Transcript Router(config)# access-list

CCNA2 Routing
Access Control
172.16.3.0
172.16.4.0
e0
e1
s0
Non-172.16.0.0
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server
172.16.4.13
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CCNA2 Routing
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Access Control Lists
Standard
Extended
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Standard Access Control Lists
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ACLs are instructions that are applied to a router interfaces.
The ACLs describe the kind of packets that are to be permitted or denied.
Permitted or Denial testing can be based on:
• source address
• destination address
• port number
ACLs are configured on the router Interfaces to control access to a network.
ACLs must be defined separately for each protocol; Unique for IP, Unique for
IPX; Unique for AppleTalk, etc.
Some times ACLs are called packet filters.
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ACL - Access Control Lists
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Reasons to create ACLs:
• limit network traffic, hence increase network performance
• provide traffic flow - limit traffic through the network
• provide for security
• ACLs establish
•which traffic is blocked
•which traffic is not blocked
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ACL - Access Control Lists
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Cisco IOS checks each packet for:
1.
destination address
2.
source address
3.
protocol
4.
port number
access-list 1 deny 192.169.1.0 0.0.255.255
access-list 1 deny 192.168.1.9 0.0.0.0
access-list 1 deny 172.16.4.0 0.0.0.255
access-list 1 permit any
Each ACL statement is checked in a sequential order (first to last) and when
there is a match, no more statements are checked.
If the results are no matches, then the packet (by default) is discarded.
Adding addition ACL statements to the end of an existing list is just a matter of
adding the new statement. BUT, if
deleting an existing ACL statement causes the entire access list to be deleted.
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ACL - Access Control Lists
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When a packet enters a router’s interface:
“IN” (inbound) checking is required. The packet:
1)
Is checked against the ACL list (if one exists)
2)
Here It can be interrogated to permit or deny.
3)
If denied the packet is dropped else,
4)
It is matched against the routing table and passed to an
“OUT” (outbound interface)
1)
“OUT” (outbound) checking is required.
Here It can be interrogated to permit or deny.
2) If denied the packet is dropped
3) If permitted then packet is allowed “OUT” (outbound).
The Outbound interface’s ACL is a different list from the inbound)
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ACL - Access Control Lists
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ACL statements operate in a logical, sequential order. When a match is made
the rest of the statements are not checked.
If none the ACL statements match, then there is an implicit deny any rule.
access-list 10 { permit | deny } { test conditions }
access-list 10 { permit | deny } { test conditions }
access-list 10 { permit | deny } { test conditions }
access-list 10 { permit | deny } { test conditions }
access-list 1 deny 192.169.1.0 0.0.255.255
access-list 1 deny 192.168.1.9 0.0.0.0
access-list 1 deny 172.16.4.0 0.0.0.255
access-list 10 deny any
access-list 1 permit any
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ACL - Access Control Lists
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Two types of IP ACL :
• standard - has access list value of 1- 99
• extended - has access list value of 100 - 199
Must be configured in global configuration mode. Router (config) #
Steps in creating ACLs:
1) create the ACL (in config mode)
2) apply the ACL to an interface
ACLs are used to filter:
• inbound traffic, or
• outbound traffic
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CCNA2 Routing
Where to place ACLs
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Standard ACLs are place as close as possible to the destination.
Extended ACLs are place as close as possible to the source.
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Where to place ACLs
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Configuring an Access List:
Router(config)# access-list <ACL number> { permit | deny } { test conditions }
Router(config)# access-list 1 permit { test conditions }
Router(config)# access-list 50 deny { test conditions }
To delete all ACL statements of an access-list
Router(config)# no access-list <ACL number>
Applying the Access List:
At an interface:
Router(config)# int E0
Router(config-if)# { protocol } access-group <ACL number> [IN | OUT]
Out is the default if not mentioned
Router(config-if)# ip access-group 1
Router(config-if)# ip access-group 50
To delete an ACL group statement (this will not delete the associated list):
Router(config)# no access-group <ACL number>
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Wildcard Mask
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A wildcard mask is matched with an IP address or protocol address.
It is a 32 bit mask divided into 4 octet, each containing 8 bits.
A 0 in the mask means to check the bit in the IP you are testing.
A 1 in the mask means ignore the bit in the IP you are testing.
This notation is the opposite from subnetting!!!!!!!
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Abbreviations
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To permit or deny any address:
0.0.0.0 255.255.255.255
Address
wildcard mask
Use the abbreviation any
Router(config)# access-list 1 permit 0.0.0.0 255.255.255.255
Router(config)# access-list 1 permit any
Router(config)# access-list 1 deny 0.0.0.0 255.255.255.255
Router(config)# access-list 1 deny any
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Abbreviations
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To match all the bits of IP address use host:
EX: 172. 30.16. 29 0. 0. 0. 0
Router(config)# access-list 1 permit 172.30.16.29 0.0.0.0
Router(config)# access-list 1 permit host 172.30.16.29
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Standard ACLs
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Criteria:
• block all traffic from a network
• allow all traffic from a network
• deny entire protocol suits
Standard ACLs only check the source address.
