Extended Access-List
Prerequisite – Access-lists [ACL], Standard Access-list
Access-list [ACL] is a set of rules defined for controlling network traffic and reducing network attacks. ACLs are used to filter traffic based on the set of rules defined for the incoming or outgoing of the network.
Extended Access-list –
It is one of the types of Access-list which is mostly used as it can distinguish IP traffic therefore the whole traffic will not be permitted or denied like in standard access-list. These are the ACL that uses both source and destination IP address and also the port numbers to distinguish IP traffic. In this type of ACL, we can also mention which IP traffic should be allowed or denied. These use range 100-199 and 2000-2699.
Features –
- Extended access-list is generally applied close to the source but not always.
- In the Extended access list, packet filtering takes place on the basis of source IP address, destination IP address, port numbers.
- In an extended access list, particular services will be permitted or denied.
- Extended ACL is created from 100 – 199 & extended range 2000 – 2699.
- If numbered with extended Access-list is used then remember rules can’t be deleted. If one of the rules is deleted then the whole access list will be deleted.
- If named with extended Access-list is used then we have the flexibility to delete a rule from the access list.
Configuration –
Here is a small topology in which there are 3 departments namely sales, finance, and marketing. The sales department has a network of 172.16.10.40/24, the Finance department has a network of 172.16.50.0/24, and the marketing department has a network of 172.16.60.0/24. Now, we want to deny the FTP connection from the sales department to finance department and deny telnet to the Finance department from both the sales and marketing departments.
Now, first configuring numbered extended access – list for denying FTP connection from sales to finance department.
Here, we first create a numbered Access-list in which we use 110 [used from extended access-list range] and deny the sales network [172.16.40.0] to make an FTP connection to the finance network [172.16.50.0].
Note – Here, as FTP uses TCP and port number 21. Therefore, we have to specify the permit or deny the condition according to the need. Also, after eq, we have to use the port number for the specified application layer protocol.
Now, we have to deny telnet connection to finance department from both sales and Marketing department which means no one should telnet to finance department. Configuring for the same.
Here, we have used the keyword any which means 0.0.0.0 0.0.0.0 i.e any IP address from any subnet mask. As telnet uses port number 23 therefore, we have to specify the port number 23 after eq.
Now, this is the most important part. As we already know there is an implicit deny at the end of every access list which means that if the traffic doesn’t match any of the rules of Access-list then the traffic will be dropped.
By specifying any any means that source having any IP address traffic will reach finance department except the traffic which it matches the above rules that we have made. Now, we have to apply the access-list on the interface of the router:
As we remember, we have to apply the extended access-list as close as possible to source but here we have applied it to close to the destination because we have to block the traffic from both sales and marketing department, therefore, we have to apply it close to the destination here otherwise we have to make separate access-list for fa0/0 and fa1/0 inbound.
Named access-list example –
Now, considering the same topology, we will make a named extended access list.
By using this command we have made an access-list named blockacl.
And then the same configuration we have done in numbered access-list.
Traffic Filtering in the Cisco Internetwork Operating System
Eric Knipp, ... Edgar DanielyanTechnical Editor, in Managing Cisco Network Security [Second Edition], 2002
Extended IP Access Lists
Although there are times when we only need to filter traffic based on the source address, more often than not we will need to match traffic with a higher level of detail. An option for more precise traffic-filtering control would be an extended IP access list. Here, both the source and destination address are checked. In addition, you also have the ability to specify the protocol and optional TCP or UDP port number to filter more precisely. In the following example, any field represented by {} is mandatory for the access list, while any field represented by [] is optional. The format of an extended IP access list is:
access-list access-list-number {permit | deny} protocol source
source-wildcard [operator source-port] destination destination-wildcard
[operator destination-port] [precedence precedence-number] [tos tos]
[established] [log | log-input]
Bold items represent keywords that are part of the access list syntax. Table4.4 lists the configuration for a standard IP access list.
