Using ACL to Mitigate IP Address Spoofing

Spoofing is a practice used to gain unauthorised access to machines/server applications, whereby an attacker unlawfully imitates another machine by manipulating IP packets. Although IP spoofing is not an attack, it is the starting point of many attacks prevalent in today’s networks.

Spoofing attack initiates from outside unsecured Internet and someone on the outside of the LAN network spoofs the company inside IP address to make it look like they are part of the inside of your network.

The key here is to deny RFC 1918 IP block that is coming into your network from the outside interface. In addition to RFC 1918 block of addresses, we also need to block any source coming from local loopback, Network, Broadcast address and multicast range –

RFC 1918 private address block is –

Other sources to block include –

Step by step configuration to mitigate IP Spoofing attack is as below –

Step 1 –

Step 2 –

Traffic is permitted to the destination address 192.168.5.0 /24 which is LAN IP Address

Step 3 –

Apply ACL to outside Interface (Gi0/0 is outside/Internet Interface)

Not only do we want to prevent people from the outside of our network ‘pretending’ they are inside our network, we also want to prevent our inside people from spoofing to a different IP address as well. The above configuration had a focus on traffic INBOUND to our network,

Step 4 –

The only range that is allowed to transmit is my internal network, in this case 192.168.5.0 /24

Step 5 –

Apply ACL to outside Interface (Gi0/0 is inside/secured Interface)

Related- Difference b/w ACL on Router and Firewall

Related- Time-based ACL