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
Labels:

Comments

Post a Comment

Popular posts from this blog

How to import Putty Saved Connections to mRemoteNG

Just started using mRemoteNG and its being very cool to connect to different remote connection with different protocols e.g Window Remote Desktop, VNC to Linux, SSH, HTTP connection etc. from a single application. As new user I configured some remote desktop connection which was quite easy to figure out. But when I wanted to add SSH connections, it came in my mind to import all of the saved connections in the putty. But I couldn't figure it out how can it be done, though it was quite easy and here are the steps. Open your mRemoteNG Create a folder if you want segregation of multiple networks Create a new connection Enter the IP address of remote server under connection in Config pane Under the config pane, select protocol " SSH version 2 ".  Once you select protocol to SSH version 2 you are given option to import putty sessions, as shown in the snap below. In the above snap, I have imported CSR-AWS session from my saved sessions in Putty.
1 comment
Read more

BGP Soft Reconfiguration vs. Route Refresh: Key Differences and Best Practices

In BGP (Border Gateway Protocol), managing route updates and reapplying new policies can sometimes be challenging, especially if you want to avoid resetting the BGP session. Two methods allow you to update routing policies without tearing down the session: BGP Soft Reconfiguration and BGP Route Refresh . While both methods serve the same purpose, they work differently and have distinct impacts on your router's resources. This post explains the key differences between Soft Reconfiguration and Route Refresh , when to use each, and why Route Refresh is preferred in most modern networks. 1. What is BGP Soft Reconfiguration? BGP Soft Reconfiguration is an older method of applying new policies (like route maps, filters, or prefix lists) without resetting the BGP session. It works by storing a local copy of all the routes received from a BGP neighbor before applying inbound policies. This local route copy allows the router to reprocess the routes when a policy change occurs. How So...
Post a Comment
Read more

BGP Local Preference Controlling Outbound Traffic in BGP

In BGP, Local Preference is used to control the outbound traffic path. It helps you decide which egress point (exit point) should be used when you have multiple connections to external networks, such as ISPs. Local Preference is an attribute that is local to your AS and is shared with all iBGP peers but not with eBGP neighbors. Higher Local Preference = More preferred outbound path. Example Scenario : You have two external links: ISP1 (via CE1) and ISP2 (via CE2). You want traffic to prefer ISP1 for all outbound traffic. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP2): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Higher Local Preference) : Create a route map to set the local preference to 200 for routes learned from CE1: route-map SET_LOCAL_PREF permit 10 set local-preference 200 In the BGP configuration for CE1, apply this route map to the neighbor: router bgp 65001 ne...
Post a Comment
Read more