DUIDs

DUIDs, or DHCP Unique Identifiers, are identifiers used by DHCPv6 clients to identify themselves to DHCPv6 servers. They are designed to be persistent across reboots and interface changes, so a server can recognize the same client even if its IPv6 address changes. The DUID is created by the client and used during DHCPv6 exchanges to bind addresses and other configuration to that particular client.

The DUID is a variable-length identifier that begins with a type field. The most common DUID types defined in RFC 6355 are: DUID-LLT (type 1), which encodes the hardware type, a timestamp, and the link-layer address; DUID-EN (type 2), which uses an enterprise number and a vendor-specific identifier; DUID-LL (type 3), which contains the hardware type and link-layer address only; and DUID-UUID (type 4), which carries a 128-bit UUID. Each type has a distinct data layout but all serve the same purpose of uniquely identifying the client.

DHCPv6 servers use the DUID to assign addresses and configuration, and clients typically preserve the DUID across reboots to maintain continuity of bindings. Some operating systems allow regenerating or replacing the DUID, which can affect existing DHCPv6 leases. Privacy considerations exist, particularly for DUID-LLT and DUID-LL, which can reveal hardware identifiers such as MAC addresses; DUID-UUID offers a more opaque option in some deployments.

In DHCPv6, the DUID is used in conjunction with an IAID (Identity Association ID) to form per-interface identity associations. The concept and formal definitions are described in RFC 6355.