UNICAST REVERSE PATH FORWARDING (URPF) IS A SECURITY FEATURE
THAT HELPS TO PREVENT IP SPOOFING ATTACKS. IT WORKS BY VERIFYING THAT THE
SOURCE IP ADDRESS OF A PACKET IS REACHABLE FROM THE INTERFACE THAT THE PACKET
WAS RECEIVED ON. IF THE SOURCE IP ADDRESS IS NOT REACHABLE, THE PACKET IS
DROPPED.
URPF IS TYPICALLY USED ON ROUTERS THAT ARE CONNECTED TO THE
PUBLIC INTERNET. THIS IS BECAUSE THE PUBLIC INTERNET IS A SHARED MEDIUM, AND IT
IS EASY FOR ATTACKERS TO SPOOF IP ADDRESSES. URPF CAN HELP TO PROTECT NETWORKS
FROM THESE ATTACKS BY PREVENTING THEM FROM FORWARDING SPOOFED PACKETS.
URPF HAS TWO MODES: STRICT MODE AND LOOSE MODE. IN STRICT
MODE, THE PACKET IS DROPPED IF THE SOURCE IP ADDRESS IS NOT REACHABLE ON THE
SAME INTERFACE THAT THE PACKET WAS RECEIVED ON. IN LOOSE MODE, THE PACKET IS
DROPPED IF THE SOURCE IP ADDRESS IS NOT REACHABLE ON ANY INTERFACE.
URPF IS A VALUABLE SECURITY FEATURE THAT CAN HELP TO PROTECT
NETWORKS FROM IP SPOOFING ATTACKS. HOWEVER, IT IS IMPORTANT TO NOTE THAT URPF
CAN ALSO DROP LEGITIMATE TRAFFIC. THIS IS BECAUSE THERE ARE CASES WHERE A
PACKET MAY BE RECEIVED ON AN INTERFACE, BUT THE SOURCE IP ADDRESS IS NOT
REACHABLE ON THAT INTERFACE. FOR EXAMPLE, THIS CAN HAPPEN IF THERE IS AN
ASYMMETRIC ROUTING PATH BETWEEN THE ROUTER AND THE SOURCE OF THE PACKET.
IF YOU ARE CONSIDERING DEPLOYING URPF, IT IS IMPORTANT TO
CAREFULLY EVALUATE THE RISKS AND BENEFITS. YOU SHOULD ALSO MAKE SURE THAT YOU
UNDERSTAND THE POTENTIAL IMPACT OF URPF ON LEGITIMATE TRAFFIC.
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