Access Lists

Access lists regulate network traffic flow and security by using permit and deny statements that filter traffic based on source address, destination address, and protocol type of a packet.  They are available for IP, IPX, AppleTalk, and other protocols.

Access lists must be entered in sequentially, you can't move individual statements once they are entered.  You can disable the access list with the no access-list [list #] command.  It is common to create the access list separately and apply it by copying and pasting it into the router configuration.

To copy an externally created access list, for example from a TFTP server, create the list on the TFTP host and save the file as plain text (ASCII).  Then, from your router, use the copy tftp running-config file_id command to copy the access list to your router.  Finally, perform the copy running-config startup-config command to save the access list to your router's NVRAM.  The first command of an edited access list file should delete the previous access list (for example, type a no access-list command at the beginning of the file) else the new rules will be appended to the end of the access list.

If you telnet into a router and apply an access list, there is the possibility that you could be blocked from the router.  To avoid the inconvenience of being locked out, use the command reload in [hh:mm].  This command will cause the router to restart and load a startup-config without the access list that blocked access.

Access List Rules

Access lists must be applied sequentially and they always end with an implicit deny statement.  Since all traffic not explicitly permitted in the access list is blocked, using the command access-list [list #] permit any at the end of the access list allows remaining traffic that hasn't been blocked with a deny statement through the interface.  This is why you can't append new rules to an access list, any statements added after the permit any or implicit deny command wouldn't be evaluated, a packet is only evaluated by an access list until it matches a statement.  You should place first on the list the lines that are most likely to find a match, this will reduce unnecessary processing and save CPU time.  To remove an access-list use the command no access-list [list #].  This command removes the entire access list.  If you don't remove an access list first, all new lines will be appended to the end of the old one.  After you create access lists, you must apply them to an interface so they can filter traffic.  They can be either outgoing or incoming filters.

   Router(config-if)#ip access-group ?

   <1-199>      IP access list (standard or extended)
   <1300-2699>  IP expanded access list (standard or extended)
   WORD         Access-list name

   
   Router(config-if)#ip access-group 1 in
  

  Router(config-if)#ip access-group 1 out
  

If the filter direction (in or out) isn't specified, it uses outbound by default. An interface can't have more than one inbound or more than one outbound access list applied to it.  Multiple lists are allowed if the lists are for different protocols.

• Routers apply lists sequentially in the order in which you type them into the router.
• Routers apply lists to packets sequentially, from the top down, one line at a time.
• Packets are processed only until a match is made and then they are acted upon based on the access list criteria contained in access list statements.
• Lists always end with an implicit deny. Routers discard any packets that do not match any of the access list statements.
• Access lists must be applied to an interface as either inbound or outbound traffic filters.
• Only one list, per protocol, per direction can be applied to an interface.

Two Main Types of Access Lists

1. Standard

Standard IP access lists filter network traffic based on the source IP address.  Using a standard access list, you can filter traffic by a host IP address, subnet, or a network address.  In order to configure standard IP access lists, you you must first create the access list and then apply it to an interface using the following syntax:

   access-list [list #] [permit|deny] [source address] [source wildcard mask]

Wildcard masks are the inverse of the subnet, the 0's are significant and the 1's are not significant.  If you compare the source address to the wildcard mask, the source address bits that match up with the 0's specifies the network (only one) and the source address bits that match the 1's are all the hosts to which the list applies.

   Router(config)#access-list 1 deny host 192.168.1.4
   Router(config)#access-list 1 permit 0.0.0.0 255.255.255.255
   Router(config)#int e0
   Router(config-if)#ip access-group 1 out

This access list allows traffic from 192.168.1.4 to enter the router, but the access list denies it from exiting on interface Ethernet 0.  The deny statement uses the default wildcard mask of 0.0.0.0 (i.e. all bits are significant and it only applies to one host).  The 0.0.0.0 255.255.255.255 can be replaced with the word any.  The list is applied to the outbound of one interface as opposed to the inbound.  This will prevent the host being blocked from other networks on the router that might not have been intended since traffic from 192.168.1.4 can enter the router and be switched to other networks on interfaces other than ethernet 0.  You should apply the standard IP access list as close to the destination network as possible, or you could inadvertently block access to portions of your network.  Use the command show access-lists to see the access lists on your router.  For just IP access lists use the command show ip access-list.

To remove the access list use the command no access-list [list #].  Use the show ip interface and the show interface commands to verify that an access list has been successfully applied to an interface.

2. Extended

Extended IP access lists can filter based on source IP address, destination IP address, protocol type, and application destination and source port numbers, whereas standard IP access lists only filter sources addresses.  You also configure extended IP access lists by creating the list and applying it to an interface using the following syntax:

 access-list [list #] [permit|deny] [protocol] [source IP addr] [source wildcard mask] [operator]
                 [port] [destination IP addr] [destination wildcard mask] [operator] [port] [log]

Example: This access list will block 192.168.1.10 from accessing TCP port www (http[80]) on host 192.168.2.2.  The host keyword is a shortcut for the 0.0.0.0 wildcard mask.  Since extended IP access lists use destination addresses, the list should be applied as close to the source as possible to reduce unnecessary traffic on the network.

   Router(config)#access-list 100 deny tcp host 192.168.1.10 host 192.168.2.2 eq www
   Router(config)#access-list 100 permit ip any any
   Router(config)#int e0
   Router(config-if)#ip access-group 100 in 

To remove the access list use the command no ip access-group [list #].  Use the same commands to monitor an extended IP access list as a standard access list i.e. show access-lists or show ip access-list.  Extended IP access lists show the number of matches per line of the access list; to clear these counters, use the command clear access-list counter [list #].

Controlling VTY Access

You can control access to VTY ports by applying a standard access list to the VTY lines.

   Router_2#conf t
   Enter configuration commands, one per line.  End with CNTL/Z.
   Router_2(config)#access-list 15 permit host 192.168.1.71
   Router_2(config)#line vty 0 4
   Router_2(config-line)#access-class 15 in

This will stop all hosts except 192.168.1.71 from telneting into the router.  This is accomplished by only allowing one host and then not permitting any other hosts since there is an implicit deny at the end of all access lists.