Effective Uses of Packet-Filtering
Devices
Filtering Based on Source Address: The Cisco
Standard ACL
1.
One of the things that packet-filtering technology is great for is the
blocking or allowing of traffic based on the IP address of the source
system.
2.
Some ways that this technology can be usefully applied are filters
blocking specific hosts (blacklisting), filters allowing specific hosts
(such as business partners), and in the implementation of ingress and
egress filters. Any of these examples can be implemented on a Cisco
router by using a "standard" access list.
Filtering Based on Source Address: The Cisco
Standard ACL
3.
The standard access list is used to specifically allow or disallow traffic
from a given source IP address only.
4.
It cannot filter based on destination or port number. Because of these
limitations, the standard access list is fast and should be preferred
when the source address is the only criteria on which you need to
filter.
5.
The syntax for a standard access list is as follows:
access-list list number 1-99 or 1300-1999 permit |deny source address
mask log
Filtering Based on Source Address: The Cisco
Standard ACL
6.
Notice that when ACLs were first created, the list number had to be 1-99. This range was expanded in IOS version 12.0(1) to include the
numbers 1300--1999.
7.
The only way that the Cisco IOS can identify the list as a standard ACL
is if a list number in one of these two ranges is used.
Filtering Based on Source Address: The Cisco
Standard ACL
8.
The mask option is a required wildcard mask, which tells the router
whether this is a single host we are filtering or an entire network
range.
9.
The log option can be appended to tell the router to specifically log
any matches of this filter.
10. These log entries can be saved in local memory or more appropriately
sent to a remote Syslog server.
Filtering Based on Source Address: The Cisco
Standard ACL
12. The previously listed access list notation is entered in global
configuration mode and can be applied to the interface in the
interface configuration mode with the access-group statement, as
shown here:
ip access-group list number in|out
Filtering Based on Source Address: The Cisco
Standard ACL
1.
The access-group command is used to specifically apply an ACL to an
interface (by its list number) either inbound or outbound.
2.
Only one access list can be applied in one direction (in or out) per
interface. This means a maximum of two applied ACLs per interface:
one inbound and one outbound.
3.
One of the confusing concepts of router ACLs is the way that applying
filters "in" or "out" works.
Filtering Based on Source Address: The Cisco
Standard ACL
4.
This is confusing because people normally visualize "in" as traffic
moving toward their internal network and "out" as traffic moving
away from their network toward outside entities.
5.
However, this premise does not necessarily hold true when talking
about the in and out keywords in Cisco router access lists.
6.
Specifically, the keywords tell the router to check the traffic moving
toward (in) or away from (out) the interface listed.
Filtering Based on Source Address: The Cisco
Standard ACL
7.
In a simple dual-interface router, this concept is more easily
illustrated.
8.
Let's assume you have an interface called e1 (hooked to your internal
network) and an external interface called s1 (hooked up to the
Internet, for example).
9.
Traffic that comes into the s1 interface moves toward your internal
network, whereas traffic that goes out of the s1 interface moves
toward the Internet.
Filtering Based on Source Address: The Cisco
Standard ACL
10. So far this seems to be pretty logical, but now let's consider the
internal e1 interface.
11. Traffic that comes into e1 moves away from your internal network
(toward the Internet), and traffic that goes out of e1 goes toward your
internal network.
Filtering Based on Source Address: The Cisco
Standard ACL
12. if something should be blocked (or permitted, for that matter) coming
in from the Internet, it would make the most sense to block it coming
in to the s1 (outside) interface.
13. In addition, if something should be filtered leaving your network, it
would be best to filter it inbound on the e1 (inside) interface.
14. Basically, you should show preference to the in keyword in your
access lists.
15. Some specific exceptions to this rule exist, which involve certain
access list types (such as reflexive ACLs) that require being placed
outbound.
Filtering Based on Source Address: The Cisco
Standard ACL
16. These examples often list the actual prompt for a router named
"router" to remind you of the command modes that the router will be
in when entering the various commands.
17. The following is an example of an actual filter that uses the previous
syntax and 190.190.190.x as the source network's IP address that you
want to deny:
router(config)#access-list 11 deny 190.190.190.0 0.0.0.255
Filtering Based on Source Address: The Cisco
Standard ACL
19. This filter's list number is 11.
20. It denies any packet with a source network address of 190.190.190
with any source host address.
21. It is applied to the interface inbound, so it filters the traffic on the way
into the router's interface.
22. The command to apply the access list to your serial 1 interface would
be
23. router(config-if)#ip access-group 11 in
Filtering Based on Source Address: The Cisco
Standard ACL
25. where we are in interface configuration mode for the interface to
which we are applying the access list, inbound.
The Cisco Wildcard Mask Explained
The wildcard mask is one of the least understood portions of the Cisco ACL
syntax. Take a look at the following example:
access-list 12 permit 192.168.1.17 0.0.0.255
In this case, 0.0.0.255 represents the wildcard mask.
It looks like a reverse subnet mask and represents the portion of the
listed IP address range to filter against the traffic in question.
Zeros mean, "Test this portion of the address," and ones mean, "Ignore
this portion of the address when testing."
The Cisco Wildcard Mask Explained
In our example, let's say a packet comes in with a source address of
192.168.2.27.
Because the first octet of the wildcard mask is a zero, the router compares the
first octet of the incoming packet to the value 192, listed in the access list.
In this case, they are the same, so the router continues to the second octet of
the wildcard mask, which is also a zero. Again, the second octet of the value of
the source address of the incoming packet is compared to the value 168.
