How to Configure a Router to block websites with Access-List

Here are the simple steps to follow to block a website to the user who are accessing Internet through your Router
Step 1: Configure a DNS server

Let’s say we want to block access to the creatively named www.block-site.com. We don’t know the IP address, and we don’t want to know it. When we configure the Access List in the router—the Cisco IOS can look it up and fill it automatically.
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.khatrinetworks.blogspot.com
Translating “www.khatrinetworks.blogspot.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.khatrinetworks.blogspot.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.block-site.com eq www
 Translating “www.block-site.com”…domain server (1.1.1.1) [OK]
Router(config)# access-list 101 permit ip any any
(Do not forget this permit tcp any any, As you know otherwise all web traffic will be denied)
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.block-site.com eq www log
Step 3: Avoid this
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.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
That is it… Your have successfully blocked the www.block-site.com

Comments

Popular Posts