Speaking as an ISP:
Most of the ISP networks I manage are multi-homed, and I don't
think uRPF provides the knobs to ensure legitimate traffic doesn't get
dropped in some cases, so we use static ACLs at the upstream edge on
ingress (and egress). These need updated any time new IP space is added
to the network (not very often).
Ingress ACL: Discard if source or destination is a bogon, RFC
1912, RFC1918; discard any traffic sourced from our own IP space
(spoofed); discard any traffic that is not destined for our IP space
(bad/mis-routed).
Egress ACL: Discard if source or destination is a bogon, RFC
1912, RFC1918; discard any traffic that is not sourced from our IP space
(spoofed).
I believe in a policy of non-blocking and being net neutral,
but if any TCP/UDP ports, IP protocols, or IP options are blocked you
might add them to the above ACLs.
On customer facing ports we use uRPF strict. Why? It's easy (one
line to implement, zero maintenance, and it works well on our Cisco
ASR1k/9k platforms)! Our customers are all single homed.
For a single homed enterprises, service providers, and end-users I'd
recommend uRPF strict. Why? Again, it's dead simple. I would love to see
this be the default on all home, prosumer, and branch office oriented
platform. Linux does this by default with the rp_filter kernel option. I
suspect that networking gear based on Linux probably leaves this at the
default setting (strict mode).
In practice I don't know that I've ever used uRPF loose mode. ACLs have
counters to verify they are working. I might have confirmed uRPF was
working as intended the first time I implemented it, but otherwise I've
used ACLs often enough to trust they are working as configured and trust
the same for uRPF.
--Blake
On 10/13/2020 5:13 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,
-Brian