Because they are also the same, the router continues to the third octet. Because
the wildcard mask specifies a zero in the third octet as well, it continues to test
the address, but the value of the third octet does not match, so the packet is
dropped.
The Cisco Wildcard Mask Explained
For the sake of example, let's continue to look at the fourth octet, even though
in actuality the packet would have been dropped at this point.
The wildcard's fourth octet is valued at 255. In binary, this equates to 11111111.
In this example, the value 0 in the access list does not match the value 27 of the
compared packet; however, because the wildcard wants us to ignore this octet,
the access list allows the packet to pass (assuming it hadn't failed on the
previous octet).
The concept might seem pretty easy with a wildcard value that deals with entire
octets, but it gets tricky when you need to deal with an address range that is
smaller than 255. The reality is that the router doesn't test octet by octet, but bit
by bit through each of the octets.
The Cisco Wildcard Mask Explained
What if you want to allow traffic from systems in the address range
192.168.1.16----192.168.1.31 only?
The first three octets of the wildcard are easy: 0.0.0. It's the last octet that is
difficult. The values between 16 and 31 are covered in the range 10000-11111.
To allow those values, you need to place zeros in your wildcard mask for the
portions that need to match exactly, and ones for the binary values that change.
Because the last four bits are the only bits that change in the desired range, the
wildcard mask reflects those four bits with ones. In binary, our wildcard mask is
as follows:
00000000.00000000.00000000.00001111
The Cisco Wildcard Mask Explained
Translated with our binary calculator, that is 0.0.0.15.
This wildcard mask works for any range of 15 addresses you are
comparing, so to make it work for 1631, you must properly reflect the
range in the IP address portion of the access list. Your final access list
looks like this:
access-list 10 permit 192.168.1.16 0.0.0.15
The Cisco Wildcard Mask Explained
zeros and ones, without interruption, as in the example listed previously.
In some cases, you need more than one ACL statement and wildcard
mask to cover a range of network addresses.
For example, if you want to block addresses in the range 232--255, you
need the command
access-list 110 deny ip 192.168.1.232 0.0.0.7 any
to block the range 232--239, and you also need to specify
access-list 110 deny ip 192.168.1.240 0.0.0.15 any
to block the range 240---255.
Blacklisting: The Blocking of Specific Addresses
One popular use of the standard access list is the "blacklisting" of particular host networks.
This means that you can block a single host or an entire network from accessing your
network.
The most popular reason for blocking a given address is mischief found by your IDS
For example, if your intranet web server should only be offering its information to your
business locations in the continental United States, and you are getting a lot of hits from a
range of IPs in China, you might want to consider blocking those addresses.
The blocking of address ranges is also a popular way to "band-aid" your system against an
immediate threat. For example, if one of your servers was being attacked from a certain IP
address, you could simply block all traffic from that host or network number
Blacklisting: The Blocking of Specific Addresses
A sample access list to block access from an outside address range would be
router(config)#access-list 11 deny 192.168.1.0 0.0.0.255
router(config-if)# ip access-group 11 in
where the network number of the outside parties to be blocked would be
192.168.1.0255. (Of course, this address range is part of the ranges reserved for
private addressing, and it's simply used as an example in this instance.) This access
list would be applied to the external router interface, inbound.
"Friendly Net": Allowing Specific Addresses
Another way you can use a standard access list is to permit traffic from a given IP
address. However, this is not recommended.
Allowing access to an address in this manner, without any kind of authentication,
can make you a candidate for attacks and scans that use spoofed addresses.
Because we can only filter on source address with a standard ACL, any inside
device with an IP address can be accessed. Also, it's impossible to protect
individual services on those devices.
If you need to set up access like this and can't do it through a solution that
requires authentication or some type of VPN, it is probably best to at least use
"Friendly Net": Allowing Specific Addresses
an access list that considers more than the source address.
This type of access may be suitable in situations requiring less security, such as
access between internal network segments.
For example, if Bob in accounting needs access to your intranet server segment,
a standard ACL would be a simple way to allow his station access.
Ingress Filtering
RFC 1918 pertains to reserved addresses. Private/reserved addresses
are ranges of IP addresses that will never be distributed for public
use.
The reserved ranges are as follows:
Class A: 10.0.0.010.255.255.255
Class B: 172.16.0.0--172.31.255.255
Class C: 192.168.0.0--192.168.255.255
Ingress Filtering
Private IP is used in internal network numbers, they are good candidates for
someone who is crafting packets or doing other malicious packettransmitting behavior, including denial of service.
Therefore, these ranges should be blocked at the outside of your network.
In addition, the loopback address 127.0.0.1 (the default address that all IP
stations use to "address" themselves) is another candidate for being
blocked, for the same reason.
While you are blocking invalid addresses, you should also block the multicast
address range 224.0.0.0239.255.255.255 and the invalid address 0.0.0.0.
Ingress Filtering
Following is a sample access list to accomplish this:
router(config)#access-list 11 deny 10.0.0.0 0.255.255.255
router(config)#access-list 11 deny 127.0.0.0 0.255.255.255
router(config)#access-list 11 deny 172.16.0.0 0.15.255.255
router(config)#access-list 11 deny 192.168.0.0 0.0.255.255
router(config)#access-list 11 deny 224.0.0.0 15.255.255.255
router(config)#access-list 11 deny host 0.0.0.0
router(config-if)# ip access-group 11 in