What is DAD (Duplicate Address Detection) and how does it work?

DAD uses ICMPv6 (Internet Control Message Protocol version 6) NS (Neighbor Solicitation) to verify that a newly generated IPv6 address is unique on the local link before the host starts using it.

* Duplicate Address Detection via an NS probe. *

* NS/NA exchange performing Duplicate Address Detection. — Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons. *

The DAD process:

1. Host generates its IPv6 GUA (Global Unicast Address) (via SLAAC (Stateless Address Autoconfiguration) or any method)
2. Host sends an ICMPv6 NS (Neighbor Solicitation) message to a specially constructed solicited-node multicast address containing the last 24 bits of the new address
3. If no NA (Neighbor Advertisement) comes back → the address is unique, host starts using it
4. If an NA is received → the address is already in use, host must generate a new interface ID

Why DAD exists:

• Even though the chance of a duplicate is astronomically low (64-bit interface ID = 18 quintillion possibilities), the IETF (Internet Engineering Task Force) recommends performing DAD
• Most operating systems perform DAD on all IPv6 unicast addresses, regardless of how they were configured (SLAAC, DHCPv6 (Dynamic Host Configuration Protocol version 6), or manual)

Solicited-node multicast address format:

• Always starts with ff02::1:ff followed by the last 24 bits of the IPv6 address
• Example: for address 2001:db8::1234:5678, the solicited-node address would be ff02::1:ff34:5678

Go deeper:

• RFC 4862 §5.4 — Duplicate Address Detection — specifies the Neighbor Solicitation probe on a tentative address before assignment.