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: 
> https://tools.cisco.com/security/center/resources/unicast_reverse_path_forwarding
> I think all router vendors support this feature. Here's a similar article by 
> Juniper: 
> https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/interfaces-configuring-unicast-rpf.html
> -mel beckman
>> On Oct 13, 2020, at 3:15 PM, Brian Knight via NANOG <nanog@nanog.org> 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,
>> -Brian

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