# 5.5 Link-local fallback

_Peios / Peios technical reference / netd / Dhcpv4_

> The 169.254/16 address netd self-assigns when DHCPv4 discovery goes unanswered — when, which address, and when it goes.

With `Address.LinkLocal` in the interface's profile, an interface whose
DHCPv4 discovery goes unanswered gets an IPv4 link-local address
(RFC 3927), so two machines on a cable with no server can still reach
each other.

## 5.5.1 When [*linklocal.when]

At the client's "nobody answered" report (§5.2), about 28 s after
discovery began, netd logs `interface <name>: no DHCP offer; link-local
<address>` and the interface desires the address. Discovery carries on
throughout.

The address is desired at prefix 16 only while no lease is held and the
profile has no `Address.Static` entry of either family (§4.2). A profile
with a static address therefore never shows one, even after the report.
[*linklocal.not-beside-a-lease-or-static]

## 5.5.2 Which address [*linklocal.address-derivation]

A pure function of the interface, so it is the same every time:

- the seed is the last four bytes of the MAC, read as a big-endian
  32-bit number, or, for a link with no MAC, the interface index times
  2654435761, wrapping at 32 bits;
- the host part is 256 plus the seed modulo 64768;
- the address is `169.254.<host / 256>.<host % 256>`.

That lands in 169.254.1.0 to 169.254.254.255, the range RFC 3927 allows
for self-assignment.

netd does not probe for a conflicting holder or announce the address
(RFC 3927 §2.2 and §2.4). Two machines whose MACs share their last four
bytes choose the same address.

## 5.5.3 When it goes [*linklocal.dropped-on-lease-or-stop]

The link-local address is forgotten when a lease binds, and when the
client is stopped (§5.4). The next reconcile removes it, because netd owns
every IPv4 address on a joined interface (§4.3).