Skip to main content

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 from this blog

What is Cisco Supervisor Engine?

Supervisor Engine is a module that is installed in the Cisco Chassis-based Catalyst Switches or Routers. Supervisor engine contains nearly all the same components of a fixed Cisco Switches or Routers. These Supervisor engines come in a variety of different types with different functionalities and are installed in the Switches/Router Chassis as per requirements of the network types.

Benefits of Supervisor Engines
By installing Latest Supervisor Engines in your existing investments (Switches and Routers) you can scale system performance and integrate next-generation services into your Networks.
Within a single multilayer switch chassis, two supervisor modules with integrated route processors can be used to provide hardware redundancy. If an entire supervisor module fails, the other module can pick up the pieces and continue operating the switch.
The supervisor engine contains the following integrated daughter cards that perform forwarding and routing and provide the protocols supported …

GNS3 Docker Error while creating node: Docker has returned an error: Cannot connect to host docker:80

Error while creating node: Docker has returned an error: Cannot connect to host docker:80 ssl:False [No such file or directory]

After adding docker template for Alpine Linux in gns3, you get above mentioned message when you want to use alpine linux in GNS3.

To get rid of this message you have to install Docker by following below link
curl -fsSL https://get.docker.com/ | sh

If you do not have curl installed then instal curl first with below command.apt-get install curl
After installing Docker you need to add your user name in the docker group with the following command. $ sudo usermod -aG docker your_username

Verify if the docker service is started with following command$ service docker status
If docker is not started then start with following command $ sudo service docker start
Logout from GNS3 Virtual Machines and log back. Start gns3 and use alpine linux.

Telnet Client is Disabled Enable it to use it from this application

This is a Cisco Network Assistant (CNA) error when you want to telnet any Cisco Devices from within CNA.
Solution to remove this error is related to Windows 7 or Windows Vista. First of all you need to Add TELNET program from Control Panel, which by Default is not added in fresh installation of Windows 7.For adding and using Telnet Program in Windows 7, Click Start > Control Panel > Uninstall a Program > Turn Windows Features On and OFF > Scroll to TELNET and Select it > Press OK After Adding Telnet Program into the Windows 7 you need to Copy Telent.exe from C:\Windows\System32 folder and paste it to C:\Windows\SysWOW64\ Now you should not get the same error.