I recently received an e-mail from a reader who wanted to
know how to block a specific Web site using the Cisco IOS. TechRepublic member Francis M. wrote, “I have a Cisco
2600, [and] we use it as [an] Internet server. I want to block some restricted Web
sites. How [do] I do that?”

This isn’t a difficult task—as long as you understand how
the Cisco IOS works. This week, I’ll walk you through the process and point out
what to watch out for along the way.

Step 1: Configure a DNS server

Let’s say we want to block access to the creatively named www.badsite.com.
We don’t know the IP address, and we don’t want to know it. That’s fine—the
Cisco IOS can look it up for us and fill it in.

To do this, we need at least one DNS server configured on
the router. To configure a DNS server, use the ip name-server command. Here’s an example:

Router(config)# ip name-server 1.1.1.1 2.2.2.2

In this case, we configured a primary and a backup DNS
server for the router to use to resolve DNS names. This doesn’t affect any
traffic flowing through the router; the router will use these DNS servers when we
ping a Web server by name. Here’s an example:

Router# ping www.techrepublic.com
Translating "www.techrepublic.com"...domain server (1.1.1.1) [OK]

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 216.239.113.101, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
Router#

In this example, the router used the domain name server we
specified (i.e., 1.1.1.1) to resolve the DNS name. It successfully translated
the DNS name to 216.239.113.101.

If we hadn’t already specified a DNS server, then the router
would have returned something like the following:

Translating "www.techrepublic.com"...domain server (255.255.255.255)
% Unrecognized host or address, or protocol not running.

Step 2: Create the ACL

To actually block the undesirable Web site, we need to
create an access control list (ACL) to define exactly what we want to block.
Here’s an example:

Router(config)# access-list 101 deny tcp any host www.badsite.com eq www
Translating "www.badsite.com"...domain server (1.1.1.1) [OK]
Router(config)# access-list 101 permit tcp any any eq www
! to allow all other web traffic

This ACL denies all Web traffic from any source going to the
specified Web site. After blocking that traffic, it will also allow all other Web
traffic from any source to any destination. Finally, because of the implied
deny, it will deny all other traffic.

What if you want to determine which IP addresses are trying
to go to the blocked Web site? You can log this information using the log keyword. Here’s an example:

Router(config)# access-list 101 deny tcp any host www.badsite.com eq www log

Step 3: Avoid this “gotcha”

Here’s one issue to keep in mind. After we entered the first
line of the ACL above, notice how the router used the DNS server to resolve the
DNS name. It then replaced the IP address that the hostname resolved to in the
ACL. Here’s a closer look at the configuration:

Router# sh run | inc access-list 101
access-list 101 deny tcp any host 66.116.109.62 eq www

This is a nice feature, but it can be problematic for a
couple of reasons. First, the IP address entered is only the first IP address
that the DNS server responded to. If this is a large Web site that has multiple
servers (such as a search engine), the ACL only contains the first IP address
that the DNS server responded with—you’ll need to manually block the other IP
addresses. Here’s an example:

C:\> nslookup www.google.com
Server:  DNSSERVER
Address:  1.1.1.1

Non-authoritative answer:
Name:    www.l.google.com
Addresses:  64.233.167.104, 64.233.167.147, 64.233.167.99
Aliases:  www.google.com

In addition, if the IP address of the blocked Web server changes,
your ACL will remain the same. You would need to manually update the ACL.

Step 4: Apply the ACL

Just because we’ve created the ACL doesn’t mean the router
is actually using it—we still have to apply the ACL. We created this ACL with
the assumption that it’s blocking traffic from our local LAN that’s going out
to the WAN (i.e., the Internet). That’s because we formatted the ACL with
source then destination.

Because of this design, we need to apply the ACL in the
OUTBOUND direction on the router. Here’s an example:

Router(config)# int serial 0/0
Router(config-if)# ip access-group 101 out

Have you used the Cisco IOS to block a Web site before? What
did your ACL look like? Do you have a good ACL to share? Post your comments in
this article’s discussion.

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David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.