Table4.4. Extended IP Access List Configuration
| Access-list list number | Defines the number of the access list. The extended access list numbers range from 100–199. |
| Permit | If conditions are met, traffic will be allowed. |
| deny | If conditions are met, traffic will be denied. |
| Protocol | Defines the Internet protocol for filtering. Available options here are keywords such as TCP or UDP, or the number of the protocol as seen in the IP header. |
| source-address | Identifies the host or network from which the packet is being sent. The source can be specified by an IP address or by using the keyword any. |
| source wildcard-mask | This defines the number of wildcard bits assigned to the source address. The source wildcard-mask can be specified by an IP address or by using the keyword any. |
| Operator source-port | Defines the name or number of a source TCP or UDP port. A list of operators is shown next. |
| Destination-address | Identifies the host or network to which the packet is being sent. The destination can be specified by an IP address or by using the keyword any. |
| Destination wildcard-mask | This defines the number of wildcard bits assigned to the destination address. The destination wildcard-mask can be specified by an IP address or by using the keyword any. |
| Operator destination-port | Defines the name or number of a destination TCP or UDP port. A list of operators is shown next. |
| precedence precedence-number | Used for filtering by the precedence level name or number [0 thru 7]. |
| tos tos-number | Used for filtering by the Type of Service level specified by a name or number [0 thru 15]. |
| established | Allows established TCP sessions through the list. |
| log | log-input | Log the event when a packet matches the access list statement. Log-input shows the same information as the log keyword, except it also adds the interface name the packet was received on. |
In the following access list, we get very specific about what host we want to access a particular network or host on a network. In the first three lines, we are permitting or allowing packets from individual hosts on subnet 172.16.130.0 to any host on network 10.0.0.0. In line 4, we are denying packets with the source address that belongs to subnet 172.16.130.0 to the destination of host 192.168.10.118. Line 5 tells us that we are permitting all IP packets with no concern of a source or destination address. The implicit deny all at the end of the list will never be matched against a packet because the previous permit statement will match all packets. In Figure4.3, we would apply this access list on the serial 0 interface in the outbound direction as follows:
Figure4.3. An Example Network
Router[conflg]# interface serial 0
Router[config-if]# ip access-group 141 out
An example of an extended access list is as follows:
access-list 141 permit icmp host 172.16.130.88 10.0.0.0 0.255.255.255
access-list 141 permit tcp host 172.16.130.89 eq 734 10.0.0.0 0.
255.255.255 range 10000 10010
access-list 141 permit udp host 172.16.130.90 10.0.0.0 0.255.255.255
eq tftp
access-list 141 deny ip 172.16.130.0 0.0.0.255 host 192.168.10.118
access-list 141 permit ip any any
Table4.5 describes the extended access list commands.
Table4.5. A Description of Access List Commands
| access-list 141 permit icmp host 172.16.130.88 10.0.0.0 0.255.255.255 | Allows host 172.16.130.88 to send ICMP messages to any host on network 10.0.0.0. |
| access-list 141 permit tcp host 172.16.130.89 eq 734 10.0.0.0 0.255.255.255 range 10000 10010 | Allows host 172.16.130.89 to initiate TCP sessions from port 734 to any port between 10000 and 10010 on any host on network 10.0.0.0. |
| access-list 141 permit udp host 172.16.130.90 10.0.0.0 0.255.255.255 eq tftp | Allows host 172.16.130.90 to send files via TFTP [UDP port 69] to any host on network 10.0.0.0. |
| access-list 141 deny ip 172.16.130.0 0.0.0.255 host 192.168.10.118 | Denies any host on network 172.16.130.0 to host 192.168.10.118. Since we configured some permit statements from hosts within these previous two subnets, this entry will deny everything between these two networks that isn't explicitly permitted in the earlier listing. |
| access-list 141 permit ip any any | Allows all hosts from any network to any network, if it has not matched one of the preceding lists. Take a good look at the order of these commands to get a feel for the importance of the list order. Remember this is processed in a top-down manner, as shown in Figure 4.2. |
Just as in our standard access list, the extended access list will require a hyphen between the words access and list. Next is the list number. Since we are referencing an extended IP access list, the numbers would range from 100 to 199. The access list number serves the same dual purpose here as we looked at earlier with the standard access list. The router must have a way to distinguish between access lists. The number performs this purpose along with tying the lines of an access list together and designates which access list the filter is part of The number also tells the router the type of access list.
