•ACLs control traffic in one direction at a time on an interface.
6
Chapter 5
How ACLs Work
•ACL statements operate in sequential, logical order, from top to bottom.
•If a condition is matched, the packet is permitted or denied and the rest
of the ACL isn’t checked.
•An implicit deny any statement is at end of all lists by default.
•This last line "deny any" is not visible but it will not allow any unmatched
packets to be permitted.
Route packet to
outbound interface
Frame arrives
at inbound
interface
L3 address
match?
No
Yes
ACL on
interface?
Yes
No
Default
Deny
No
Any
matches?
Yes
No
7
ACL on
interface?
Yes
Yes
Permit?
Default
Deny
No
Any
matches?
Yes
No
Permit?
Yes
Forward packet
Chapter 5
Wildcard Masks
•
•
•
ACLs statements include masks, also called wildcard
masks. The mask determines how much of an IP source or
destination address to apply to the address match. The
numbers 1 and 0 in the mask identify how to treat the
corresponding IP address bits. They are different to
subnet masks, and follow different rules.
Wildcard masks and subnet masks are both 32 bits long
and use binary 1s and 0s. Subnet masks use binary 1s and
0s to identify the network, subnet, and host portion of an
IP address.
Wildcard masks use binary 1s and 0s to filter individual or
groups of IP addresses to permit or deny access to
resources based on an IP address. By carefully setting
wildcard masks, you can permit or deny a single or several
IP addresses
8
Chapter 5
Wildcards (Inverse Mask)
•
•
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
9
Chapter 5
Wildcard Masks
Source IP
172.16.10.10
Wildcard
0.0.0.0
Source IP 10101100. 00010000. 00001010.
Wildcard
00001010
00000000. 00000000. 00000000. 00000000
Must
match
Must
match
Must
match
Must
match
Range of matching addresses: 172.16.10.10 only
10
Chapter 5
Wildcard Masks
Source IP
172.16.10.0
Wildcard
0.0.0.255
Source IP 10101100. 00010000. 00001010.
Wildcard
00000000. 00000000. 00000000.
Must
match
Must
match
Must
match
00000000
11111111
Don’t
Care
Range of matching addresses: 172.16.10.0 to 172.16.10.255
11
Chapter 5
Wildcard Masks
Source IP
172.16.10.0
Wildcard
0.0.255.255
Source IP 10101100. 00010000. 00001010.
Wildcard
00000000. 00000000.
Must
match
Must
match
11111111.
Don’t
Care
00000000
11111111
Don’t
Care
Range of matching addresses: 172.16.0.0 to 172.16.255.255
12
Chapter 5
Wildcard Mask Keywords
•
The keywords host and any help identify the most common
uses of wildcard masking, eliminating the need to enter
wildcard masks when identifying a specific host or
network.
•The host option substitutes for the 0.0.0.0 mask:
Instead of entering 192.168.10.10 0.0.0.0, use host 192.168.10.10.
•The any option substitutes for the IP address and 255.255.255.255
mask:
instead of entering 0.0.0.0 255.255.255.255, can use the keyword any
by itself.
13
Chapter 5
Standard ACLs
•
•
Standard ACLs allow you to permit or deny traffic
from source IP addresses.
The destination of the packet and the ports involved do
not matter.
•The example allows all traffic from network
192.168.30.0/24 network.
•Because of the implied deny any at the end, all other
traffic is blocked with this ACL.
•Standard ACLs are created in global configuration mode.
14
Chapter 5
Extended ACLs
•Extended ACLs filter IP packets based on several
attributes, for example, protocol type, source IP address,
destination IP address, source TCP or UDP ports, destination
TCP or UDP ports, and optional protocol type information for
finer granularity of control.
•In the example, ACL 103 permits traffic originating from
any address on the 192.168.30.0/24 network to any
destination host using port 80 (HTTP).
•Extended ACLs are created in global configuration mode.
