nanog mailing list archives
Re: Ingress filtering on transits, peers, and IX ports
From: Brian Knight via NANOG <nanog () nanog org>
Date: Tue, 13 Oct 2020 17:49:42 -0500
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? Thanks, -Brian 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 beckmanOn 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
Current thread:
- Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Mel Beckman (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Matt Harris (Oct 13)
- RE: Ingress filtering on transits, peers, and IX ports Jean St-Laurent via NANOG (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Marcos Manoni (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Nikolas Geyer (Oct 13)
- Re: Ingress filtering on transits, peers, and IX ports Brandon Martin (Oct 14)
- Re: Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 14)
- Re: Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 14)
- RE: Ingress filtering on transits, peers, and IX ports Jean St-Laurent via NANOG (Oct 15)
- Re: Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 19)
- Re: Ingress filtering on transits, peers, and IX ports Randy Bush (Oct 19)
- Re: Ingress filtering on transits, peers, and IX ports Baldur Norddahl (Oct 20)
- Re: Ingress filtering on transits, peers, and IX ports Brian Knight via NANOG (Oct 22)
- Re: Ingress filtering on transits, peers, and IX ports Mel Beckman (Oct 13)