uPRF strict more

Brian Johnson brian.johnson at netgeek.us
Fri Oct 1 13:31:15 UTC 2021

For strict-mode... Completely agree.

As has been previously said, this is a tool that all players involved need to understand. This is no different than everyone correctly using BGP in their application for their outcomes.

> On Sep 29, 2021, at 12:07 PM, Adam Thompson <athompson at merlin.mb.ca> wrote:
> We just ran into a typical case where uRPF caused a partial outage for one of my customers: the customer is multi-homed, with another provider that I'm also​ connected to.  Customer advertised a longer-prefix to the other guy, so I started sending traffic destined for Customer to the Other Provider... who then promptly dropped it because they had uRPF enabled on the peering link, and they were seeing random source IPs that weren't mine.  Well... yeah, that can happen (semi-legitimately) anytime you have a topological triangle in peering.
> I've concluded over the last 2 years that uRPF is only​ useful on interfaces pointing directly at non-multi-homed customers, and actively dangerous anywhere else.
> -Adam
> Adam Thompson
> Consultant, Infrastructure Services
> 100 - 135 Innovation Drive
> Winnipeg, MB, R3T 6A8
> (204) 977-6824 or 1-800-430-6404 (MB only)
> athompson at merlin.mb.ca <mailto:athompson at merlin.mb.ca>
> www.merlin.mb.ca <http://www.merlin.mb.ca/>
> From: NANOG <nanog-bounces+athompson=merlin.mb.ca at nanog.org> on behalf of Amir Herzberg <amir.lists at gmail.com>
> Sent: September 28, 2021 20:06
> To: Randy Bush <randy at psg.com>
> Cc: North American Network Operators' Group <nanog at nanog.org>
> Subject: Re: uPRF strict more
> Randy, great question. I'm teaching that it's very rarely, if ever, used (due to high potential for benign loss); it's always great to be either confirmed or corrected... 
> So if anyone replies just to Randy - pls cc me too (or, Randy, if you could sum up and send to list or me - thanks!)
> Amir
> -- 
> Amir Herzberg
> Comcast professor of Security Innovations, Computer Science and Engineering, University of Connecticut
> Homepage: https://sites.google.com/site/amirherzberg/home <https://sites.google.com/site/amirherzberg/home>
> `Applied Introduction to Cryptography' textbook and lectures: https://sites.google.com/site/amirherzberg/applied-crypto-textbook <https://sites.google.com/site/amirherzberg/applied-crypto-textbook>
> On Tue, Sep 28, 2021 at 8:50 PM Randy Bush <randy at psg.com <mailto:randy at psg.com>> wrote:
> do folk use uPRF strict mode?  i always worried about the multi-homed
> customer sending packets out the other way which loop back to me;  see
> RFC 8704 §2.2
> do vendors implement the complexity of 8704; and, if so, do operators
> use it?
> clue bat please
> randy

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