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{{short description|Multicast routing technique to minimize loops and enhance security}}
{{No footnotes|date=May 2019}}
'''Reverse-path forwarding''' ('''RPF
| title=Reverse Path Forwarding
| date=2010
| publisher=[[Juniper Networks]]
| access-date=2021-05-12}}</ref>
In standard unicast [[IP routing]], the router forwards the packet away from the source to make progress along the distribution tree and prevent routing loops. In contrast, the router's multicast forwarding state runs more logically by organizing tables based on the reverse path, from the receiver back to the root of the distribution tree at the source of the multicast. This approach is known as reverse-path forwarding.
== Multicast RPF ==
Multicast RPF, typically denoted simply as RPF, is used in conjunction with a multicast [[routing protocol]] such as [[Multicast Source Discovery Protocol]] or [[Protocol Independent Multicast]] to ensure loop-free forwarding of multicast packets. In multicast routing, the decision to forward traffic is based upon source address and not on destination address as in unicast routing. It does this by utilizing either a dedicated multicast routing table or, alternatively, the router's unicast routing table.
When a multicast packet enters a router's interface, the router looks up the list of networks that are reachable via that interface (i.e., it checks the paths by which the packet could have arrived). If the router finds a matching routing entry for the source IP address of the multicast packet, the RPF check passes and the packet is forwarded to all other interfaces that are participating in that multicast group. If the RPF check fails, the packet is dropped. As a result, the forwarding of the packet is decided based upon the reverse path of the packet rather than the forward path. By only forwarding packets that come into the interface that also holds the routing entry for the source of the packet, loops are prevented.
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# the unicast routing table is correct and stable and,
# the path used from a sender to a router and the reverse path from the router back to the sender are symmetric.
If the first assumption is false, the RPF check will fail because it depends upon the router's unicast routing table as a fallback. If the second assumption is false, the RPF check would reject multicast traffic on all but the shortest path from the sender to the router which would lead to a non-optimal multicast tree. In cases where the links are unidirectional, the reverse-path approach can fail altogether.
== Unicast RPF
'''Unicast RPF''' (uRPF), as defined in RFC 3704, is an evolution of the concept that traffic from known invalid networks should not be accepted on interfaces from which
| url=https://tools.cisco.com/security/center/resources/unicast_reverse_path_forwarding
| title=Understanding Unicast Reverse Path Forwarding
| publisher=[[Cisco Systems]]
| access-date=2021-05-12}}</ref>
uRPF extends this idea by utilizing the knowledge all routers must have
In cases of symmetric routing, routing where packets flow
Unfortunately, it is often the case on the larger Internet backbone that routing is asymmetric and the routing tables cannot be relied upon to point to the best route for a source to get to a router. Routing tables specify the best forward path and only in the symmetric case does that equate to the best reverse path.
RFC 3704 gives more details on how to extend
===Strict mode===
In strict mode, each incoming packet is tested against the FIB and, if the ''incoming'' interface is not the best reverse path, the packet check will fail. By default failed packets are discarded.{{efn|name=command|Example command on Cisco devices: ip verify unicast source reachable-via {rx} - Strict mode, {any} - loose mode}}
===Feasible mode===
In feasible mode, the FIB maintains alternate routes to a given
===Loose mode===
In loose mode each incoming packet's source address is tested against the FIB. The packet is dropped only if the source address is not reachable via ''any'' interface on that router.{{efn|name=command}}
== {{anchor|Filtering}}
RPF is often
Reverse path filters are typically used to disable asymmetric routing where an IP application has a different incoming and outgoing routing path. Its intent is to prevent a packet entering one interface from leaving via the other interfaces. Reverse-path Filtering is a feature of the [[Linux Kernel]].<ref>{{cite web
| url=https://www.theurbanpenguin.com/rp_filter-and-lpic-3-linux-security/
| title=rp_filter and LPIC-3 Linux Security
| date=2020-08-27
| website=theurbanpenguin.com
| access-date=2021-05-12}}</ref>
== See also ==
* [[Flooding (computer networking)]]
==Notes==
{{Notelist}}
==References==
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== External links ==
* {{cite IETF |RFC=2827 |title=Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing}}
* {{cite IETF |RFC=3704 |title=Ingress Filtering for Multihomed Networks}}
▲* [http://www.juniper.net/techpubs/software/erx/erx50x/swconfig-routing-vol1/html/ip-multicast-config7.html#120398 Juniper Networks on multicast RPF]
[[Category:Routing]]
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