Router(config)# access-list <ACL number> { deny | permit } source [ source wildcard] [log]
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Standard ACLs
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What does this statement accomplish?
Access-list 33 permit 172.16.0.0 0.0.255.255 log
Permits all traffic from 172.16.0.0 and sends messages to the console every
time the access list is executed.
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Standard ACLs
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What does this statement accomplish?
Access-list 44 deny 172.16.13.7 0.0.0.0 log
Denies traffic from host 172.16.13.7 and sends message to the console every
time the access list is hit.
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Standard ACLs
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What does this statement accomplish?
Access-list 55 deny 172.16.64.3 0.0.0.255
Denies all traffic from network 172.16.64.0
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Standard ACLs
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The log command:
Prints messages to the console which includes the ACL number, whether
the packet was permitted or denied, the source address, and the number
of packets.
The message is generated for the first packet that matches, and then at
five-minute intervals, including the number of packets permitted or
denied in the prior five-minute interval.
Log is used for debugging only not to be left active on live networks.
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Standard ACLs
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Example of applying the access-list:
Router(config-if)# ip access-group 33 in
Router(config-if)# ip access-group 44 out
Router(config-if)# ip access-group 44
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Standard ACLs
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To monitor IP access list: (in EXEC mode)
show access-list
Displays all access lists & their parameters configured on the router.
(Does not show you which interface the list is set on.)
show access-list <ACL number>
Shows only the parameters for the access list <ACL number>. (Does not
show you the interface the list is set on.)
show ip access-list
Shows only the IP access lists configured on the router
show ip interface
Shows which interfaces have access lists set (containing an accessgroup).
show running-config
Shows the routers entire configuration
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CCNA2 Routing
Standard ACLs
172.16.3.0
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172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
R(config)# Interface E0
R(config-if)# ip access-group 1 out
R(config)# Interface E1
R(config-if)# ip access-group 1 out
R(config)# access-list 1 permit 172.16.0.0 0.0.255.255
What does it do?
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Standard ACLs
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One Access list used by multiple Interfaces
172.16.3.0
172.16.4.0
e0
e1
s0
R(config)# Interface E0
Non-172.16.0.0
server
172.16.4.13
172.16.4.13
R(config-if)# ip access-group 1 out
R(config)# Interface E1
R(config-if)# ip access-group 1 out
R(config)# access-list 1 permit 172.16.0.0 0.0.255.255
Allows only traffic from source network 172.16.0.0 to be forwarded & and
non-172.16.0.0 traffic is blocked.
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CCNA2 Routing
Standard ACLs
172.16.3.0
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172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
R(config)# Interface E0
R(config-if)# ip access-group 1 out
R(config)# access-list 1 deny 172.16.4.13 0.0.0.0
R(config)# access-list 1 permit any
What does this do?
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Standard ACLs
172.16.3.0
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172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
R(config)# Interface E0
R(config-if)# ip access-group 1 out
R(config)# access-list 1 deny 172.16.4.13 0.0.0.0
R(config)# access-list 1 permit any
Denies traffic from a specific device, 172.16.4.13 & allows all other traffic thru E0 to
network 172.16.3.0.
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Standard ACLs
172.16.3.0
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172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 1 out
access-list 1 deny 172.16.4.0 0.0.0.255
access-list 1 permit any
What does this do?
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Standard ACLs
172.16.3.0
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172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 1 out
access-list 1 deny 172.16.4.0 0.0.0.255
access-list 1 permit any
Denies traffic from the subnet, 172.16.4.0 & allows all other traffic thru E0 to
network 172.16.3.0.
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Extended ACLs
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Criteria:
• checks both the packet’s source & destination addresses
• check for specific protocol
• check for specific port numbers
• permit or denied applications – pings, telnets, FTP, etc.
•ACL values range between 100 – 199 (for IP)
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Extended ACLs
Port Numbers (decimal)
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IP Protocol
20
FTP data
21
FTP program (control)
23
Telnet
25
(SMTP)
Simple Mail Transport Protocol
53
DNS
69
TFTP
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Extended ACLs
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Router(config)# access-list <ACL number> { permit | deny } protocol source source-mask
destination destination-mask operator operand {established}
ACL number
100 – 199
permit | deny
Packet is allowed or blocked
protocol
IP, TCP, UDP, ICMP, GRE or IGRP
source -- Source address source-wildcard mask
destination -- Destination address destination-wildcard mask
Continued
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
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Extended ACLs
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Router(config)# access-list <ACL number> { permit | deny } protocol source source-mask
destination destination-mask operator
operand {established}
operator
lt, gt, eq, neq
Operand
Port number
established
Allows TCP traffic to pass if the packet uses an established connection ( for example, has
ACK bits set ).
access-list 101 permit tcp 172.16.4.0 0.0.0.255 any eq 25
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Extended ACLs
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Configuring an extended ACL to an interface:
Router(config-if)# ip access-group <ACL number> { in | out }
NOTE: out is the default.