Desinging & Planning…
Placement of Access Lists
Often you have a few options about how to apply your access lists and stili achieve the same affect on the traffic flowing through the router, in the case of the previous example, access list 141 was applied outbound on the serial 0 interface. Because access list 141 was designed to only filter traffic originating from the 172.16.130.0 network, and not traffic from 172.17.0.0, this list could have been applied in the inbound direction on Ethernet 0. Both approaches will have the same affect on the traffic flowing through the router.
There is a minor difference between these two approaches, though. When the ACL is applied outbound on the Serial0 interface, the traffic enters the Ethernet0 interface and is processed against the routing table. The packet is then passed to the outbound interface, where it is checked against any outbound ACLs. If the outbound interface is Serial 0, it checks packets against access list 141 and will permit or deny the traffic based on the rules defined in that list.
When the ACL is applied inbound on the Ethernet0 interface, the traffic is permitted or denied before it is processed against the routing table. On a router under heavy traffic loads, this could make a considerable difference in the delay that is introduced because the router does not have to process packets that will be dropped by the outbound interface.
Although inbound filtering has the advantage with respect to route processing, that does not necessarily make it the better way to apply access lists. Under different circumstances, you may want to prevent access to an external subnet from both Ethernet interfaces. In this case, it may be easier to apply the access lists in the outbound direction of Serial0 because packets from both Ethernet interfaces will have to pass through Serial0 to get to the external subnet. In other words, you are applying the access list to the bottleneck in traffic. Otherwise, you will have to keep two separate access lists, one specific for Ethernet0 and the other specific for Ethernet1. If the router is under light traffic loads, it may be easier to maintain a single access list.
There is disagreement among network and security professionals about which approach is better, but neither approach should be considered better than the other in all cases. It is up to you to decide which is best for your situation.
Keywords permit or deny
A keyword permit or deny specifies to the router the action to be performed. For example, the keyword permit would allow the packet to exit or enter the interface, depending on whether you specify the filtering to be performed in or out. Again, this option provides the same function as in our standard access list. The last line of our extended access list example could have read as follows:
access-list 141 permit ip any any
Protocol
You have the option of filtering several different protocols using the extended access list. The protocol field in the IP header is an 8-bit number that defines what protocol is used inside the IP packet. TCP and UDP are only two of the possible protocols that can be filtered on, although they are most common. Other protocols, such as ICMP and EIGRP, have their own protocol numbers because they are not encapsulated inside TCP or UDP. If we use a question mark when defining an access list, we can see the protocol numbers that have been defined by name inside the router.
Router [config]#access-list 191 permit?
An IP protocol number
ahp Authentication Header Protocol
eigrp Cisco's EIGRP routing protocol
esp Encapsulation Security Payload
gre Cisco's GRE tunneling
icmp Internet Control Message Protocol
igmp Internet Gateway Message Protocol
igrp Cisco's IGRP routing protocol
ip Any Internet Protocol
ipinip IP in IP tunneling
nos KA9Q NOS compatible IP over IP tunneling
ospf OSPF routing protocol
pcp Payload Compression Protocol
pim Protocol Independent Multicast
tcp Transmission Control Protocol
udp User Datagram Protocol
Protocols not on the preceding list may also be filtered with extended access lists, but they must be referenced by their protocol number. A full list of assigned IP protocol numbers can be found at www.iana.org/assignments/protocol-numbers.
