Anti-spoofing filters

Terry Slattery
Principal Architect

I’ve been doing more consulting work and am surprised by the number of organizations that don’t use anti-spoofing filters within their networks.  An anti-spoofing filter is placed on the input side of a router interface of a user subnet and only allows packets through that are within the address range of that subnet.  The intent is to exclude packets that have invalid source addresses.  Just make sure that the filter doesn’t exclude the multicast and broadcast addresses and routing protocol updates that are needed for normal network operations.

I would want to log all denies in an anti-spoofing filter and send them to the security group.  The report would allow them to identify that a system has been compromised or misconfigured.  While it is more work to configure the anti-spoofing filters, they provide an additional level of visibility into what is happening in the network.

I think of anti-spoofing filters as the inverse of Netflow.  Netflow tells you what traffic is being forwarded while an anti-spoofing filter tells you that certain traffic is not being forwarded.  Modern security recommendations are “security in depth” and anti-spoofing filters are another valuable part of the security toolkit.



Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under


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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.