Router(config)# int E0
Router(config-if)# ip access-group 101 in
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Extended ACLs
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172.16.3.0
172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 101
access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 21
access-list 101 permit ip 172.16.4.0 0.0.0.255 0.0.0.0 255.255.255.255
What does this do?
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Extended ACLs
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172.16.3.0
172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 101
access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 21
access-list 101 permit ip 172.16.4.0 0.0.0.255 0.0.0.0 255.255.255.255
Blocks FTP traffic from all hosts on 172.16.4.0 to any device on 172.16.3.0 & allows
all other traffic to 172.16.3.0
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Extended ACLs
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172.16.3.0
172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 101
access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 23
access-list 101 permit ip any any
What does this do?
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Extended ACLs
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172.16.3.0
172.16.4.0
e0
e1
s0
Non-172.16.0.0
server
172.16.4.13
Interface E0
ip access-group 101
access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 23
access-list 101 permit ip any any
Denies only telnet traffic from 172.16.4.0 to 172.16.3.0 network, and permits all
other traffic thru E0 to any address.
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CCNA2 Routing
Extended/Standard ACL numbers for IP
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NOTE:
Standard ACL numbers: 1-99;
1300-1999
Extended ACL numbers: 100-199;
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2000-2699
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CCNA2 Routing
Standard/Extended ACL
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You can not add ACL statements into the body of the access-list
(ONLY at the end of the list).
Otherwise the access list must be deleted first, and then rewritten.
Therefore it is prudent to write your access-list in text format using
“notepad”, and then transfer it to your router.
Outbound filters do not affect traffic originating from within its own
router.
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Configuring Named ACLs
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NOTE:
• A NAMED ACL is an alphanumeric string instead of the ACL
number (1 - 199 )
• NAMED ACLs are not compatible with Cisco IOS release prior to
Release 11.2
• Named ACLs can be used for either standard & extended
•You cannot configure the same name for multiple ACLs.
•use Name ACL when you want to intuitively identify ACLs
•use Name ACL when you have more than 99 standard & 100
extended ACLs have been configured on a router for a given
protocol
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Configuring Named ACLs
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Router(config)# ip access-list { standard | extended } name { deny | permit } { commands }
ip access-list standard internetfilter deny 172.10.15.0 0.0.0.255
permit 128.88.0.0 0.0.255.255
permit 36.0.0.0 0.0.255.255
ip access-list extended marketing-group permit tcp any 171.69.0.0 0.255.255.255
eq telnet
deny udp any 171.69.0.0 0.255.255.255 lt 1024
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Named ACL
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A named ACL will allow the deletion of statements, but will only allow
for the statements to be inserted a the end of the list.
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More Details
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access-list 1 permit 172.16.0.0 0.0.255.255
10101100.00010000.00000000.00000000
00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010000.xxxxxxxx. xxxxxxxx
Matched value
Incoming packet with address of 172.18.4.2. Will it be permitted?
Source :
10101100.00010010.00000100.00000010
Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010010.xxxxxxxx. xxxxxxxx Result
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access-list 1 permit 172.16.0.0 0.0.255.255
10101100.00010000.00000000.00000000
00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010000.xxxxxxxx. xxxxxxxx
Match value
Incoming packet with address of 172.18.4.2. Will it be permitted?
Source :
10101100.00010010.00000100.00000010
Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010010.xxxxxxxx. xxxxxxxx Result
Does the result equal the match value?
No! Hence the incoming packet will not be permitted.
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access-list 1 permit 172.16.0.0 0.0.255.255
10101100.00010000.00000000.00000000
00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010000.xxxxxxxx. xxxxxxxx
Match value
Incoming packet with address of 172.16.4.2. Will it be permitted?
Source :
10101100.00010000.00000100.00000010
Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxxx
10101100.00010000.xxxxxxxx. xxxxxxxx Result
Does the result equal the match value?
Yes! Hence the incoming packet will be permitted.
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access-list 1 permit 172.16.0.0 0.0.255.254
10101100.00010000.00000000.00000000
00000000.00000000.xxxxxxxx. xxxxxxx0
10101100.00010000.xxxxxxxx. xxxxxxx0
Matched value
Incoming packet with address of 172.16.4.1. Will it be permitted?
