So I have put together what I think is a reasonable and complete ACL. From my time in the enterprise world, I know that a good ingress ACL filters out traffic sourcing from:

* Bogon blocks, like,, RFC1918 space, etc (well-documented in
* RIR-assigned blocks I am announcing to the rest of the world

However, I recognized a SP-specific case where we could receive legitimate traffic sourcing from our own IP blocks: customers running multi-homed BGP where we have assigned PA space to them. So I added "permit" statements for traffic sourcing from these blocks.

Also, we have direct peering links that are numbered within our assigned prefixes. So we can use the same ACL with these peer interfaces and continue to have BGP work, I added "permit" statements for these point-to-point subnets.

So the order of the statements is:

* Permit where source is direct peer PtP networks
* Permit where source is BGP customer PA prefix
* Deny where source is bogon
* Deny where source is our advertised prefixes
* Permit all other traffic

I considered BGP customer PI prefixes to be out of scope for ingress filtering, since the customer is likely to be multi-homing. Should we consider filtering them?

The Team Cymru Secure IOS Template [] also references an ICMP fragment drop entry on the ingress ACL. I think that's good for an enterprise network, but as an SP, I'm very hesitant to include this. Is this included in anyone else's transit / peer / IX ACL?

Is there anything else that I'm not thinking of?



On 2020-10-14 09:25, Brian Knight via NANOG wrote:
Hi Marcos,

Thanks for your reply.  But I am looking for guidance on traffic
filtering, not BGP prefix filtering.

I have looked at BCP 84, and it's a good overview of the methods
available to an ISP.  My questions are more operational in nature and
are not covered by the document.  Of the choices presented in BCP 84,
what do folks really use?  If it's an ACL, what challenges have there
been with updates?  etc.


On 2020-10-13 18:52, Marcos Manoni wrote:
Hi, Brian

Check RFC3704/BCP84 Ingress Filtering for Multihomed Networks (Updated
by: RFC8704 Enhanced Feasible-Path uRPF).

    Ingress Access Lists require typically manual maintenance, but are
    the most bulletproof when done properly; typically, ingress access
    lists are best fit between the edge and the ISP when the
    configuration is not too dynamic if strict RPF is not an option,
    between ISPs if the number of used prefixes is low, or as an
    additional layer of protection

Ingress filters Best Practices
*Don’t accept BOGON ASNs
*Don’t accept BOGON prefixes
*Don’t accept your own prefix
*Don’t accept default (unless you requested it)
*Don’t accept prefixes that are too specific
*Don’t accept if AS Path is too long
*Create filters based on Internet Routing Registries

And also BGP Best Current Practices by Philip Smith


El mar., 13 oct. 2020 a las 19:52, Brian Knight via NANOG
(<>) escribió:

Hi Mel,

My understanding of uRPF is:

* Strict mode will permit a packet only if there is a route for the source IP in the RIB, and that route points to the interface where the packet was received

* Loose mode will permit a packet if there is a route for the source IP in the RIB. It does not matter where the route is pointed.

Strict mode won't work for us, because with our multi-homed transits and IX peers, we will almost certainly drop a legitimate packet because the best route is through another transit.

Loose mode won't work for us, because all of our own prefixes are in our RIB, and thus the uRPF check on a transit would never block anything.

Or am I missing something?



On 2020-10-13 17:22, Mel Beckman wrote:

You can also use Unicast Reverse Path Forwarding. RPF is more efficient than ACLs, and has the added advantage of not requiring maintenance. In a nutshell, if your router has a route to a prefix in its local RIB, then incoming packets from a border interface having a matching source IP will be dropped.

RPF has knobs and dials to make it work for various ISP environments. Implement it carefully (as is be standing next to the router involved :)

Here's a Cisco brief on the topic:

I think all router vendors support this feature. Here's a similar article by Juniper:

-mel beckman

On Oct 13, 2020, at 3:15 PM, Brian Knight via NANOG <> wrote:

We recently received an email notice from a group of security researchers who are looking at the feasibility of attacks using spoofed traffic. Their methodology, in broad strokes, was to send traffic to our DNS servers with a source IP that looked like it came from our network. Their attacks were successful, and they included a summary of what they found. So this message has started an internal conversation on what traffic we should be filtering and how.

This security test was not about BCP 38 for ingress traffic from our customers, nor was it about BGP ingress filtering. This tested our ingress filtering from the rest of the Internet.

It seems to me like we should be filtering traffic with spoofed IPs on our transit, IX, and peering links. I have done this filtering in the enterprise world extensively, and it's very helpful to keep out bad traffic. BCP 84 also discusses ingress filtering for SP's briefly and seems to advocate for it.

We have about 15 IP blocks allocated to us. With a network as small as ours, I chose to go with a static ACL approach to start the conversation. I crafted a static ACL, blocking all ingress traffic sourced from any of our assigned IP ranges. I made sure to include:

* Permit entries for P-t-P WAN subnets on peering links

* Permit entries for IP assignments to customers running multi-homed BGP

* The "permit ipv4 any any" at the end :)

The questions I wanted to ask the SP community are:

* What traffic filtering do you do on your transits, on IX ports, and your direct peering links?

* How is it accomplished?  Through static ACL or some flavor of uRPF?

* If you use static ACLs, what is the administrative overhead like? What makes it easy or difficult to update?

* How did you test your filters when they were implemented?

Thanks a lot,


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