15
Chapter 5
Numbering & Naming ACLs
Numbered ACLs - Assign a number based on which
protocol is to be filtered filtered:
•
(1 to 99) and (1300 to 1999): Standard IP ACL
•
(100 to 199) and (2000 to 2699): Extended IP ACL
Named ACLs - assign a name by providing the name of
the ACL: (only after IOS Release 11.2!)
• Names can contain alphanumeric characters.
• Recommended that the name be written in
CAPITAL LETTERS.
• Names cannot contain spaces or punctuation and
mustbegin with a letter.
• Possible to add or delete entries within the ACL.
16
Chapter 5
ACL Placement
•
•
Standard ACLs should be placed close to the
destination. (standard ACLs can only permit/deny
according to SOURCE)
Extended ACLs should be placed close to the source.
Extended 17
ACL’s can act on a wider range of parameters
Chapter 5
ACL Best Practice
Using ACLs requires attention to detail and great care. Mistakes can be
costly in terms of downtime, troubleshooting efforts, and poor network
service. Before starting to configure an ACL, basic planning is required:
•
•
•
Base ACLs on the security policy of the
organisation.
Prepare a description of what ACLs are required
to do.
Use a text editor to create, edit and save ACLs.
•
•
DON’T create them on the fly!
Test ACLs on a development network before
18
implementing
them on a production network.
Chapter 5
Editing Numbered ACLs
•
•
When configuring an ACL, the statements are added in the order
that they are entered at the end of the ACL. However, there is no
built-in editing feature that allows you to edit a change in an ACL selectively inserting or deleting lines is not possible.
Therefore, any ACL is best constructed in a text editor such as MS
Notepad, allowing the ACL to be edited and then pasted into the
router as follows:
1.
2.
3.
Display the ACL using the sh run command.
Highlight the ACL, copy it, and then paste it into MS Notepad.
Edit the list as required. Once the ACL is correctly displayed in
MS Notepad, highlight it and copy it.
In global configuration mode, remove the old access list using
the no access-list command. Then paste the new ACL into the
configuration of the router.
19
Chapter 5
Configuring Standard ACLs
192.168.30.0/24
192.168.10.1/24
Fa0/0
PC1
192.168.10.10/24
Computer
Fa0/1
Fa0/2
192.168.11.1/24
R1
Fa0/1
Fa0/1
PC2
192.168.11.10/24
Fa0/2
Computer
•Both ACLs have the same effect, due to the implicit deny at
the end of all ACLs:
20
Chapter 5
Creating Standard ACLs
access-list # permit/deny source IP wildcard
To delete:
21
Chapter 5
Packet
Header
Segment
Header
Data
Standard ACL
Source is
192.168.10.1?
Yes
access-list 2 deny 192.168.10.1
Deny
access-list 2 permit 192.168.10.0 0.0.0.255
access-list 2 deny 192.168.0.0 0.0.255.255
No
Source is
192.168.10.0
0.0.0.255?
No
Yes
Permit access-list 2 permit 192.0.0.0 0.255.255.255
Source is
192.168.0.0
0.0.255.255?
No
Yes
Deny
Source is
192.0.0.0
0.255.255.255?
Source is
22 Any other address?
Yes
Permit
No
Implicit Deny
Chapter 5
Verify Standard ACLs
•The remark keyword is used for documentation and makes access lists
a great deal easier to understand. Each remark is limited to 100
characters.
•When reviewing the ACL in the configuration, the remark is also
displayed.
23
Chapter 5
Applying Standard ACLs
10.1.1.1/30
S0/0/0
192.168.10.1/24
Fa0/0
PC1
192.168.10.10/24
Computer
Fa0/1
Fa0/2
192.168.11.1/24
R1
Fa0/1
Fa0/1
PC2
192.168.11.10/24
Fa0/2
Computer
•After a standard ACL is configured, it is linked to an
interface using the ip access-group command:
•Direction refers to the direction in which packets must
be are flowing in order for the ACL to check them.