Source :
10101100.00010000.00000100.00000001
Wildcard mask: 00000000.00000000. xxxxxxxx. xxxxxxx0
10101100.00010000.xxxxxxxx. xxxxxxx1 Result
Incoming packet with address of 172.16.4.4. Will it be permitted?
Source :
10101100.00010000.00000100.00000100
Wildcard mask: 00000000.00000000. xxxxxxxx. xxxxxxx0
10101100.00010000. xxxxxxxx. xxxxxxx0 Result
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access-list 1 permit 172.16.0.0 0.0.255.254
10101100.00010000.00000000.00000000
00000000.00000000.xxxxxxxx. xxxxxxx0
10101100.00010000.xxxxxxxx. xxxxxxx0
Matched value
Incoming packet with address of 172.16.4.5. Will it be permitted?
Source :
10101100.00010000.00000100.00000101
Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxx0
10101100.00010000.xxxxxxxx. xxxxxxx1 Result
So the access list perform what operation? Permits 172.16.4.4, and denies
172.16.4.1 and 172.16.4.5
Permits all even addresses from the network 172.16.0.0
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CCNA2 Routing
Permit/Deny BLOCKS of addresses
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One can permit or deny a block of addresses. However, the blocks must be
a power of 2! (Example, 2, 4, 8, 16, 32, 64, 128, etc.)
When you need to specify a range of addresses, you choose the closet
block size for your needs.
You want to block access to part of network that is in the range from
172.16.8.0 through 172.16.15.0. This is a block size of 8. Hence:
172.16.8.0 0.0.7.255
Note the wildcard part is always 1 less than the block size!
Also in this case for a block of 8, the beginning address must either start at
0, 8, 16, etc.
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Permit/Deny BLOCKS of addresses
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One has a subnet whose addresses range from 171.17.2.128 to 171.17.2.191.
One wants to divide this network so the top half are permitted and the
bottom half is denied to any other network. What is the access lists?
The block range
is:
64
What is the access-list for the bottom?
access-list 101 deny ip 171.17.2.128 0.0.0.31 any
What is the access-list for the top?
access-list 101 permit ip 171.17.2.160 0.0.0.31 any
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Permit/Deny BLOCKS of addresses
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What does this do?
access-list 10 deny 172.16.64.0 0.0.63.255
Denies a block of 64 address starting at 172.16.64.0
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Virtual Terminal ACL
Module 11
You can control access via the VTY ports controlling telnet sessions
coming into the router.
You write the ACL as usual, but use access-class to apply it.
As an example:
Router(config t)# access-list 1 permit 172.16.1.0 0.0.0.255
Router(config t)# line vty 0 4
Router(config-line)# login
Router(config-line)# password cisco
Router(config-line)# access-class 1 in
Note: only numbered access lists can be applied to VTY virtual lines!
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Established option
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The ‘establish’ option in an access-list occurs only with TCP
datagrams. There are cases when you want to stop host B from
initiating a connection with a host A while permitting A to initiate
connections with B.
establish
¾ M a cintos h SE
¾ M a cintos h SE
A
response
B
establish
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Established option
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As a practical example:
172.16.3.0
e1
e0
INTERNET
172.16.4.0
172.16.3.13
Allow host 172.16.3.13 with Internet connection, but don’t allow the
internet to initialize any sessions.
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Established option
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172.16.3.0
e1
e0
INTERNET
172.16.4.0
172.16.3.13
Router(config)# int E1
Router(config-if)# ip access-group 101 in
Router(config)# access-list 101 permit tcp any 172.16.0.0 0.0.255.255
established
Router(config)# access-list 101 permit tcp any host 172.16.3.13 eq. www
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Established option
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172.16.3.0
e1
e0
INTERNET
172.16.4.0
172.16.3.13
Router(config)# int E1
Router(config-if)# ip access-group 101 in
Router(config)# access-list 101 permit tcp any 172.16.3.13 0.0.0.0 eq www established
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Established option
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172.16.3.0
e1
e0
INTERNET
172.16.4.0
172.16.3.13
Note: established argument is limited to tcp which means UDP, ICMP and all
other IP protocols will not match, and will be denied, unless specifically
allowed. Hence
Router(config)# int E1
Router(config-if)# ip access-group 101 in
Router(config)# access-list 101 permit tcp any 172.16.0.0 0.0.255.255 eq www established
Router(config)# access-list 101 permit icmp any any
Router(config)# access-list 101 permit udp any any eq 53
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CCNA2 Routing
Where to Place ACLs?
Module 11
ACL Rules:
Standard ACL
Place the ACL as near the destination as possible.
Extended ACL
Put the ACL as close as possible to the source
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4/9/2016
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CCNA2 Routing
Module 11
Access Lists
Standard
Extended
End of Session
Perrine modified by Brierley
4/9/2016
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