What is Duplicate Address Detection (DAD) in ICMPv6?

Before using an IPv6 address, a device sends an NS targeting that very address; if a neighbor already uses it, that neighbor replies with an NA. No reply means the address is unique and safe to use.

Duplicate Address Detection (DAD):

A process used in IPv6 to ensure an address is unique before using it.

* The device probes its own tentative address with an NS — a returned NA means someone else has it, while silence means it is safe to use. *

How DAD Works:

Device                          Network
   │                               │
   │─── NS (target: own address) ─→│  "Anyone using this address?"
   │                               │
   │←── NA (if duplicate) ─────────│  "Yes, I'm using it!"
   │                               │

Process:

1. Device assigns itself an IPv6 address (GUA or LLA)
2. Device sends Neighbor Solicitation (NS) with its own address as target
3. If another device has that address, it responds with Neighbor Advertisement (NA)
4. No response = Address is unique, safe to use
5. Response received = Duplicate! Don't use that address

When DAD is performed:

• When a new IPv6 address is configured
• Applies to both Global Unicast (GUA) and Link-Local (LLA) addresses
• DAD is not required, but RFC 4861 recommends DAD on unicast addresses

Key insight: DAD prevents IP address conflicts on IPv6 networks, similar to gratuitous ARP in IPv4.

Go deeper:

• RFC 4861 — Neighbor Discovery for IP version 6 (IPv6) — specifies Duplicate Address Detection and recommends it on unicast addresses.
• Neighbor Discovery Protocol — Wikipedia — DAD among the autoconfiguration safeguards of NDP.