24
Chapter 5
Standard ACLs to Control VTY Access
10.1.1.1/30
S0/0/0
192.168.10.1/24
Fa0/0
PC1
192.168.10.10/24
Computer
Fa0/1
Fa0/2
192.168.11.1/24
R1
Fa0/1
Fa0/1
PC2
192.168.11.10/24
Fa0/2
Computer
•Restricting VTY access allows the definition of which IP addresses are
allowed Telnet access to the router EXEC process. This technique can be
used with with SSH to further improve administrative access security.
25
Chapter 5
Verifying ACLs
There are many show commands that will verify
the content and placement of ACLs on the
router:
•
•
•
•
show ip interface
show access-lists
show access-list <ACL-number>
show running-config
26
Chapter 5
Extended ACLs
•Extended ACLs are used more often than standard ACLs
because they provide a greater range of control and,
therefore, add additional security.
•What do you think your test will include?
•Like standard ACLs, extended ACLs check the packet
source addresses, but they can also check the destination
address, protocols and port numbers (or services).
27
Chapter 5
Extended ACLs
At the end of the
extended ACL
statement, an
administrator can
specify a TCP or
UDP port number.
Using Port Numbers:
Using Keywords:
28
Chapter 5
Extended ACLs
•Use the ‘?’ to
display a list of
layer-4 protocols
and their
associated port
numbers
29
Chapter 5
Creating Extended ACLs
S0/1/0
209.165.200.255/27
R2
S0/0/0
10.1.1.1/30
192.168.10.0/24
Fa0/0
R1
ISP
S0/0/1
10.2.2.1/30
S0/0/1
10.2.2. 2/30
R3
192.168.11.0/24
Fa0/0
•Administrator needs to restrict Internet access to allow
only website browsing. ACL 103 applies to traffic leaving the
192.168.10.0 network, and ACL 104 to traffic coming into the
network.
30
Chapter 5
Extended ACLs - Established
•Allow access to traffic that originated in the network only
•Allow external network testing
In
WAN
Out
Computer
A(config)#access-list 101 Permit TCP Any Any Established
A(config)#access-list 101 Permit ICMP Any Any Echo-Reply
A(config)#access-list 101 Permit ICMP Any Any Unreachable
31
Chapter 5
Applying Extended ACLs
S0/1/0
209.165.200.255/27
R2
S0/0/0
10.1.1.1/30
192.168.10.0/24
Fa0/0
R1
ISP
S0/0/1
10.2.2.1/30
S0/0/1
10.2.2. 2/30
R3
192.168.11.0/24
Fa0/0
•ACL 103 is allowing internal users to access the Internet – it
is applied to the S0/0/0 outbound.
•ACL 104 is allowing established Internet traffic to enter
network 192.168.10.0 – it is applied to S0/0/0 inbound.
32
Chapter 5
Applying Extended ACLs
10.1.1.1/30
S0/0/0
192.168.10.1/24
Fa0/0
PC1
192.168.10.10/24
Computer
Fa0/1
Fa0/2
192.168.11.1/24
R1
Fa0/1
Fa0/1
PC2
192.168.11.10/24
Fa0/2
Computer
•Deny FTP traffic from subnet 192.168.11.0 going to subnet 192.168.10.0,
but permitting all other traffic. FTP requires ports 20 and 21, therefore
both eq 20 and eq 21 must be specified to deny FTP
33
Chapter 5
Applying Extended ACLs
10.1.1.1/30
S0/0/0
192.168.10.1/24
Fa0/0
PC1
192.168.10.10/24
Computer
Fa0/1
Fa0/2
192.168.11.1/24
R1
Fa0/1
Fa0/1
PC2
192.168.11.10/24
Fa0/2
Computer
•Deny Telnet traffic from 192.168.11.0 leaving interface
Fa0/0, but allow all other IP traffic from any other source
to any destination out Fa0/0. Note the use of the any
keywords, meaning from anywhere going to anywhere.
