5.7 Identity and memory

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A DHCP server recognises a client by its client identifier, and a reservation on the server matches on it. netd keeps the identifier stable across boots and NIC swaps in the same slot, and remembers per network what it was given.

5.7.1 The DUID #

The machine's DHCP unique identifier, shared by DHCPv4 and DHCPv6 (RFC 4361), is decided once per netd process, the first time a client needs it:

  1. the registry's Machine\System\Network Duid, as hex, when it is set and parses;
  2. otherwise the file /var/state/netd/duid, when it holds at least 4 bytes;
  3. otherwise a new DUID-LL: 00 03 00 01 followed by the MAC of the interface whose client is starting. It is written to /var/state/netd/duid; a failure to write is logged and changes nothing else.

When the registry's Duid is absent, empty or not hex, the result of 2 or 3 is written to it, as colon-separated lower-case hex, replacing a value that was not hex.

Hex in Duid (and ClientId) can be written with :, - or spaces between digits, or none. A value with any other character, an odd number of digits, or no digits is not hex.

5.7.2 The client identifier #

Per interface, Interfaces\<id> ClientId:

  • when the value is set and is non-empty hex, it is sent as written;
  • when it is absent, the identifier is generated and written back: ff, then a 4-byte IAID, then the DUID. The IAID is the interface id's text folded into 4 bytes by XOR, byte i of the text into byte i mod 4;
  • when it is set but not hex, netd logs <id>: ClientId "<value>" is not hex; regenerating, where <id> is the interface id, generates one, and overwrites the value.

ClientId is a value both netd and the operator write. Writing it is how an operator makes netd present the identifier a server reservation expects. It is read when a client starts, so it takes effect at the next start (a carrier loss, a profile edit, a restart of netd).

5.7.3 The address a network gave #

While an interface holds a lease on an identified network (§7.1), netd writes the lease address to that network's Networks\<id> RequestedAddress whenever it differs from what is there.

When a client starts, it takes as its previous address (§5.1):

  1. the RequestedAddress of the network currently identified on the interface, when there is one; otherwise
  2. the RequestedAddress of the network the interface's Status LastNetwork names (§8.1).

With one, the client opens with an INIT-REBOOT REQUEST for it. A server that agrees answers with an ACK and the machine keeps the address in one round trip. One that refuses answers with a NAK and the client discovers afresh. Silence for two transmissions does the same (§5.2).

RequestedAddress is also the operator's: writing it is a soft reservation, an address the client asks for first and the server can refuse.

Nothing about a lease itself — its timers, its server, its options — is persisted. A machine that reboots asks for its address again.

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