topic Enabling unicast reverse path forwarding ASA in Network Security

Enabling unicast reverse path forwarding ASA

Anish Chauhan — Tue, 12 Mar 2019 03:04:28 GMT

I've experienced a few issues with enabling anti-spoofing through the unicast reverse path forwarding feature on ASAs.

The version in question is 9.1.3 on an HA pair of ASA5550's.

When enabling this through the commands: ip verify reverse-path interface interface_name it seemed to cause a couple of issues:

(it was enabled on all interfaces to begin with)

1. The first issue was that all outbound traffic was dropped altogether.

2. The second issue was that for connected remote access VPN clients, they were able to connect fine but subsequent outbound internet connectivity was unavailable until this page was refreshed several times. This issue went away as soon as this feature was removed.

I know that unicast reverse path forwarding typically should only uncover problems with the flow of network due to diverse routing etc. but I was wondering whether anyone has uncovered any strangle anomolies when implementing this.

Thanks, Anish

Enabling unicast reverse path forwarding ASA

jumora — Thu, 14 Nov 2013 03:24:08 GMT

Can you please post the configuration and show route plus show arp.

Enabling unicast reverse path forwarding ASA

Marius Gunnerud — Thu, 14 Nov 2013 08:23:36 GMT

The only times I have had issues with uRPF is when I made configuration mistakes. You might already know this but uRPF makes decisions based on what is in your routing table.

Do you by chance have asymmetric routing in your network?

Enabling unicast reverse path forwarding ASA

jumora — Thu, 14 Nov 2013 14:00:29 GMT

For outside traffic, for example, the ASA can use the default route to satisfy Unicast RPF protection. If traffic enters from an outside interface, and the source address is not known to the routing table, the ASA uses the default route to correctly identify the outside interface as the source interface.

Enabling unicast reverse path forwarding ASA

jumora — Thu, 14 Nov 2013 14:01:58 GMT

If the packet comes from a different ARP entry than the associated to the default gateway IP it drops it

Enabling unicast reverse path forwarding ASA

jumora — Thu, 14 Nov 2013 14:05:05 GMT

Well it could be said other IP other than default gateway if it does not have the route to that address.

Enabling unicast reverse path forwarding ASA

Marius Gunnerud — Thu, 14 Nov 2013 14:06:52 GMT

Another possibility is that if the ASA receives a packet on an interface, but it actually expects to receive it on another, that packet will be considered as a spoof and it will be dropped.

Enabling unicast reverse path forwarding ASA

Anish Chauhan — Sun, 24 Nov 2013 21:32:51 GMT

Hi All,

Thanks so much for your replies. It turned out that there was neither diverse/asymmetric routing in place but the drops were all packets that should have been dropped such as broadcasts. We never got to the bottom of the VPN anomaly as it seemed to resolve itself but that seemed to be AV linked.

Thanks, Anish

Enabling unicast reverse path forwarding ASA

Julio Carvajal — Mon, 25 Nov 2013 05:18:57 GMT

Hello Anish,

Just as a comment remember that the ASA only supports RPF Strict mode so whenever a packet violates that check the traffic will be drop.

Asymetric routing was the cause of the problem.

Packet-captures and logs for the next time

Any other question you have? Otherwise you can mark it as answered~

Rate all of the helpful posts!!!

Regards,

Jcarvaja

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