34
Chapter 5
Chap 5 – Access Control Lists
Learning Objectives
•
•
•
•
•
Explain how ACLs are used to secure a medium-size
Enterprise branch office network.
Configure standard ACLs in a medium-size Enterprise
branch office network.
Configure extended ACLs in a medium-size Enterprise
branch office network.
Describe complex ACLs in a medium-size Enterprise branch
office network.
Implement, verify and troubleshoot ACLs in an enterprise
network environment.
35
Chapter 5
Any
Questions?
36
Chapter 5
Chapter 5.2.8 – Standard
ACLs
Lab Topology
209.165.200.224/27
Fa0/0
192.168.20.1/254
S0/0/0
WWW/TFTP
192.168.20.254/24
S0/1/0
R2
.2
10.2.2.0/30
Fa0/1
192.168.11.0/24
S2
Computer
Computer
PC1
192.168.10.10
PC2
192.168.11.10
37
Computer
Ext Host
209.165.202.158/27
S0/0/1
•Allow only PC 1
to Telnet to R3
S1
209.165.202.129/27
ISP
209.165.201.1/27
.2
192.168.10.0/24
Fa0/1
Fa0/0
S0/0/1
DCE
.1
S0/0/0
DCE .1
R1
.226
.225
10.1.1.0/30
Fa0/0
S0/0/1
DCE
•The 192.168.11.0/24 network
is allowed access to all
destinations, except to any
networks connected to the
ISP.
R3
Fa0/0
•The 192.168.10.0/24 network
is allowed access to all
locations, except the
192.168.11.0/24 network.
WWW
209.165.201.30/27
192.168.30.0/24
S3
•The 192.168.30.0/10
network is allowed access to
all destinations.
•Host 192.168.30.128 is not
allowed access outside of the
LAN.
Computer
Computer
PC3
192.168.30.10
PC4
192.168.30.128
Chapter 5
Chapter 5.3.4 – Extended
ACLs
Lab Topology
209.165.200.224/27
Fa0/0
192.168.20.1/254
S0/0/0
WWW/TFTP
192.168.20.254/24
10.1.1.0/30
S0/0/0
DCE
R1
.226
.225
•Outside hosts are allowed to
establish a web session with the
internal web server on port 80 only.
S1
S2
Computer
Computer
PC1
192.168.10.10
PC2
192.168.11.10
38
209.165.202.129/27
Computer
Ext Host
209.165.202.158/27
10.2.2.0/30
.2
•Only ping replies are allowed
through R2.
192.168.11.0/24
ISP
209.165.201.1/27
S0/0/1
DCE
.1
S0/0/1
WWW
209.165.201.30/27
•All IP addresses of the
192.168.30.0/24 network are
blocked from accessing all IP
addresses of the 192.168.20.0/24
network.
R3
Fa0/1
192.168.10.0/24
Fa0/1
Fa0/0
•Only established TCP sessions are
allowed in.
.1
Fa0/0
S0/1/0
R2
.2
S0/0/1
DCE
Fa0/0
192.168.30.0/24
•The first half of 192.168.30.0/24
is allowed access to all other
destinations.
•For the 192.168.10.0/24 network,
block Telnet access to all locations
and TFTP access to the corporate
Web/TFTP server at 192.168.20.254.
All other access is allowed.
•For the192.168.11.0/24 network,
allow TFTP access and web access to
the corporate Web/TFTP server at
192.168.20.254. Block all other traffic
from the 192.168.11.0/24 network to
the 192.168.20.0/24 network. All
other access is allowed.
S3
Computer
Computer
PC3
PC4
192.168.30.10 192.168.30.128
•The second half of
192.168.30.0/24 network is allowed
access to the 192.168.10.0/24 and
192.168.11.0/24 networks.
•The second half of
192.168.30.0/24 is allowed web and
ICMP access to all remaining
destinations.
•All other access is implicitly
denied.
Chapter 5
Chapter 5.2.8 /5.3.4 –
Standard/Extended
ACLs
39
Lab Topology
Chapter 5