It is important to remember that the IP keyword in the protocol field matches all protocol numbers.You must use a systematic approach here when designing your access list. For example, if your first line in the access list permits IP for a specific address, and the second line denies UDP for the same address, the second statement would have no effect. The first line would permit IP, including all the above layers. An option here may be to reverse the order of the statements. With the statements reversed, UDP would be denied from that address and all other protocols would be permitted.
Source Address and Wildcard-mask
The source address and source wildcard-mask perform the same function here as in a standard IP access list. So, in the preceding example we could have used the wildcard mask instead of the host and any keywords. The access list would then look as follows:
access-list 141 permit ip 172.16.130.88 0.0.0.0 i0.0.0.0 0.255.255.255
access-list 141 permit ip 172.16.130.89 0.0.0.0 i0.0.0.0 0.255.255.255
access-list 141 permit ip 172.16.130.90 0.0.0.0 i0.0.0.0 0.255.255.255
access-list 141 permit ip 172.16.130.0 0.0.0.255 192.168.10.118 0.0.0.0
access-list 141 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
In the first three lines, we are permitting or allowing packets from individual hosts on subnet 172.16.130.0 to any host on network 10.0.0.0. In line 4, we are permitting packets with the source address that belongs to subnet 172.16.130.0 to the destination of host 192.168.10.118. Line 5 tells us that we are permitting all packets regardless of the source or destination address. Remember that standard IP access lists have a default mask of 0.0.0.0. This does not apply to extended access lists so we must specify one.
Destination Address and Wildcard-mask
The destination address and wildcard-mask have the same effect and structure as the source address and wildcard-mask. So, here the keywords host and any are also available. You can utilize these keywords to specify any destination address as well as a specific destination without using the wildcard mask. Remember that extended access lists try a match on both source and destination. A common mistake here is trying to build an extended access list with the idea of only filtering the source address, and forgetting to specify the destination address.
Source and Destination Port Number
Many times, we don’t want to deny all access to a particular server. When you put a Web server out on the Internet, you want everyone to be able to access it on port 80 [WWW], but you don’t want to allow access to any other ports, because it gives hackers the opportunity to exploit other services you may not be aware of [although you should know of them in the first place]. Restricting access to this level of detail is another benefit of extended ACLs. We have the option of specifying a source and destination port number in the access list. Let’s look at a simple example:
Router[config]# interface Serial 0
Router[config-if]# ip access-group 111 in
Router[config]#access-list 111 permit tcp any host 172.17.11.19 eq 25
Router[config]#access-list 111 permit tcp any host 172.17.11.19 eq 23
These commands are explained in Table 4.6.
Table4.6. Router Commands
| access-list 111 permit tcp any host 172.17.11.19 eq 25 | Permits SMTP from anywhere to host 172.17.11.19. |
| access-list 111 permit tcp any host 172.17.11.19 eq 23 | Permits Telnet from anywhere to host 172.17.11.19. |
| interface Serial 0 | Enters interface submode. |
| ip access-group 111 in | Applies access list inbound on interface. |
In line 1, we are permitting TCP packets from any source to the destination of host 172.22.11.19 if the destination port is 25 [SMTP]. In line 2, we are permitting TCP packets from any source to the destination of host 172.22.11.19 if the destination port is 23 [Telnet]. The implicit deny statement at the end of this access list will prevent all other traffic from making it into our network.
Let’s take a look at filtering with TCP and UDP. When using TCP, for example, the access list will examine the source and destination port numbers inside the TCP segment header. So, when using an extended access list, you have the capability to filter to and from a network address and also to and from a particular port number. You have several options when deciding which operator to use, such as:
■eq equal to
■neq not equal to
■gt greater than
■lt less than
■range specifies an inclusive range or ports [Here, two port numbers are specified.]